JP3563141B2 - Solid phase diffusion method of Zn and method of manufacturing LED - Google Patents

Description

[0001]
[Industrial applications]
The invention according to this application relates to a method for solid-phase diffusion of Zn into a semiconductor.
[0002]
[Prior art]
As an example of a conventional solid phase diffusion method of Zn, an example of a solid phase diffusion method of Zn into a III-V compound semiconductor is disclosed in a document: Japanese Patent Application Laid-Open No. 62-98721.
[0003]
According to the solid-phase diffusion method described in this document, first, an oxide doped with Zn, for example, a ZnO.SiO ₂ mixed film is formed on a substrate of an N-type III-V compound semiconductor by sputtering. Apply film. Next, the ZnO.SiO ₂ film is covered with an annealing cap film and annealed. By annealing, Zn is solid-phase diffused into the substrate using the ZnO.SiO ₂ film as a diffusion source without damaging the boundary surface of the substrate.
[0004]
[Problems to be solved by the invention]
However, according to the Zn diffusion method disclosed in the above document, the Zn diffusion concentration in the substrate cannot be sufficiently increased. Generally, in order to form an LED having a low sheet resistance and a high light emission intensity having a shallow diffusion region having a junction depth of 1 μm or less, the Zn diffusion concentration needs to be 10 ²⁰ / cm ⁻³ or more. However, it is extremely difficult to reach this concentration by the method disclosed in the above document. For example, when a diffusion source formed by a sputtering method using a target having a molar ratio of ZnO: SiO ₂ = 1: 1 is used, the diffusion concentration of Zn in the substrate does not reach 10 ²⁰ / cm ⁻³ . .
[0005]
On the other hand, if a ZnO film as a diffusion source is provided directly on the substrate, the diffusion concentration can be increased. However, since the ZnO film reacts with the substrate, the surface of the substrate becomes rough and a boundary surface cannot be obtained. As a result, even if an electrode is formed on the surface of the substrate, the contact resistance increases.
[0006]
For this reason, it is possible to perform high-concentration Zn diffusion such that the contact resistance with the electrode after diffusion is low and the Zn surface concentration reaches 10 ²⁰ cm ⁻³ without the surface of the substrate damaging the boundary surface. It has been desired to realize a solid-phase diffusion method.
[0007]
[Means for Solving the Problems]
<First invention>
According to the Zn diffusion method of the first invention according to this application, Zn-doped SiO ₂ doped with Zn in advance is formed on an n-type compound semiconductor substrate. Film, forming a layered structure by sequentially stacking a ZnO film and annealing cap film, by performing heat treatment to the product layer structure, Zn doped SiO ₂ As a diffusion source film and the ZnO film, the substrate, and a step of solid-phase diffusing Zn, Zn-doped SiO ₂ The thickness of the film is set to 100 to 500 ° .
[0008]
Further, according to the solid phase diffusion method for Zn of the first invention, Zn-doped SiO ₂ doped with Zn in advance is formed on a substrate of an n-type compound semiconductor. Forming a laminated structure in which a film, a ZnO film, and an annealing cap film are sequentially laminated, and performing a heat treatment on the laminated structure to obtain a Zn-doped SiO ₂ film. As a diffusion source film and the ZnO film, the substrate, and a step of solid-phase diffusing Zn, Zn-doped SiO ₂ The film is characterized in that the molar ratio between Zn and Si in the film is in the range of 0.1 ≦ Zn / (Zn + Si) ≦ 0.5.
[0010]
In the above-described diffusion method, it is preferable to use an n-type GaAs _1-X P _X epitaxial substrate (where X represents a composition ratio and 0 ≦ X ≦ 0.4) as the n-type compound semiconductor substrate.
[0011]
<Second invention>
Further, according to the LED manufacturing method of the second invention according to the present application, Zn is doped in advance on a substrate of an n-type compound semiconductor, doped with Zn, and has a thickness of 100 to 500 °. Doped SiO ₂ Spreading and forming a diffusion source consisting of film and the ZnO film, on expansion Chigen, forming annealing cap film, by heat-treating the substrate formed with A kneel cap film, a Zn diffusion source into the substrate by the steps of forming a diffusion region, and the diffusion source and the step of removing the annealing cap film on the diffusion source, a first main electrode electrically connected to the n-type portion of the substrate, the diffusion region Forming a second main electrode which is electrically connected to the second main electrode.
Further, the method of manufacturing the LED is such that Zn-doped SiO ₂ doped with Zn in advance is sequentially laminated on an n-type compound semiconductor substrate. A Zn-doped SiO ₂ film having a molar ratio between Zn and Si in the range of 0.1 ≦ Zn / (Zn + Si) ≦ 0.5 Forming a diffusion source composed of a film and a ZnO film; forming an annealing cap film on the diffusion source; and heat-treating the substrate on which the annealing cap film is formed to diffuse Zn from the diffusion source to the substrate. Forming a diffusion region, removing a diffusion source and an annealing cap film on the diffusion source, a first main electrode electrically connected to the n-type portion of the substrate, and electrically connecting to the diffusion region. Forming a second main electrode connected to the second main electrode.
[0012]
In the above-described LED manufacturing method according to the second aspect of the invention, it is preferable to use an n-type GaAs _1-X P _X epitaxial substrate (where X represents a composition ratio and 0 ≦ X ≦ 0.4) as the substrate.
[0013]
In the first and second aspects of the present invention, the term "substrate" includes, for example, an epitaxial layer grown on a normal substrate and a substrate that can diffuse Zn.
[0014]
[Action]
According to the method for diffusing Zn of the first invention according to this application, the solid-phase diffusion of Zn using a Zn-doped oxide film and a Zn oxide film, which are sequentially laminated on an n-type compound semiconductor substrate, as a diffusion source. Do. As a result, it is possible to suppress damage to the surface of the substrate and to perform solid-phase diffusion of high-concentration Zn onto the substrate.
[0015]
Further, according to the LED manufacturing method of the second invention according to the present application, solid-phase diffusion of Zn is performed using a Zn-doped oxide film and a Zn oxide film sequentially stacked on a substrate as a diffusion source. As a result, it is possible to suppress the damage to the surface of the substrate and to perform high-concentration solid-phase diffusion of Zn to the substrate. As a result, a light emitting layer having a low sheet resistance in the diffusion region can be formed even if the coupling depth of the diffusion region is as small as about 1 μm. For this reason, it is possible to suppress current concentration in a region immediately below the vicinity of the electrode, and to make light emission intensity uniform on the upper surface of the diffusion region.
[0016]
【Example】
Hereinafter, an example of a solid phase diffusion method of Zn and a method of manufacturing an LED according to the first invention of the present application will be described with reference to the drawings. It should be noted that the drawings referred to merely schematically show the size, shape, and arrangement of each component so that these inventions can be understood. Therefore, these inventions are not limited only to the illustrated examples.
[0017]
<First embodiment>
In the first embodiment, an example of the solid phase diffusion method for Zn of the first invention will be described. FIG. 1 is a diagram for explaining a solid phase diffusion method of Zn according to the first embodiment.
[0018]
In the first embodiment, a Zn-doped SiO ₂ film (hereinafter, referred to as a “GaAsP substrate”) 10 is formed on a n-type GaAs _0.8 P _0.2 epitaxial substrate (hereinafter, simply referred to as a “GaAsP substrate”) by a sputtering method. A ZnO.SiO ₂ film 12 is also formed. The molar ratio between Zn and Si contained in the ZnO.SiO ₂ film 12 is Zn / (Zn + Si) = 0.3. The molar ratio between Zn and Si is preferably 0.1 ≦ Zn / (Zn + Si) ≦ 0.5. The reason that the molar ratio is set to 0.1 or more is that if the molar ratio of Zn is extremely low, the ZnO · SiO ₂ film 12 mainly functions as a SiO ₂ barrier film, so that the diffusion of Zn is excessively suppressed. This is because, as a result, it is difficult to perform high concentration diffusion. The reason for setting the molar ratio to 0.5 or less is that if the molar ratio of Zn is extremely high, the ZnO · SiO ₂ film 12 substantially acts as a ZnO film and reacts with the surface of the substrate. As a result, the boundary surface of the substrate is damaged.
[0019]
According to the results of experiments conducted by the inventor of the present application, when the thickness of the ZnO · SiO ₂ film 12 is 100 to 500 °, the diffusion concentration of Zn increases. Therefore, the thickness of the ZnO.SiO ₂ film 12 is desirably 100 to 500 °.
[0020]
In this embodiment, the thickness of the ZnO.SiO ₂ film is set to 150 °.
[0021]
Next, a ZnO film as a Zn oxide film is formed on the ZnO · SiO ₂ film 12. If the thickness of this ZnO film 14 is 200 ° or more, solid phase diffusion of Zn can be performed as a substantially infinite diffusion source. In this embodiment, the thickness of the ZnO film is 1500 °.
[0022]
Next, a 1000-nm-thick SiN film is formed as an annealing cap film 16 on the ZnO film to cover the diffusion source. FIG. 1 shows a cross section of the laminated structure in a state where the annealing cap film is formed.
[0023]
Next, heat treatment (annealing) is performed on the laminated structure, so that Zn is solid-phase diffused into the substrate 10 using the ZnO · SiO film 12 and the ZnO film 14 as the diffusion source 20.
[0024]
The annealing is desirably performed at a temperature of 750 ° C. or less. When annealing is performed at a temperature higher than 750 ° C., cracks and pinholes are significantly generated in the film of the diffusion source, and the damage of the film of the diffusion source is increased. Also, when annealing is performed at a high temperature, a reaction between the film of the diffusion source and the surface of the substrate occurs, which makes it difficult to obtain a boundary surface after removing the film of the diffusion source. Therefore, the annealing temperature is more desirably about 600 to 700 ° C. In this example, annealing was performed at a temperature of 700 ° C. for 60 minutes to form a Zn diffusion region having a bond depth of about 1.5 μm.
[0025]
It is desirable that the composition ratio X of P in the n-type GaAs _1-X P _X epitaxial substrate is 0 ≦ X ≦ 0.4. This is because when the composition ratio X is larger than 0.4, it becomes an indirect semiconductor. Further, the composition ratio X may be 0, that is, an n-type GaAs epitaxial substrate.
[0026]
Next, the graph of FIG. 2 shows the result of analyzing the Zn concentration profile of the diffusion region obtained in the first example by secondary ion mass spectrometry (SIMS). In SIMS analysis, cesium (Ce) was used at 5.5 kV.
[0027]
The horizontal axis of the graph in FIG. 2 represents the depth (μm) from the surface of the compound semiconductor on the substrate, and the vertical axis represents the Zn concentration (number / cm ⁻³ ) in logarithm. A curve I in the graph indicates a Zn concentration profile of the diffusion region obtained in the first embodiment. As shown by the curve I, in this example, the Zn concentration at a depth from the surface to around 1.5 μm achieves 10 ²⁰ / cm ⁻³ or more.
[0028]
Further, in the graph of FIG. 2, as a comparative example, the result of SIMS analysis of the Zn concentration profile of the diffusion region obtained by diffusing Zn by a conventionally known method is also shown as a curve II. In the comparative example, a ZnO · SiO ₂ film having a thickness of 1500 ° and a SiN film having a thickness of 1000 ° are sequentially laminated on a substrate, and a heat treatment is performed at a temperature of 750 ° C. for 30 minutes to diffuse Zn. As shown in the curve II, in the comparative example, even in the vicinity of the surface of the substrate, the Zn concentration does not exceed 5 × 10 ¹⁹ / cm ⁻³ at most, and does not reach 10 ²⁰ / cm ⁻³ .
[0029]
In this regard, in the first embodiment, solid-phase diffusion of Zn at a high concentration of 10 ²⁰ / cm ⁻³ or more can be easily realized with good reproducibility. Further, since the reaction between the surface of the compound semiconductor substrate of the substrate and the ZnO.SiO ₂ film constituting the diffusion source is weak, a boundary surface can be obtained after removing the diffusion source.
[0030]
<Second embodiment>
In the second embodiment, an example of the method for manufacturing the LED of the second invention will be described.
3A to 3C are first-half cross-sectional process diagrams for describing the method of manufacturing the LED of the second embodiment. 4A and 4B are cross-sectional process diagrams of the latter half following FIG. 3C.
[0031]
According to the LED manufacturing method of the second embodiment, first, Al ₂ having an opening 32 a on an n-type GaAs _0.8 P _0.2 epitaxial substrate (hereinafter, also simply referred to as a GaAsP substrate) 30. An O ₃ diffusion mask pattern 32 is formed (FIG. 3A).
[0032]
Next, a SiO ₂ film (hereinafter, referred to as a ZnO · SiO ₂ film) 34 and a ZnO film 36 doped with Zn, which is P-type Zn, are formed on the entire surface of the substrate on which the diffusion mask pattern 32 is provided by a sputtering method. To form a diffusion source 40 composed of a ZnO.SiO ₂ film 34 and a ZnO film 36. Here, the thickness of the ZnO · SiO ₂ film 34 is set to 150 °, and the thickness of the ZnO film 36 is set to 1500 °. Next, an annealing cap film 38 made of SiN is formed on the diffusion source 40 to cover the diffusion source 40 (FIG. 3B).
[0033]
Next, the substrate on which the annealing cap film 38 is formed is heat-treated. By this heat treatment, Zn is selectively diffused from the diffusion source 40 to the substrate 30 through the opening 32a to form a diffusion region 42 (FIG. 3C).
[0034]
Next, the diffusion source and the annealing cap film on the diffusion source are removed. In this embodiment, since the ZnO.SiO ₂ film is provided on the substrate, the reaction between the surface of the substrate and the diffusion source can be suppressed. As a result, by removing the diffusion source, the surface 44 of the diffusion region exposed to the opening 32a becomes a boundary surface (FIG. 4A).
[0035]
Next, an Al electrode 48 is formed as a P-side electrode electrically connected to a part of the surface 44 of the diffusion region exposed to the opening 32a. Next, after polishing the back surface of the substrate, an Au alloy electrode 46 is formed on the bottom surface of the substrate as an N-side electrode. In this LED, the surface 44 of the exposed diffusion region becomes the light emitting surface 44 (FIG. 4B).
[0036]
Next, the graph of FIG. 5 shows the luminous intensity distribution of the luminous surface when the LED manufactured in the second embodiment is caused to emit light by applying a voltage of about 1.6 V. The horizontal axis of the graph represents the distance of the light emitting surface of the LED from the electrode, and the vertical axis represents the light emission intensity (arbitrary unit) of the LED. A curve III in the graph of FIG. 5 shows a light emission intensity distribution of the LED manufactured in this example. As shown by the curve III, the LED manufactured in this example has a uniform light emission intensity over the entire light emitting surface in a range of about 100 μm in width.
[0037]
Further, the graph of FIG. 5 shows the light emission intensity distribution of the LED of the comparative example as a curve IV. In the LED of this comparative example, a diffusion region having a junction depth of about 1.5 μm was formed by performing a heat treatment at a temperature of 750 ° C. for 1 hour at a temperature of 750 ° C. in forming a diffusion region by using a conventionally known vapor phase diffusion method. . In the comparative example shown by the curve IV, only the emission intensity immediately near the Al electrode is strong, and the emission intensity sharply decreases as the distance from the Al electrode increases. As described above, in the conventional LED using the gas phase diffusion, the unevenness of the in-plane distribution of the emission intensity becomes remarkable particularly when the coupling depth Xj of the diffusion region is as small as 1.5 μm or less.
[0038]
In this regard, in the LED of the second embodiment, the diffusion concentration of Zn is high even if the coupling depth of the diffusion region is small. For this reason, even if the coupling depth is shallow, current is not concentrated in a region immediately below the vicinity of the electrode, and the light emission intensity in the light emitting surface of the LED can be made uniform. As a result, the light emission intensity hardly decreases even when the LED is separated from the electrode, so that an LED with a high light amount can be obtained. Further, when the coupling depth is small, there is an advantage that the side diffusion of Zn in the light emitting layer can be suppressed, and a high-density LED array can be formed.
[0039]
<Third embodiment>
Third Embodiment In a third embodiment, an example of the method for manufacturing the LED of the second invention will be described.
6A to 6C are first-half cross-sectional process drawings for describing the method of manufacturing the LED of the third embodiment. FIGS. 7A and 7B are cross-sectional process drawings of the latter half following FIG. 6C.
[0040]
According to the LED manufacturing method of the third embodiment, first, a p-type Zn-doped SiO ₂ film is formed on the entire surface of an n-type GaAsP epitaxial substrate (hereinafter, also simply referred to as a GaAsP substrate) 50. (Hereinafter referred to as a ZnO.SiO ₂ film) 54 and a ZnO film 56 are sequentially stacked by a sputtering method using a sputtering method to form a diffusion source (shown in FIG. 1) composed of the ZnO.SiO ₂ film 54 and the ZnO film 56. ) Are formed. In this embodiment, the thickness of the ZnO · SiO ₂ film 54 is set to 150 °, and the thickness of the ZnO film 56 is set to 1500 °. Next, photolithography and etching are performed to define an island-shaped diffusion source pattern 60 (FIG. 6A).
[0041]
Next, an annealing cap film 58 made of SiN is formed on the entire surface of the substrate on which the diffusion source pattern 60 is defined (FIG. 6B).
[0042]
Next, the substrate on which the annealing cap film 58 is formed is heat-treated. By this heat treatment, Zn is selectively diffused from the diffusion source pattern 60 to the substrate 50 to form a diffusion region 62 (FIG. 6C).
[0043]
Next, a resist pattern 64 having an opening 64a is formed on the diffusion source pattern 60 (FIG. 7A).
[0044]
Next, etching is performed through the resist pattern using buffered hydrofluoric acid to remove the portion of the annealing cap film 58 exposed in the opening 64a and the diffusion source pattern 60, thereby exposing the surface of the diffusion region 62. This surface becomes a boundary surface similarly to the surface 44 obtained in the second embodiment.
[0045]
Next, an Al electrode 68 is formed as a P-side electrode electrically connected to a part of the surface 64 of the diffusion region exposed to the opening 64a. Next, after polishing the back surface of the substrate, an Au alloy electrode 66 is formed as an N-side electrode. Thus, an LED can be obtained (FIG. 7B).
[0046]
Like the LED of the second embodiment, the LED of the third embodiment has a high Zn diffusion concentration even when the diffusion region has a small coupling depth. For this reason, the light emission intensity within the light emitting surface of the LED can be made uniform even at a shallow coupling depth. As a result, the light emission intensity hardly decreases even when the LED is separated from the electrode, so that an LED with a high light amount can be obtained. When the coupling depth is small, the side diffusion of Zn in the light emitting layer can be suppressed. Therefore, a high-density LED array can be created.
[0047]
In each of the embodiments described above, examples in which each invention according to this application is formed using specific materials and under specific conditions have been described. However, these inventions can be subjected to many changes and modifications. For example, in each of the above-described embodiments, the film serving as the diffusion source is formed by using the sputtering method. However, in these inventions, the film serving as the diffusion source is formed by using a method other than the sputtering method, for example, the CVD method. Can also be considered.
[0048]
In each of the embodiments described above, the SiN film is formed as the annealing cap film. However, in these inventions, for example, an Al ₂ O ₃ film, a SiO ₂ film, a SiON film, or an AlN film is formed as the annealing cap film. You may.
[0049]
In the embodiments described above, as a Zn doped oxide film, was used doped with Zn to SiO _2, of these aspects, for example doped with Zn in for Al ₂ O ₃ Zn doped oxide film May be used.
[0050]
In each of the embodiments described above, a GaAsP substrate is used as a substrate or a substrate. However, in these inventions, a compound semiconductor such as GaAs or GaAlAsInP may be used as a material.
[0051]
【The invention's effect】
According to the method for diffusing Zn of the first invention according to this application, the solid-phase diffusion of Zn using a Zn-doped oxide film and a Zn oxide film, which are sequentially laminated on an n-type compound semiconductor substrate, as a diffusion source. Do. As a result, it is possible to suppress the reaction between the surface of the substrate and the diffusion source and perform solid-phase diffusion of high-concentration Zn to the substrate.
[0052]
Further, according to the LED manufacturing method of the second invention of the present application, a Zn-doped oxide film and a Zn oxide film are sequentially stacked on an n-type compound semiconductor substrate, and the Zn solid phase is used as a diffusion source. Perform diffusion. As a result, it is possible to suppress the reaction between the surface of the substrate and the diffusion source, and perform high-concentration solid-phase diffusion of Zn to the substrate. As a result, a diffusion region having a high Zn concentration and a low sheet resistance can be formed even when the coupling depth of the diffusion region is small. Therefore, it is possible to produce an LED having a uniform light emission intensity on the upper surface of the diffusion region.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view for explaining a solid phase diffusion method of Zn according to a first embodiment.
FIG. 2 is a graph showing an analysis result of a Zn concentration profile of a diffusion region obtained in a first example.
FIGS. 3A to 3C are first-half cross-sectional process drawings for explaining the method of manufacturing the LED of the second embodiment;
4 (A) and (B) are cross-sectional process drawings in the latter half following FIG. 3 (C).
FIG. 5 is a plan view and a graph of light emission intensity distribution of the LED manufactured in the second embodiment.
FIGS. 6A to 6C are first-half cross-sectional process drawings for describing the method of manufacturing the LED of the third embodiment.
7 (A) and (B) are cross-sectional process drawings in the latter half following FIG. 6 (C).
[Explanation of symbols]
10: GaAsP substrate 12: ZnO · SiO ₂ film 14: ZnO film 16: annealing cap film (SiN film)
20: diffusion source 30: n-type GaAsP substrate 32: mask pattern 32a: opening 34: ZnO.SiO ₂ film 36: ZnO film 38: annealing cap film (SiN film)
40: diffusion source 42: diffusion region 44: surface 46: Au alloy electrode 48: Al electrode 50: substrate 54: ZnO.SiO ₂ film 56: ZnO film 58: annealing cap film (SiN film)
60: diffusion source pattern 62: diffusion region 64: resist pattern 64a: opening 66: Au alloy electrode 68: Al electrode

Claims (6)

Zn-doped SiO ₂ doped with Zn in advance on an n-type compound semiconductor substrate Forming a laminated structure in which a film, a ZnO film, and an annealing cap film are sequentially laminated;
By subjecting the laminated structure to a heat treatment, the Zn-doped SiO ₂ As film and diffusion source said ZnO layer, said substrate, said Zn saw including a step of solid-phase diffusion,
The Zn-doped SiO ₂ A solid phase diffusion method for Zn, wherein the thickness of the film is 100 to 500 ° . Zn-doped SiO ₂ doped with Zn in advance on an n-type compound semiconductor substrate Forming a laminated structure in which a film, a ZnO film, and an annealing cap film are sequentially laminated;
By subjecting the laminated structure to a heat treatment, the Zn-doped SiO ₂ Using the film and the ZnO film as diffusion sources, solid-phase diffusing the Zn into the substrate.
The molar ratio between Zn and Si of the Zn doped SiO ₂ film,
A solid phase diffusion method for Zn, wherein the range is 0.1 ≦ Zn / (Zn + Si) ≦ 0.5. The method for solid-phase diffusion of Zn according to claim 1 or 2 ,
A solid phase diffusion method for Zn, wherein an n-type GaAs _1-X P _X epitaxial substrate (where X represents a composition ratio and 0 ≦ X ≦ 0.4) is used as the n-type compound semiconductor substrate. Zn-doped SiO ₂ sequentially deposited on an n-type compound semiconductor substrate , doped with Zn in advance and having a thickness of 100 to 500 ° Forming a diffusion source comprising a film and a ZnO film ;
Forming an annealing cap film on the diffusion source;
And heat-treating the substrate formed with the said annealing cap film, by from said diffusion source Ru is diffused Zn into the substrate, forming a diffusion region,
Removing the diffusion source and an annealing cap film on the diffusion source;
Forming a first main electrode electrically connected to the n-type portion of the substrate and a second main electrode electrically connected to the diffusion region. Method. Zn-doped SiO ₂ doped in advance with Zn , which is sequentially laminated on an n-type compound semiconductor substrate A Zn-doped SiO ₂ film, wherein the molar ratio between Zn and Si is in the range of 0.1 ≦ Zn / (Zn + Si) ≦ 0.5. Forming a diffusion source comprising a film and a ZnO film ;
Forming an annealing cap film on the diffusion source;
And heat-treating the substrate formed with the said annealing cap film, by from said diffusion source Ru is diffused Zn into the substrate, forming a diffusion region,
Removing the diffusion source and an annealing cap film on the diffusion source;
Forming a first main electrode electrically connected to the n-type portion of the substrate and a second main electrode electrically connected to the diffusion region. Method. The method for manufacturing an LED according to claim 4 or 5 ,
As the substrate, n-type GaAs _1-X P _X epitaxial substrate (where X represents a composition ratio, 0 ≦ X ≦ 0.4) solid phase diffusion method Zn, which comprises using a.

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