(12) INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
`
`(19) World Intellectual Property Organization ”Va | I
`International Bureau
`
`(43) International Publication Date
`15 November 2007 (15.11.2007)
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`(51)
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`International Patent Classification:
`H01L 31/00 (2006.01)
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`(81)
`
`I
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`
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`(21)
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`International Application Nulnber:
`PCT/US2007/003459
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`(22)
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`International Filing Date: 9 Februaiy 2007 (09.02.2007)
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`(25)
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`Filing Language:
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`(26)
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`Publication Language:
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`(30)
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`Priority Data:
`11/381,681
`
`English
`
`English
`
`(84)
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`4 May 2006 (04.05.2006)
`
`US
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`(71)
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`(72)
`(75)
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`(74)
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`Applicant (for all designated States except US): SUN-
`POVVER CORPORATION [US/US]; 3939 N. lst Street,
`San Jose, CA 95134 (US).
`
`Inventor; and
`Inventor/Applicant (for US only): COUSINS, Peter,
`John [AU/PH]; 302 Apo Street, Ayala Alabang, Alabang,
`1780 Muntinlupa City (PH).
`
`Agents: WOODWARD, Henry, K. et al.; Beyer Weaver
`LLP, P.o. Box 70250, Oakland, CA 94612—0250 (US).
`
`(10) International Publication Number
`
`WO 2007/130188 A2
`
`Designated States (unless otherwise indicated, for every
`kind of national protection available): AE. AG, AL, AM,
`AT, AU, AZ, BA, BB, BG, BR, BW, BY, BZ, CA, CH, CN,
`CO, CR, CU, CZ, DE, DK, DM, DZ, EC, EE, EG, ES, FI,
`GB, GD, GE, GH, GM, GT, HN, I—fR, HU, ID, IL, TN, IS,
`JP, KE, KG, KM, KN, KP, KR, KZ, LA, LC, LK, LR, LS,
`LT, LU, LV, LY, MA, MD, MG, MK, MN, MW, MX, MY,
`MZ, NA, NG, NI, NO, NZ, OM, PG, PH, PL, PT, RO, RS,
`RU, SC, SD, SE, SG, SK, SL, SM, SV, SY, TJ, TM, TN,
`TR, TT, TZ, UA, UG, US, UZ, VC, VN, ZA, ZM, ZW.
`
`Designated States (unless otherwise indicated, for every
`kind of regional protection available): ARTPO (BW, GH,
`GM, KE, LS, MW, NIZ, NA, SD, SL, SZ, TZ, UG, ZM,
`ZW), Eurasian (AM, AZ, BY, KG, KZ, MD, RU, TJ, TM),
`European (AT, BE, BG, CH, CY, CZ, DE, DK, EE, ES, Fl,
`FR, GB, GR, HU, IE, IS, IT, LT, LU, LV, MC, NL, PL, PT,
`RO, SE, SI, SK, TR), OAPI (BF, BJ, CF, CG, CI, CM, GA,
`GN, GQ, GW, ML, MR, NE, SN, TD, TG).
`
`Published:
`without international search report and to be republished
`upon receipt of that report
`
`For two—letter codes and other abbreviations, refer to the ”Guid—
`ance Notes on Codes and Abbreviations ” appearing at the beg in—
`ning ofeach regular issue ofthe PCT Gazette.
`
`(54) Title: SOLAR CELL HAVING DOPED SEMICONDUCTOR HETEROJUNCTION CONTACTS
`
`
`
`(57) Abstract: A silicon solar cell has doped amorphous silicon contacts formed on a tunnel silicon oxide layer on a surface of a
`silicon substrate. High temperature processing is unnecessary in fabricating the solar cell.
`
`
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`W02007/130188A2|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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`WO 2007/130188
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`PCT/US2007/003459
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`SOLAR CELL HAVING DOPED SEMICONDUCTOR
`HETEROJUNCTION CONTACTS
`
`BACKGROUND OF THE INVENTION
`
`[0001] This invention relates generally to photovoltaic solar cells, and more
`
`5
`
`particularly the invention relates to a solar cell structure which is efficient in operation
`
`and economical to manufacture.
`
`[0002] The use of photovoltaic cells for the direct conversion of solar radiation into
`
`electrical energy is well known, see Swanson, US. Patent No. 4,234,352 for example.
`
`Briefly, the photovoltaic cell comprises a substrate of semiconductive material having
`
`10
`
`a pm junction defined therein.
`
`In the planar silicon cell the p-n junction is formed
`
`near a surface of the substrate which receives impinging radiation. Radiated photons
`
`create mobile carriers (holes and electrons) and the substrate which can be directed to
`
`an electrical circuit outside of the cell. Only photons having at least a minimum
`
`energy level (e.g., l.l electron volt for silicon) can generate an electron-hole pair in
`
`15
`
`the semiconductor pair. Photons having less energy are either not absorbed or are
`
`absorbed as heat, and the excess energy of photons having more than 1.1 electron volt
`
`energy (e.g. photons have a wavelength of LI um and less) create heat. These and
`
`other losses limit the efficiency of silicon photovoltaic cells in directly converting
`
`solar energy to electricity to less than 30%.
`
`20
`
`[0003] Solar cells with interdigitated contacts of opposite polarity on the back surface
`
`of the cell are known and have numerous advantages over conventional solar cells
`
`with front side metal grids and blanket or grid metallized backside contacts, including
`
`improved photo-generation due to elimination of front grid shading, much reduced
`
`grid series resistance, and improved "blue" photo-response since heavy front surface
`doping is not required to minimize front contact resistance because there are no front
`
`25
`
`contacts.
`
`In addition to the performance advantages, the back-contact cell structure
`
`allows simplified module assembly due to coplanar contacts.
`
`See Swanson US.
`
`Patent No. 4,927,770 for example.
`
`[0004] While interdigitated back-contact (lBC) solar cells have been fabricated, cost
`
`30
`
`considerations have limited commercialization of the lBC solar cell. Heretofore,
`
`conventional microelectronics (integrated circuit) processing has been employed in
`
`

`

`WO 2007/130188
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`PCT/US2007/003459
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`fabricating IBC solar cells, including the use of backside difl‘usions, contacts, and
`
`metal
`
`lines fabricated by conventional microelectronics photolithography, thin film
`
`metallization, and etching processes. This fabrication process is capable of producing
`
`high efficiency solar cells, but the process is not cost effective for application in
`
`5
`
`conventional
`
`low-cost,
`
`flat—plate solar panels.
`
`The key problem with practical
`
`realization of an IBC solar cell by this process is the high cost of fabrication,
`
`including etching, doping and mask alignment, and the use of thick metal conductor
`
`deposition by vacuum evaporation or sputtering. Further, the processing must be
`
`carried out in a clean room environment. Thus IBC solar cells fabricated using these
`
`10 methods have been restricted to application in high concentration solar cells or in very
`
`high value one-sun applications.
`
`[0005] Copending application serial no. 11/306,510 combines a semiconductor
`
`substrate with acceptor and donor polymer contacts to provide a solar cell which is
`
`economically fabricated. Importantly, fabrication of the solar cell, is improved in cost
`
`15
`
`and in reduced temperature cycling through use of inkjet application of the polymer
`
`contacts without the need for photoresist masking, etching, and dopant diffusion and
`
`annealing as is required in prior art solar cells.
`
`[0006] The present invention utilizes a semiconductor such as amorphous silicon as
`
`donor and acceptor contact in a silicon solar sell which can be readily and cost
`
`20
`
`effectively fabricated.
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`

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`WO 2007/130188
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`PCT/US2007/003459
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`SUMMARY OF THE INVENTION
`
`[0007] The invention utilizes doped amorphous silicon, Si-Ge, or III - V compounds
`
`as a donor or an acceptor contact in silicon solar cell.’ The contact material can be
`
`vapor deposited along with the dopant as necessary for donor or acceptor application.
`
`5
`
`As used herein, “amorphous” silicon includes “poly crystalline” silicon.
`
`[0008] When deposited on a single crystal silicon substrate, a tunnel oxide is first
`
`grown and separates a deposited amorphous silicon from the substrate to prevent re-
`
`crystallization of the amorphous silicon.
`
`[0009] In an interdigitated back contact (IBC) cell, the front surface can be textured
`
`10
`
`by chemical or physical abrasion to provide a radiation capturing surface with an anti-
`
`reflective and passivating coating such as silicon nitride, doped silicon carbide, or a
`
`thin coating of amorphous silicon over the textured surface.
`
`[0010] The invention and object and features thereof will be more readily apparent
`
`from the following detailed description and appended claims when taken with the
`
`15
`
`drawings.
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`

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`WO 2007/130188
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`PCT/US2007/003459
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`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0011] Fig.
`
`l
`
`is a side view in section of an interdigitated back contact solar cell
`
`including doped amorphous contact
`invention
`
`in accordance with one embodiment of the
`
`5
`
`[0012] Figs 2A - 2D are side views in section illustrating the solar cell of Fig.
`
`1
`
`during fabrication.
`
`

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`WO 2007/130188
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`PCT/US2007/003459
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`DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
`
`[0013] In accordance with one embodiment of the invention, an interdigitated back
`
`contact (IBC) solar cell comprising a silicon semiconductor body having first and
`second opposing major surfaces receives radiation through the firstsurface and has
`
`5
`
`first and second patterns of acceptor doped amorphous silicon and donor doped
`
`amorphous silicon, respectively, on the second or back surface for receiving electron
`
`and hole carriers created in the silicon substrate by radiated photons. The structure is
`
`similar to prior art back contact solar cells which utilize doped P and N conductivity
`
`contacts formed in the substrates for receiving the holes and electrons created by
`
`10
`
`radiation. However,
`
`the use doped P and N contacts in the substrate requires
`
`photoresist masking, etching, dopant diffusion, and high temperature processing in the
`
`fabrication of the solar cell. The use of acceptor and donor
`
`amorphous silicon
`
`contacts on the structure,
`
`in accordance with an embodiment of the invention,
`
`obviates the need for photoresist masking and dopant diffusion and the high
`
`15
`
`temperature processing required in annealing the diffused dopants. A tunnel silicon
`
`oxide can be placed between the contacts and the substrate to prevent epitaxial growth
`
`of the amorphous silicon on the substrate.
`
`[0014] Consider now Fig. 1 which is a side view in section of an interdigitated back
`
`contact (IBC) solar cell in accordance with one embodiment of the invention. The
`
`20
`
`cell includes a light n-type monocrystalline or polycrystalline substrate 10 having a
`
`front surface which receives radiation and a textured surface on which is formed a
`
`thin (cg. 10-150 angstrom) tunnel silicon oxide layer 12 with a passivation coating 14
`
`over tunnel oxide 12 which can comprise silicon nitride, doped silicon carbide, or a
`
`doped amorphous silicon layer.
`
`25
`
`[0015] On the back surface of substrate 10 is a second tunnel oxide layer 16 over
`
`which is formed P+ amorphous silicon contacts 18. A dielectric such as silicon oxide
`
`20 separates P+ amorphous silicon 18 from N+ amorphous silicon 22 which is formed
`
`in openings through P+ amorphous silicon layer 18 and in contact with tunnel oxide
`
`16. While the amorphous silicon layers 18, 22 are formed by low temperature vapor
`
`3O
`
`deposition, tunnel oxide 16 prevents any re-crystallization of the amorphous silicon
`
`by epitaxial growth from silicon substrate 10. Metal contact 24 engages P+
`
`

`

`WO 2007/130188
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`PCT/US2007/003459
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`amorphous silicon layer 18, and metal contacts 26 engage N+ amorphous silicon
`
`layers 22.
`
`[0016] The back contact heterojunction enhances the rear passivation of the solar cell
`
`by the inclusion of tunnel oxide 16, heterojunction field provided by the amorphous
`
`5
`
`silicon contacts, and contact passivation. As will be described further herein below, a
`
`process benefit
`
`in making the device isthat high temperature dopant drive is not
`
`required.
`
`[0017] Figs. 2A-2D are section views illustrating the solar cell of Fig.
`
`1 during
`
`fabrication.
`
`Initially, as shown in Fig. 2A, silicon substrate 10, which can be either
`
`10
`
`intrinsic or light doped, has a thin tunnel oxide 16 grown thereon which can have a
`
`thickness of from 10-20 angstrom, for example. Amorphous silicon layer l8 is then
`
`deposited with a boron dopant and a dopant concentration of ION-102' or 101320 -
`
`10E2l atoms per cubic centimeter and to a thickness of 500 to 2000 angstroms. The
`
`growth of a doped silicon layer by vapor deposition is a known silicon process.
`
`15
`
`Inclusion ‘of an intrinsic layer under the p-type doped silicon layer can be made if a
`
`PIN structure is desired.
`
`[0018] Thereafter, an insulating layer of silicon oxide 20 is deposited by low pressure
`
`chemical vapor deposition (LPCVD, PECVD, APCVD), or by a spin on glass process.
`
`Silicon oxide layer 16 is 500 to 1000 angstroms in this illustrative embodiment.
`
`20
`
`[0019] Thereafter, as illustrated in Fig. 23,
`
`the front surface of substrate 10 is
`
`textured by chemical or mechanical abrasion. This process step can precede the
`
`process steps of Fig. 2A, if desired. A photoresist mask is then formed on the back
`
`surface of the substrate 10 and etched to form openings through silicon oxide layer 20
`
`and amorphous silicon l8 to substrate [0. The thin tunnel oxide is removed in the
`
`25
`
`etching process also, and a new layer of tunnel silicon oxide is then applied on the
`
`exposed surface of substrate l0 through the etched openings by chemical growth.
`
`In
`
`forming tunnel oxide 16 in Fig. 2B, tunnel oxide layer 12 can be simultaneously
`
`formed on the front surface. Following the growth of the thin tunnel oxide in the
`
`etched openings, again to a thickness of 10-20 angstroms, an N+ doped amorphous
`
`30
`
`silicon layer 22 is deposited over the back surface, as shown in Fig. 2C.
`
`

`

`WO 2007/130188
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`PCT/US2007/003459
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`[0020] The layer 22 is doped with an N dopant such as phosphorous with a
`
`concentration of 1020-102| or 10E20 -lOE21 atoms per cubic centimeter. This can be
`
`deposited using plasma enhanced chemical vapor deposition (PECVD, LPCVD,
`
`APCVD). N+ amorphous silicon 22 is then masked and selectively etched to expose
`
`5
`
`the underlying P+ amorphous silicon 18 for reception of metal contacts.
`
`In Fig. 2D,
`
`metal contacts 24 and 26 are made to P+ amorphous silicon 18 and N+ amorphous
`
`silicon 22 by metal deposition and photoresist masking and etching. The contacts can
`
`be formed by first scattering a seed layer of a conductive metal such as aluminum or
`
`copper and then pattern plating the seed metal to increase thickness. The cell is then
`completed by depositing a passivating layer 14 on tunnel oxide 12 on the fi’ontsurface
`
`10
`
`of substrate 10 using silicon nitride, doped silicon carbide, or N+ doped amorphous
`
`silicon.
`
`[0021] A heterojunction solar cell
`
`in accordance with the invention, using doped
`
`amorphous silicon contacts is readily fabricated using conventional semiconductor
`
`l5
`
`processing techniques without
`
`the for high temperature processing. While the
`
`invention has been described with reference to an interdigitated back contact solar cell
`
`in which both P+ and N+ contacts are employed, the invention can be applied to solar
`
`cells having a single doped amorphous silicon on the back surface. Further, while the
`
`heterojunction is provided by amorphous silicon, other high band gap material such as
`
`20
`
`gennanium-silicon alloy, doped silicon carbide, or other lII-V compound material can
`be employed in the contact structures. Thus, while the invention has been described
`
`with reference to specific embodiments, the description is illustrative of the invention
`
`and is not to be construed as limiting the invention. Various modifications and
`
`applications may occur to those skilled in the art without departing from the spirit and
`
`25
`
`scope of the invention as defined by the appended claims.
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`

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`WO 2007/130188
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`PCT/US2007/003459
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`CLAIMS
`
`What is claimed is:
`
`l.
`
`A solar cell comprising:
`
`a) a semiconductor body having first and second opposing major surfaces,
`
`b) a first dielectric layer on the first surface and a second dielectric layer 0n
`
`the second surface, the second dielectric layer comprising a tunnel oxide,
`
`c) a first pattern of acceptor doped semiconductor material over the tunnel
`
`oxide on the second surface, and a second pattern of donor doped semiconductor
`
`material over the tunnel oxide on the second surface and interleaved with the first
`
`10‘
`
`pattern, and
`
`d) a first conductive pattern interconnecting the acceptor doped semiconductor
`
`material and a second conductive pattern interconnecting the donor doped
`
`semiconductor material.
`
`15
`
`2.
`
`The solar cell as defined by claim 1 wherein the semiconductor material is
`
`selected from the group consisting of amorphous silicon, silicon-germanium, and III -
`
`V compound semiconductors.
`
`3.
`
`The solar cell as defined by claim 2 wherein the semiconductor material
`
`20
`
`comprises amorphous silicon.
`
`4.
`
`The solar cell as defined by claim 3 wherein the acceptor doped amorphous
`
`silicon is boron doped.
`
`25
`
`5.
`
`The solar cell as defined by claim 4 wherein the donor doped amorphous
`
`silicon is phosphorous doped.
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`

`

`WO 2007/130188
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`PCT/US2007/003459
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`6.
`
`The solar cell as defined by claim 5 wherein the first major surface is textured.
`
`7.
`
`The solar cell as defined by claim 6 wherein the semiconductor body
`
`comprises silicon and the tunnel oxide comprises silicon oxide.
`
`8.
`
`The solar cell as defined by claim 7 wherein the first and second conductive
`
`patterns are selected from the group consisting of aluminum and copper.
`
`9.
`
`The solar cell as defined by claim 3 wherein the semiconductor body
`
`10
`
`comprises silicon and the tunnel oxide comprises silicon oxide.
`
`10.
`
`The solar cell as defined by claim 9 wherein the first and second conductive
`
`patterns are selected from the group consisting of aluminum and copper.
`
`15
`
`11.
`
`In a silicon solar cell in which mobile carriers can be created in a silicon
`
`substrate by impinging radiation, a contact for receiving mobile carriers comprising a
`
`tunnel oxide on a surface of the substrate, and a doped semiconductor material layer
`
`on the tunnel oxide.
`
`20
`
`12.
`
`The contact as defined by claim 1 I wherein the semiconductor material is
`
`selected from the group consisting of amorphous silicon, silicon-germanium, and Ill -
`
`V compounds semiconductors.
`
`13.
`
`The contact as defined by claim 12 wherein the semiconductor material
`
`25
`
`comprises amorphous silicon.
`
`14.
`
`The contact as defined by claim 13 wherein the tunnel oxide comprises silicon
`
`oxide.
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`

`

`WO 2007/130188
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`PCT/US2007/003459
`
`15.
`
`The contact as defined by claim 14 wherein the silicon oxide has a thickness in
`
`the range of 10-20 angstroms.
`
`16.
`
`The contact as defined by claim 14 wherein the amorphous silicon is doped
`
`5
`
`with a donor dopant.
`
`17.
`
`The contact as defined by claim 16 wherein the donor dopant is phosphorous.
`
`18.
`
`The contact defined by claim 14 wherein the amorphous silicon is doped with
`
`10
`
`an acceptor dopant.
`
`19.
`
`The contact as defined by claim 18 wherein the acceptor dopant is boron.
`
`20.
`
`In a method of fabricating a silicon solar cell, the steps of fabricating carrier
`
`15
`
`acceptor contacts comprising:
`
`a) providing a silicon substrate having first and second opposing major
`
`surfaces,
`
`b) fOnning a tunnel silicon oxide layer on the first major surface, and
`
`e) forming a doped amorphous silicon layer on the tunnel silicon oxide.
`
`20
`
`25
`
`21.
`
`The method steps of claim 20 wherein the doped amorphous silicon has
`
`dopant in excess of 10'9 atoms per cc.
`
`22.
`
`The method steps of claim 21 wherein the dopant is a donor dopant.
`
`23.
`
`The method steps of claim 22 wherein the dopant comprises phosphorus.
`
`l0
`
`

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`WO 2007/130188
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`PCT/US2007/003459
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`24.
`
`The method steps of claim 21 wherein the dopant is an acceptor dopant.
`
`25.
`
`The method steps of claim 24 wherein the dopant comprises boron.
`
`5
`
`26.
`
`The method steps of claim 20 wherein the tunnel silicon oxide has a thickness
`
`on the order of 10-20 angstroms.
`
`10
`
`ll
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