Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
  • C23C14/06—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material characterised by the coating material
  • C23C14/08—Oxides
  • C23C14/086—Oxides of zinc, germanium, cadmium, indium, tin, thallium or bismuth
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
  • C23C14/22—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material characterised by the process of coating
  • C23C14/54—Controlling or regulating the coating process
  • C23C14/548—Controlling the composition
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10P—GENERIC PROCESSES OR APPARATUS FOR THE MANUFACTURE OR TREATMENT OF DEVICES COVERED BY CLASS H10
  • H10P14/00—Formation of materials, e.g. in the shape of layers or pillars
  • H10P14/20—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials
  • H10P14/22—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials using physical deposition, e.g. vacuum deposition or sputtering
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10P—GENERIC PROCESSES OR APPARATUS FOR THE MANUFACTURE OR TREATMENT OF DEVICES COVERED BY CLASS H10
  • H10P14/00—Formation of materials, e.g. in the shape of layers or pillars
  • H10P14/20—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials
  • H10P14/29—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials characterised by the substrates
  • H10P14/2901—Materials
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10P—GENERIC PROCESSES OR APPARATUS FOR THE MANUFACTURE OR TREATMENT OF DEVICES COVERED BY CLASS H10
  • H10P14/00—Formation of materials, e.g. in the shape of layers or pillars
  • H10P14/20—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials
  • H10P14/29—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials characterised by the substrates
  • H10P14/2901—Materials
  • H10P14/2902—Materials being Group IVA materials
  • H10P14/2905—Silicon, silicon germanium or germanium
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10P—GENERIC PROCESSES OR APPARATUS FOR THE MANUFACTURE OR TREATMENT OF DEVICES COVERED BY CLASS H10
  • H10P14/00—Formation of materials, e.g. in the shape of layers or pillars
  • H10P14/20—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials
  • H10P14/29—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials characterised by the substrates
  • H10P14/2901—Materials
  • H10P14/2913—Materials being Group IIB-VIA materials
  • H10P14/2914—Oxides
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  • H10P—GENERIC PROCESSES OR APPARATUS FOR THE MANUFACTURE OR TREATMENT OF DEVICES COVERED BY CLASS H10
  • H10P14/00—Formation of materials, e.g. in the shape of layers or pillars
  • H10P14/20—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials
  • H10P14/34—Deposited materials, e.g. layers
  • H10P14/3402—Deposited materials, e.g. layers characterised by the chemical composition
  • H10P14/3424—Deposited materials, e.g. layers characterised by the chemical composition being Group IIB-VIA materials
  • H10P14/3426—Oxides
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  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10P—GENERIC PROCESSES OR APPARATUS FOR THE MANUFACTURE OR TREATMENT OF DEVICES COVERED BY CLASS H10
  • H10P14/00—Formation of materials, e.g. in the shape of layers or pillars
  • H10P14/20—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials
  • H10P14/34—Deposited materials, e.g. layers
  • H10P14/3402—Deposited materials, e.g. layers characterised by the chemical composition
  • H10P14/3434—Deposited materials, e.g. layers characterised by the chemical composition being oxide semiconductor materials
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  • H10P14/00—Formation of materials, e.g. in the shape of layers or pillars
  • H10P14/20—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials
  • H10P14/34—Deposited materials, e.g. layers
  • H10P14/3438—Doping during depositing
  • H10P14/3441—Conductivity type
  • H10P14/3444—P-type
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  • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
  • H10F71/00—Manufacture or treatment of devices covered by this subclass
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
  • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

• the present invention relates to methods of fabricating doped metal oxide films and to systems for fabricating the same.
• TCOs Transparent conducting oxides
• LEDs light emitting diodes
• fabricating a p-n homojunction using wide-gap semiconductors is faced with problems as they either form n-type or p-type but not both types.
• each dopant source containing the same dopant element may result in formation of both p-type and n-type semiconductors.
• a nitrogen (dopant element) doped Zinc Oxide (ZnO) film using N 2 as the dopant source led to n-type conduction whilst using NO 2 or NO as the dopant source led to p-type conduction, and using P 2 O 5 as the dopant source led to n-type conduction whilst Zn 3 P 2 led to p- type conduction.
• Physical vapour deposition methods such as magnetron sputtering have been used to fabricate doped metal oxide films.
• Such methods involve radio frequency (RF) sputtering of one or more targets comprising the metal oxide and the dopant source within a vacuum chamber to deposit a doped metal oxide film on a substrate.
• RF radio frequency
• two separate chambers, each equipped with target materials comprising the respective dopant material are required for forming the n-type and p-type metal oxide films. Accordingly, the substrate will have to be transferred from one chamber to another after deposition which can be tedious and time consuming.
• a p- and n-type doped metal oxide film that is of at least the same quality or better quality than existing films may usefully be provided.
• a method of fabricating a doped metal oxide film comprising the steps of:
• a system for fabricating a doped metal oxide (MO) film comprising: a vacuum chamber having a mount for mounting a semi ⁇ conductor substrate therein; a plasma generator capable of generating plasma from one or more targets, said plasma comprising at least metal (M) , oxygen (0) and dopant ions; at least one gas conduit for supplying gas into said chamber; a controller for controlling the supply of said gas to said chamber and for controlling said plasma generator; wherein in use, a semi-conductor substrate is mounted on said mount and an inert carrier gas is supplied to said chamber via said gas conduit, and wherein said controller operates said plasma generator to form plasma comprising at least metal (M) , oxygen (0) and dopant ions to form a doped MO film layer on said mounted
• a system for fabricating a phosphorous-doped zinc oxide (ZnO) film comprising: a vacuum chamber having a mount for mounting a semi ⁇ conductor substrate therein; a plasma generator capable of generating plasma from one or more targets, said plasma comprising at least zinc (Zn) , oxygen (0) and phosphorous (P) ions; at least one gas conduit for supplying gas into said chamber; a controller for controlling the supply of said gas to said chamber and for controlling said plasma generator; wherein in use, a semi-conductor substrate is mounted on said mount and an inert carrier gas is supplied to said chamber via said gas conduit, and wherein said controller operates said plasma generator to form plasma comprising at least zinc (Zn) , oxygen (O) and phospho
• the term "about”, typically means +/- 5% of the stated value, more typically +/- 4% of the stated value, more typically +/- 3% of the stated value, more typically, +/- 2% of the stated value, even more typically +/- 1% of the stated value, and even more typically +/- 0.5% of the stated value.
• range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range.
• range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.
• the method of fabricating a doped metal oxide (MO) film comprises the steps of:
• step (d) controlling, during step (c) , the amount of 0 ions relative to said dopant ions within said plasma to form at least one of an n-type MO film and a p-type MO film on said substrate.
• the system for fabricating a doped metal oxide (MO) film comprises: a vacuum chamber having a mount for mounting a semi- conductor substrate therein; a plasma generator capable of generating plasma from one or more targets, said plasma comprising at least metal (M) , oxygen (0) and dopant ions; at least one gas conduit for supplying gas into said chamber; a controller for controlling the supply of said gas to said chamber and for controlling said plasma generator; wherein in use, a semi-conductor- substrate is mounted on said mount and an inert carrier gas is supplied to said chamber via said gas conduit, and wherein said controller operates said plasma generator to form plasma comprising at least metal (M) , oxygen (0) and dopant ions to form a doped MO film layer on said mounted substrate, and wherein the amount of 0 ions relative to M ions within said plasma is controlled to form at least one of an n-type MO film and a p-type MO film on said substrate.
• a plasma generator capable of generating plasma from one or more targets, said plasma compris
• the vacuum chamber may be maintained at a pressure range selected from the group consisting of: lOOmPa to 7,000 mPa, lOOmPa to 5,000 mPa, lOOmPa to 4,000 mPa, lOOmPa to 3,000 mPa, lOOmPa to 2,000 mPa, 50OmPa to 7,000 mPa, lOOOmPa to 7,000 mPa, 200OmPa to 7,000 mPa, and 300OmPa to 7,000 mPa.
• the semiconductor substrate can be made from any semiconductor material such as zinc oxide, aluminum oxide, silicon or glass.
• the substrate is silicon.
• silicon has good processability and is relatively cheap when compared to the other semiconductor materials.
• the substrate is zinc oxide.
• ZnO is suitable for growing thin films of doped metal oxide owing to its excellent match with the doped metal oxide film in terms of thermal expansion and crystalline structure.
• the semiconductor substrate during the forming step (c) , may be maintained at a temperature range selected from the group consisting of: 200 0 C to 500 0 C, 300 0 C to 500 0 C, 400 0 C to 500 0 C, 200 0 C to 400 0 C and 200 0 C to 300 0 C.
• the metal oxides can be transparent conducting oxides (TCOs) such as zinc oxide, indium oxide, tin oxide, indium tin oxide, cadmium oxide and indium cadmium oxide.
• TCOs transparent conducting oxides
• the metal component of the metal oxide may be selected from Group HB (such as zinc and cadmium) or Group III (such as gallium and indium) of the
• the metal component of the metal oxide is zinc.
• ZnO has a high exciton energy (60meV) which renders it suitable for use in optoelectronic devices.
• the dopant may be selected from Group I (such as lithium, sodium and potassium) or Group V (such as nitrogen, arsenic and phosphorous) of the Periodic Table of Elements.
• the dopant may be Phosphorous (P) .
• the dopant can be delivered into the metal oxides in the form of compounds containing the dopant element.
• dopant sources for P include Zn 3 P 2 and P2O 5
• dopant sources for N include N 2 , NO 2 and NO.
• One or more target materials can be provided within the chamber to generate at least M, O and dopant ions within the plasma.
• the target material can be selected from at least one of M, MO and dopant material.
• the target materials can also be provided within the chamber to generate at least Zn, 0 and P ions within the plasma.
• the target material in this case, can be selected from at least one of Zn, P, ZnO, P 2 O5 and Zn 3 P 2 -
• the single target may comprise a plurality of target materials sintered together, for example, the materials ZnO and P 2 O 5 can be sintered into a single target for magnetron sputtering to generate Zn, 0 and P ions.
• the target can also be in the form of separate targets, each of a target material selected from the above, for example, a first target comprising pure ZnO and a second target comprising a mixture of ZnO and P 2 O 5 sintered together.
• the target materials within said chamber can have a higher molar ratio of M atoms or Zn atoms relative to 0 atoms.
• the molar ratio of M atoms or Zn atoms relative to 0 atoms in said target materials can be selected from the group consisting of: about 1.01:1, about 1.05:1, about 1.1:1, about 1.2:1, about 1.5:1, about 2:1,about 3:1, about 2.01:3, about 2.05:3, about 2.1:3, about 2.2:3, about 2.5:3, about 3:3 and about 4:3. Controller
• the controller controls the supply of said gas to said chamber and controls said plasma generator.
• the controller regulates the flow of oxygen gas into said chamber in controlling step (d) . Additionally, the controller can also regulate the flow of inert gas carrier into said chamber.
• the flow of the gases into said chamber can be regulated by a mass flow controller or a pressure flow controller.
• a mass flow controller is a MKS Type 247D model by MKS Instruments.
• the controller may also comprise a RF power controller for controlling the RF power of sputtering to the targets to control the generation of plasma.
• the RF power of the sputtering can be in the range of 0-600 Watts.
• An example of a suitable RF power controller is a RFG 600 SE model by Coaxial Power System Ltd.
• the amount of oxygen gas by volume in the chamber, in the regulating step (dl) may be in the range selected from the group consisting of 0% to less than 7%, 0 % to 3
• the amount of oxygen within said chamber can be maintained at the range between 0% to 3% by volume in the regulating step (dl) .
• the amount of oxygen within said chamber can be maintained at the range between 3% to less than 7% by volume in the regulating step (dl) .
• the volume ratio of oxygen gas to inert carrier gas in the chamber may be in the range selected from the group consisting of: 5:95 to 20:80, 5:95 to 15:85, 5:95 to 10:90, 10:90 to 20:80, and 15:85 to 20:80.
• both an n-type MO film and a p-type MO film can be fabricated to form a p-n junction within the same chamber using the same dopant source simply by varying the amount of oxygen within the chamber.
• Fig. 1 shows a schematic view of an apparatus for fabricating a phosphorous-doped zinc oxide (ZnO) film using a method in accordance with a disclosed embodiment (MSS3A sputtering system) .
• Fig. 2 shows a phase diagram of a Zn-O-P ternary system.
• Fig. 3 shows the energy levels of P-doped ZnO in ZnO:P 2 ⁇ 5 and ZnO:Zn3P 2 systems.
• Fig. 4 shows the current (I) - voltage (V) characteristics of (a) p-type ZnO and (b) p-n homojunction formed from p-type and n-type ZnO.
• Fig. 5 shows a schematic diagram of a LED based on P doped ZnO p-n homojunction.
• Fig. 6 shows a luminance spectra of ZnO p-n homojunction loaded forward bias (20V) at room temperature. Best Mode
• Non-limiting examples of the invention, including the best mode, and a comparative example will be further described in greater detail by reference to specific
• Example 1 Preparation of p-tpe and n-type Phosphorous (P) Doped Zinc Oxide (ZnO) film using P2O 5 as dopant source.
• FIG. 1 A system 10 for fabricating P-doped ZnO film by physical vapour deposition (PVD) is shown in Fig. 1.
• the system is a radio frequency (RF) magnetron sputtering system (coaxial MSS3A/LL) .
• the system comprises a vacuum chamber 100 having a mount 120 for mounting a silicon substrate 140 therein, a RF magnetron sputter 160 for generating plasma from a target 180, gas conduits 200,220 for supplying Ar and O 2 gases respectively into the chamber 100, and mass flow controllers 210,230 for controlling the supply of Ar and O2 gases respectively into the chamber 100.
• a RF power controller (Model: RFG 600 SE by Coaxial Power System Ltd) (not shown in the figures) for controlling RF power of the RF magnetron sputter is also provided.
• the silicon substrate 140 is an n-type semiconductor substrate and is pre-sputtered with a 100 nm layer of pure ZnO.
• the mount 120 rotates the substrate 140 during the deposition process to achieve a uniform coating.
• the substrate 140 is biased by a biasing voltage 240 provided by the RF power.
• the substrate 140 is connected to a heater 260 to control the temperature of the substrate 140 during deposition.
• the substrate is placed at a distance of 100mm beneath the target 180.
• the target 180 comprises a mixture of ZnO powder (95 wt%) and P 2 O 5 powder (5 wt%) sintered together.
• a rotary pump 300 is connected to the chamber and to a molecular turbo pump 280.
• the rotary pump 300 provides an initial vacuum within the chamber while the molecular turbo pump 280 provides a further vacuum within the chamber to achieve the desired working pressure.
• the total working pressure in the chamber 100 was maintained at 15 mTorr (2000 mPa) .
• the substrate 140 Prior to deposition, the substrate 140 was rotated and the target 180 was sputtered for 20 minutes to clean its surface at a RF power of 40 W.
• deposition of p doped ZnO film commenced as the target was sputtered at the conditions described above.
• the sputtering results in the formation of a plasma comprising Zn, 0 and P ions in the presence of Argon gas.
• the argon gas carries the Zn, P and O ions to the surface of the substrate to form the P doped ZnO film.
• the temperature of the substrate was maintained at 300 0 C.
• O 2 gas is supplied into the chamber and regulated to maintain a percentage by volume of 5% for a time period of 10 minutes in the chamber to grow 400 nm layer of p- type ZnO film on the substrate.
• the oxygen supply is cut off and the target was sputtered for 100-120 minutes under the same conditions to deposit 400 ran layer of n-type P doped ZnO substrate on the p-type ZnO film.
• Example 2 Preparation of p-fcype and n-type Phosphorous (P) Doped Zinc Oxide (ZnO) film using Zn 3 O 2 as dopant source.
• the P doped ZnO film is deposited using the same method and conditions described in Example 1. However, in this Example, the two targets were employed - pure ZnO target and pure Zn 2 P 3 targets 180, both sputtered at RF powers of 4OW and 15 W respectively.
• Example 3 - Zn-O-P ternary system The possible doping sources for ZnO include Zn 3 P 2 , P 2 O 5 and P.
• a Zn-O-P ternary system 20 as shown in Fig. 2 phosphorus doping in ZnO can be carried out either along the ZnO-P 2 O 5 binary 400 or the ZnO-Zn 3 P 2 binary 420.
• P-ZnO binary 440 is not stable as it either belongs to a ZnO-P 2 O 5 -Zn 3 P 2 ternary 460 or a P-P 2 O 5 -Zn 3 P 2 ternary 480 depending on the phosphorus concentration. Accordingly, only ZnO-P 2 O 5 system and ZnO-Zn 3 P 2 system and the dopant behaviours of P 2 O 5 (under both oxygen rich and oxygen poor) and Zn 3 P 2 (under both oxygen rich and oxygen poor) were investigated.
• the dopant sources were optimized and the possible acceptor and donor levels predicted theoretically.
• a density functional theory (DFT) calculation using CASTEP code was employed.
• Oxygen rich (zinc poor) and oxygen poor (zinc rich) conditions for the ZnO-P 2 O 5 system and ZnO-Zn 3 P 2 systems were used to analyse formation energies of P with all possible charged interstitial and substitutional states.
• the energy levels of P in ZnO forbidden gap are shown in Fig. ,3.
• P is a donor which is located at 0.3 eV below the bottom of conduction band under the oxygen rich growth conditions.
• thermochemistry For all doping systems, the thermal (or temperature) effect was considered by employing commercial a thermochemistry software, FACTSage. Based on the calculation results, it was found that the thermal effect becomes the winning factor for the shallow acceptor only in the ZnOIZn 3 P 2 doping system under zinc rich growth condition at an elevated temperature (400 0 C) .
• ZnO films are deposited using the methods of Examples 1 and 2.
• I-V characteristic of p-type ZnO vs n-type Si substrate, p-type ZnO vs. n-type ZnO are shown in Fig. 3.
• P doped ZnO samples under the ZnOrP 2 Zn 3 system displayed good rectification behaviour.
• the Hall coefficient of +0.406 Cm 3 C "1 measured by van der Pauw electrode configuration, suggested the conduction to be p-type.
• Combining the Hall coefficient and conductivity measurement resulted in a carrier density of 3.84xlO 19 cm “3 , Hall mobility of the positive holes of 6.69 Cm 2 V -1 S "1 and the resistivity of 0.024 Ohm.cm.
• the Seebeck coefficient was found to be positive, further confirming p-type conductivity.
• the X-ray powder diffraction patterns of ZnOrP thin films deposited by the two doping systems showed a strong peak at 34.4325° (2 ⁇ ) consistent with c-axis oriented wurtzite Zn
• the same element (P) can create either p-type or n-type conduction of ZnO using different dopant sources containing the same dopant element. Furthermore, p- and n-type P doped ZnO thin films can be obtained by adjusting the oxygen concentration during sputtering in the ZnOrZn 3 P 2 system. This will make fabrication of ZnO p-n junction devices very convenient and cheap, as the same deposition chamber and the same dopant source are used for both purposes.
• a prototype of ZnO p-n homojunction or diode was fabricated and its schematic configuration shown in Fig. 5.
• An n- type P doped ZnO thin film was grown on the p-type ZnO at room temperature using pure Ar plasma.
• the I-V curve for a p-n homojunction of ZnO is shown in Fig. 4.
• no luminescence was observed under the forward bias because of the low current level and high defect density in the interface of this starting p-n homojunction.
• a weak UV luminescence was observed at loaded forward bias (20 V) as shown in Fig. 6 at wavelength 525nm. The strongest peak appear at wavelength 370nm which corresponds to about 3.349 eV.
• a prototype ZnO p-n homojunction or diode is further fabricated using the methods of Examples 1 and 2.
• the theoretical calculations were performed using a Kohn-Sham density-functional theory (DFT) , within a generalized gradient approximation (GGA) using the plane-wave total energy method as implemented in the CASTEP code.
• DFT Kohn-Sham density-functional theory
• GGA generalized gradient approximation
• the crystal structures were determined under the condition that the total energy was minimized from all atomic configurations.
• the self-consistent total energy in the ground state was effectively obtained by the density-mixed scheme.
• Atomic positions were optimized to minimize the total energy using the quasi-Newton method with the Broyden-Fletcher-Goldfarb-Shanno hessian update scheme (BFGS) .
• BFGS Broyden-Fletcher-Goldfarb-Shanno hessian update scheme
• Ultrasoft pseudopotentials were employed to treat "shallow" core electrons as valence states by including multiple sets of occupied states in each angular momentum channel. This leads to high accuracy and transferability of the potentials.
• the next uppermost states of O (2s, 2p) , P (3s, 3p) and Zn (3d, 4s) were explicitly treated as a part of the valence states, respectively.
• a 72-atom wurtzite supercell doped with one phosphorus atom was used in the calculation and a 108-atom supercell was also used to confirm the convergence.
• a plane-wave cut-off of 400 eV was chosen, relaxing all atoms and leaving the lattice constants frozen until the SCF convergence per atom was below 2'10- 6 eV.
• a uniform charge-neutralising background was applied. This approach leads to an error due to the electrostatic interactions between the charge and its images, which was corrected by using Madelung energy.
• the method is not limited to P doped metal oxides but can be used to form other types of doped metal oxides. It will be appreciated that the doped metal oxides films, particularly P doped ZnO films, resulting from the method are stable and of good reproducibility.
• the method involves fabricating both an n-type MO film and a p-type MO film to form a p-n junction within the same chamber using the same dopant source simply by varying the amount of oxygen within the chamber.

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