Method for purifying powder, method for producing powder, powder, semiconductor device, and method for producing semiconductor device

Abstract

The present invention enables low-cost production of the following: a powder which is composed of fine particles containing zinc oxide as a main component and in which fine particles of one selected conductivity type are present in a particularly high proportion; and a high-performance semiconductor device using the powder.　In a powder composed of the fine particles, p-type fine particles and n-type fine particles are present. Therefore, once obtained, the powder is subjected to a step for increasing the proportion of one conductivity type (p-type) (step for enriching a specific conductivity type (p-type)). In this step, electrophoresis is used. Since the p-type fine particles and the n-type fine particles differ in zeta potential, these fine particles can be separated from each other by electrophoresis. Accordingly, a p-type layer can be formed in a light-emitting element or a light-receiving element by sintering the powder in which the p-type fine particles are present in an increased proportion.

Description

Method for purifying powder, method for manufacturing powder, powder, semiconductor device, method for manufacturing a semiconductor device

[0001] The present invention relates to a method for purifying a powder mainly composed of zinc oxide, a method for producing the powder, the powder, a semiconductor device using the same, and a method for producing the same.

[0002] Solar cells, which convert visible light into electrical energy, require both high photoelectric conversion efficiency and low manufacturing cost. Conversely, light-emitting elements that emit light when an electric current is passed through them are also known and can be used in displays, for example. These light-emitting elements also require both high luminous efficiency and low manufacturing cost.

[0003] Direct transition semiconductor materials with a band gap corresponding to the wavelength of light they absorb or emit are used as materials for semiconductor devices such as solar cells (photodetectors) and light-emitting elements. In particular, zinc oxide (ZnO) is known as such a semiconductor material that corresponds to short-wavelength light. ZnO has the advantages of being inexpensive and having low toxicity to the human body. On the other hand, while semiconductor devices such as those described above generally require a pn junction (a stacked structure of p-type and n-type layers), controlling the conductivity type of ZnO is extremely difficult compared to silicon, etc. Generally, it is easy to form the n-type, but it is extremely difficult to form the p-type. For this reason, the manufacture of semiconductor devices using ZnO pn junctions (p-type layers) has not been easy.

[0004] In contrast, Patent Document 1 describes a method for producing fine particles by a gas evaporation method, in which zinc (Zn) is evaporated in a chamber by generating a discharge in a reduced-pressure atmosphere with a zinc (Zn) target as one electrode, thereby generating fine particles on the inner wall of the chamber. Since these fine particles are formed by the bonding of Zn with oxygen in the atmosphere, the main component is ZnO. In this process, the generation of oxygen vacancies in ZnO can also be suppressed by controlling the amount of zinc evaporated. Furthermore, nitrogen (N), which is also present in the atmosphere, is added to these fine particles, and the nitrogen functions as an acceptor in the ZnO. Therefore, these fine particles are high-quality p-type ZnO fine particles, and a p-type layer (pn junction) can be formed using these fine particles.

[0005] Patent documents 2 to 4 describe how these fine particles can be sintered at low temperatures to form a p-type layer, and how this p-type layer can be used to manufacture semiconductor devices such as light-emitting elements and photodetectors. This makes it possible to manufacture light-emitting elements and photodetectors using ZnO at low cost. In this case, a high-quality n-type layer can be formed by a method different from the p-type layer, such as magnetron sputtering or CVD, and combined with the above-mentioned p-type layer. In particular, by forming the p-type layer, which was previously difficult to form, using fine particles (powder) in this way, these semiconductor devices can be manufactured at low cost. Furthermore, such p-type ZnO fine particles are also effective as photocatalytic materials, as described in Patent Document 5, for example, in addition to semiconductor devices.

[0006] Japanese Patent No. 4072620, International Publication WO2013 / 125719, Japanese Patent Publication No. 2013-175507, Japanese Patent No. 7470361, Japanese Patent Publication No. 2023-106792

[0007] As described in Patent Document 4, a p-type layer in a photoelectric conversion element can be formed using powder obtained by the manufacturing method described in Patent Document 1. However, it has been found that this powder contains a mixture of p-type and n-type fine particles. In the technology described in Patent Document 4, a photoelectric conversion element with good characteristics was obtained by employing a structure that can reduce the influence of n-type fine particles (especially leakage current) in this case.

[0008] However, analysis of the powder used to form the p-type layer in the photoelectric conversion element revealed that the proportion of n-type fine particles was actually high, while the proportion of p-type fine particles was around 0.5 or even lower. It was found that even with such a small proportion of p-type fine particles, it is possible to form a p-type layer using these particles to obtain a photoelectric conversion element with good characteristics.

[0009] Therefore, increasing the proportion of p-type fine particles in this powder is expected to further improve the characteristics of the photoelectric conversion element. This is true not only for photoelectric conversion elements but also for other applications where p-type ZnO is particularly effective, as described in Patent Document 5.

[0010] Therefore, it was necessary to increase the proportion of specific conductive type of fine particles in this powder and improve the performance of semiconductor devices using it.

[0011] The present invention has the following configuration in order to solve the above problems. The present invention is a method for purifying powder, in which zinc oxide is the main component and p-type fine particles and n-type fine particles are mixed, and the ratio of the p-type fine particles and the n-type fine particles is increased, characterized in that a DC electric field is applied to a medium in which the fine particles are dispersed by mixing the powder, a region is created in the medium in which the ratio of the p-type fine particles or the n-type fine particles is locally increased, and the fine particles in the region are extracted. The DC electric field may be formed by a plurality of electrodes. The ratio of the p-type fine particles may be increased. The medium may be water. The present invention relates to a method for producing a powder containing p-type fine particles mainly composed of zinc oxide, comprising: a powder extraction step in which a mixed gas containing oxygen gas and nitrogen gas is used as an atmospheric gas in a chamber, and zinc is heated and evaporated using an electrical discharge in the chamber, thereby extracting fine particles with an average particle diameter of 50 nm to 500 nm deposited on the inner wall of the chamber to obtain a powder; and a specific conductive type concentration step in which the proportion of the p-type fine particles in the powder is increased using the powder purification method. The present invention relates to a powder in which, by applying the powder purification method, the proportion of either the p-type fine particles or the n-type fine particles is increased compared to before the application of the purification method. The present invention relates to a powder produced by the powder production method. The ratio of the number of p-type fine particles to the sum of the number of p-type fine particles and n-type fine particles may be 0.52 or more. The present invention relates to a powder in which both have an average particle diameter of 50 nm to 1 μm and a nitrogen concentration in the crystal of 10 １６ cm －３ ~4 x 10 ２０ cm －３The present invention relates to a powder containing a mixture of p-type zinc oxide fine particles and n-type zinc oxide fine particles, characterized in that the ratio of the number of p-type zinc oxide fine particles to the total number of p-type zinc oxide fine particles to the total number of n-type zinc oxide fine particles is 0.52 or more. The present invention relates to a semiconductor device having a p-type layer, which is a p-type semiconductor layer, on a substrate, wherein the p-type layer is composed of fine particles mainly composed of zinc oxide that are sintered, and the fine particles are divided into p-type fine particles and n-type fine particles, characterized in that the ratio of the number of p-type fine particles to the sum of the number of p-type fine particles and n-type fine particles is 0.52 or more. The present invention relates to a method for manufacturing a semiconductor device, comprising: a powder extraction step for producing a powder containing a mixture of p-type fine particles and n-type fine particles; a p-type concentration step for applying an electric field using two electrodes to a medium in which the fine particles are dispersed by mixing the powder, thereby accumulating the p-type fine particles and n-type fine particles on one electrode and the other electrode, respectively, according to the zeta potential of the fine particles in the medium, thereby creating a region in the medium where the proportion of p-type fine particles is locally increased, and extracting the fine particles from this region to obtain a powder in which the proportion of p-type fine particles is increased; and a p-type fine particle layer formation step for forming the p-type layer by sintering the powder obtained in the p-type concentration step. In the powder extraction step, a mixed gas containing oxygen gas and nitrogen gas may be used as the atmosphere gas in a chamber, and zinc may be heated and evaporated using an arc discharge in the chamber to obtain the powder by taking out the fine particles deposited on the inner wall of the chamber. The medium may be water.

[0012] As the present invention is configured as described above, a powder consisting mainly of zinc oxide fine particles, in which the proportion of fine particles of one selected conductive type is particularly high, and a high-performance semiconductor device using this powder can be obtained at low cost.

[0013] This shows the results of measuring the relationship between nitrogen concentration and arc discharge current in zinc oxide fine particles obtained by gas evaporation. This is a schematic diagram showing a specific conductivity type concentration step in a powder manufacturing method according to an embodiment of the present invention. This is a schematic diagram showing the band structure when n-type and p-type ZnO are in contact with a solvent. This shows the results of measuring the relationship between gas pressure and zeta potential when ZnO powder is produced by gas evaporation. These are SCM(a) and AFM(b) mapping results for a sample containing a mixture of n-type ZnO fine particles and p-type ZnO fine particles. This is a cross-sectional view showing the structure of a semiconductor device (light-emitting element) according to an embodiment. This shows the results of comparing the luminescence characteristics of Example 1 and Comparative Example 1. This shows the results of measuring the relationship between luminescence intensity and p-type abundance from the luminescence characteristics of each example and Comparative Example 1. This is a cross-sectional view showing the structure of a semiconductor device (photodetector) according to an embodiment.

[0014] The powder according to the embodiment of the present invention is a powder containing a mixture of p-type ZnO fine particles (p-type fine particles) and n-type ZnO fine particles (n-type fine particles), with a particularly high proportion of p-type fine particles. This powder is obtained by applying the purification method according to the embodiment of the present invention to such a powder mixture of p-type and n-type fine particles. In this case, it is also possible to increase the proportion of n-type particles, but as described in Patent Documents 1 to 5, increasing the proportion of p-type particles is considered to be particularly important industrially, so the following will mainly describe the case where the proportion of p-type particles is increased.

[0015] This powder is produced, for example, by subjecting a powder consisting of fine particles obtained by the manufacturing method described in Patent Document 1 to a step (specific conductivity type concentration step) that increases the proportion of one conductivity type (p-type). The manufacturing method is described below.

[0016] Here, the fine particles produced by the manufacturing method described in Patent Document 1 are fine particles composed of ZnO with an average particle size of about 10 to 500 nm, and have different properties from bulk ZnO. The manufacturing method is described in Patent Document 1. In this method, as shown in Figure 1, a target made of zinc (Zn) is placed in a chamber. In a reduced-pressure atmosphere containing oxygen inside the chamber, a discharge (arc discharge or RF discharge) is generated between the target and an electrode placed in a vacuum close to the target, thereby evaporating Zn from the surface of the target. The evaporated Zn is oxidized by the oxygen in the atmosphere to become ZnO, which adheres to the inner wall of the chamber as fine particles, becoming ZnO fine particles. As the raw material for the target, a zinc ingot with a low concentration, such as 4N (purity 99.99%), can be used. Even when using such a low-purity, inexpensive ingot, high-quality crystals can be obtained in the resulting p-type ZnO fine particles.

[0017] Furthermore, the average particle size measured here was determined by scanning electron microscopy in accordance with JIS H7804:2005. Since these fine particles are generally not spherical, the particle size was calculated as (major axis + minor axis) / 2.

[0018] Furthermore, as described in Patent Document 1, the amount of zinc evaporation can be controlled, for example, by controlling the discharge current. Specifically, as the atmosphere inside the chamber, for example, a mixture of oxygen gas and nitrogen gas in a molar ratio of 4:1 can be used. This gas atmosphere is suitable for generating arc discharges, with a ratio of 20 × 10 ３Reduce the pressure to the Pa level. As a result, while oxidizing Zn to ZnO, nitrogen (N) serving as an acceptor can be simultaneously doped into the fine particles at a high concentration. Therefore, the formed ZnO fine particles can be made p-type. Here, for example, as described in "Growth of p-Type ZnO," Takafumi Yachibayashi, Journal of Applied Physics, Vol. 82, No. 10, p. 852 (2013), in ZnO, even if an element that should originally serve as an acceptor is doped, a level that substantially serves as a donor is simultaneously formed due to the self-compensation effect, and generally it is difficult to obtain a p-type stably and with good reproducibility. In contrast, according to this production method, by substituting oxygen (O) sites in ZnO with nitrogen, p-type ZnO fine particles can be obtained stably. At this time, by adjusting the discharge current, as described in Patent Document 1, the crystallinity of ZnO in the fine particles can also be increased. That is, high-quality p-type ZnO fine particles can be obtained. Also, Patent Document 1 states that from the intensity of the peak attributed to nitrogen acceptors in the emission spectrum, the nitrogen concentration of these p-type ZnO fine particles is 10 １６ cm －３ ~10 ２０ cm －３ as described.

[0019] Figure 1 shows the result of measuring the average nitrogen concentration in ZnO fine particles obtained by the production method described in Patent Document 1 using the inert gas fusion - thermal conductivity method. The horizontal axis is the arc discharge current. From this result, nitrogen is added to ZnO at a high concentration of 10 ２０ cm －３ or more (up to 4 × 10 ２０ cm －３ ). However, as described above, even when nitrogen is added at such a high concentration, a considerable amount of n-type fine particles existed in the obtained powder. The reason is that only a part of the added nitrogen functions as an acceptor (it does not function to supply holes by substituting oxygen at the oxygen sites of ZnO), so in Figure 1, the nitrogen is 10 ２０ cm －３Although added, the actual hole concentration is estimated to be less than 1 / 10 of this, and this is thought to be due to the fact that crystal defects in ZnO function as donors. For this reason, the ratio of p-type and n-type fine particles in the powder produced by the manufacturing method described in Patent Document 1 is actually about the same, and in some cases the ratio of p-type was about 0.4. However, because the electrical conductivity of the n-type particles formed in this case is low, it was possible to manufacture a p-type layer in a photoelectric conversion element using this powder, as described in Patent Document 4.

[0020] Therefore, after obtaining this powder by the manufacturing method described in Patent Document 1, further improvement in the properties of the p-type layer can be expected by performing a step to increase the proportion of p-type fine particles (decrease the proportion of n-type fine particles). Specifically, first, the manufacturing method described in Patent Document 1 can be performed as a powder extraction step to obtain a powder mainly composed of ZnO. A step to increase the proportion of one conductive type (p-type) (specific conductive type (p-type) concentration step: purification method) is then applied to this powder. Here, electrophoresis is used in this step. Since the zeta potentials of p-type fine particles and n-type fine particles are different, they can be separated by electrophoresis. Figure 2 is a schematic diagram of this step (purification method). Note that the configuration shown here is described to illustrate the basic principle of this step, and other configurations can be used as long as p-type fine particles and n-type fine particles can be separated using electrophoresis. In Figure 2(a), the suspension obtained by mixing the powder in a medium (ultrapure water) and stirring is filled into a U-shaped tube (container) 100. Here, an electrode (positive electrode) 101 is provided at one end (left side) of the U-shaped tube 100, and an electrode (negative electrode) 102 is provided at the other end (right side), and the suspension is filled between the positive electrode 101 and the negative electrode 102. As a result, an electric field (DC electric field) is applied to the suspension by the electrode (positive electrode) 101 and the electrode (negative electrode) 102. Here, it is assumed that n-type fine particles 201 (white circles) and p-type fine particles 202 (black circles) are mixed in the suspension in approximately equal proportions overall.

[0021] Figure 3 schematically shows the band structures of n-type and p-type ZnO in a medium (solvent: water) and when they are in contact. Here, the left side shows the band diagrams (E) of n-type and p-type ZnO in bulk. Ｃ : Conduction band lower edge energy, E Ｖ : Valence band upper energy, E Ｆ E in the solvent is at the Fermi level, and this ZnO is in contact with the solvent (water) shown in the center. ｒｅｄｏｘ The graph shows the redox potential of the solvent, and the right side shows the band structures when n-type and p-type ZnO are in contact with the solvent, respectively. As shown on the right side of Figure 2, similar to the Schottky junction between a semiconductor and a metal, when n-type and p-type ZnO are in contact with the solvent, E Ｆ and E ｒｅｄｏｘ The bands become continuously equal at the interface, and the band structure bends on the ZnO side so that an electric field is formed near the interface, eliminating the flow of electrons and holes between ZnO and the solvent. The direction of this bending is opposite for n-type and p-type nanoparticles. As a result, near the surface of the ZnO nanoparticles (the interface with water), holes (positive charge) accumulate in the n-type (upper side in the figure), and electrons (negative charge) accumulate in the p-type (lower side in the figure). Consequently, anions from the solvent accumulate on the surface of the n-type nanoparticles, and cations accumulate on the surface of the p-type nanoparticles. Consequently, an electric dipole is formed on the surface of each nanoparticle, and its direction is opposite for n-type and p-type nanoparticles. Therefore, the n-type nanoparticle 201 has a negative (low) zeta potential, and the p-type nanoparticle 202 has a positive (high) zeta potential.

[0022] In the manufacturing method described in Patent Document 1 (gas evaporation method), it was found through the evaluation method for p-type and n-type microparticles described later that the lower the pressure during generation, the longer the lifetime of nitrogen radicals formed in the gas, which increases the ratio of nitrogen acceptors incorporated into the microparticles, and thus the higher the proportion of p-type microparticles. Figure 4 shows the relationship between the zeta potential measured by electrophoresis on a suspension of the obtained powder mixed with pure water, and the pressure during the generation of the microparticles (powder). The same measurement was performed on samples from three lots (three types of plots). From these results, it can be confirmed that the lower the pressure, the higher the zeta potential. In other words, it was confirmed that p-type microparticles have a higher zeta potential than n-type microparticles.

[0023] Therefore, as shown in Figure 2(b), when a positive voltage is applied to electrode 101 and a negative voltage to electrode 102, the n-type fine particles 201 move towards electrode 101 and the p-type fine particles 202 move towards electrode 102. As a result, a region with a high concentration of n-type fine particles 201 is formed near electrode 101, and a region with a high concentration of p-type fine particles 202 is formed on the electrode 102 side.

[0024] In order to extract only the region with a high concentration of p-type fine particles 202 in the suspension, as shown in Figures 2(c) and 2(d), a partition 103 is provided on the side of the U-tube 100 closer to the electrode 102. By discharging the suspension on the electrode 101 side of the partition 103 from the outlet 104, a suspension with a higher concentration of p-type fine particles 202 remains between the partition 103 and the electrode 102. When this suspension is extracted, the concentration of p-type fine particles 202 in this suspension is higher than in the state before processing (Figure 2(a)). In other words, the relative abundance of p-type fine particles 202 to n-type fine particles 201 in this suspension is increased. By evaporating the solvent of the suspension with a higher proportion of p-type fine particles 202, a powder with a higher proportion of p-type fine particles 202 is obtained.

[0025] Note that the position of the partition 103 in Figure 2(c) is arbitrary as long as it is spaced apart from electrodes 101 and 102. That is, the partition 103 may be placed at position B, for example. Alternatively, the partition 103 may be placed closer to the positive electrode 101, and the outlet may be placed even closer to the positive electrode 101.

[0026] Alternatively, for example, if the proportion of n-type microparticles in the original powder is higher than that of p-type microparticles, many n-type microparticles 201 will move towards the electrode 101, so even if only the bottom portion of the U-shaped tube 100 in Figure 2 is extracted, the proportion of p-type microparticles 202 will increase. In other words, which region to extract after applying the electric field can be appropriately set according to the proportion of n-type microparticles 201 and p-type microparticles 202 in the original powder, and it is not necessarily required to extract the region near the electrode 102. Furthermore, the apparatus configuration for electrophoresis is arbitrary, as long as it is possible to separate the n-type microparticles 201 and p-type microparticles 202 by applying an electric field of a certain polarity to each microparticle and determining the difference in zeta potential. Alternatively, the method for extracting the region in the suspension where p-type microparticles 202 (or n-type microparticles 201) are concentrated (diluted) is arbitrary. Also, as long as it is possible to separate the p-type microparticles 202 and n-type microparticles 201 by electrophoresis as described above, the type of solvent (medium) is also arbitrary. As such a medium, in addition to pure water, an aqueous electrophoretic solution capable of electrophoresis can be used. Furthermore, as a non-aqueous electrophoretic solution, organic solvents such as ethanol, acetonitrile, methanol, acetone, etc., and organic solvents (toluene, isopropyl alcohol, etc.) can be used. Electrolyte solutions and buffer solutions such as sodium acetate, sodium citrate, urea, phosphate buffer, TrisHCl buffer, phosphate-buffered saline (PBS), borate buffer, sodium acetate buffer, HEPES buffer, sodium citrate buffer, sodium bicarbonate buffer, MOPS buffer, sodium cacodylate buffer, glycine-NaOH buffer, etc., as well as gel media (agarose gel, polyacrylamide gel, etc., and other capillary electrophoresis solutions) can also be used as appropriate. Similarly, the method of forming the electric field is arbitrary, as long as a (DC) electric field can be applied to the suspension.

[0027] By repeatedly performing the above p-type concentration process on the powder, the concentration of p-type fine particles 202 in the powder can be further increased. In the above example, the concentration of p-type fine particles 202 was increased, but conversely, a suspension with an increased concentration of n-type fine particles 201 can be obtained similarly. However, as mentioned above, compared to the p-type case, the n-type ZnO layer can generally be easily obtained without using n-type ZnO fine particles; therefore, increasing the concentration of p-type fine particles 202 as described above is particularly effective.

[0028] This section describes the evaluation results of powders obtained from suspensions in which the concentration of p-type fine particles was increased using the method described above. This evaluation was performed using a scanning capacitance microscope (SCM: Bruker NanoScope V Dimension Icon XR). Since measurement is not easy in the state of pure powder, the sample actually measured was a sintered layer obtained by mixing 0.05 g of this powder (ZnO fine particles) and 0.1 g of binder with 0.3 ml of IPA, coating a glass substrate with a conductive film at 4000 rpm for 10 sec, and then heat-treating (sintering) it on a hot plate at 280°C for 60 sec. Furthermore, the surface of this sample was etched (milled) using an Ar ion beam and then evaluated.

[0029] In SCM, the capacitance at each point in a sample is measured as a pseudo-MOS structure using a probe, similar to those used in atomic force microscopy (AFM), as the electrode. The capacitance C of the MOS structure (semiconductor surface) changes with the formation of storage layers, depletion layers, and inversion layers on the surface in response to the applied voltage. However, the order in which these layers are formed is reversed between n-type and p-type semiconductors as the applied voltage changes, resulting in the capacitance change being in the opposite direction for n-type and p-type semiconductors. Therefore, SCM can be used to distinguish between n-type and p-type regions on the surface of a sample.

[0030] Furthermore, if the binder used is an insulator, the capacitance of this insulator can be considered to be approximately constant with respect to the applied voltage. Therefore, when measuring the rate of change of capacitance C with respect to the applied voltage V (dC / dV) using an SCM, a small value of dC / dV corresponding to the binder (approximately zero) can be used as a boundary to distinguish between n-type and p-type layers with dC / dV above and below this boundary.

[0031] Figure 5(a) shows the actual mapping result of dC / dV for a sample in which n-type fine particles 201 and p-type fine particles 202 are mixed as described above. Here, shading corresponding to the value of dC / dV for an AC signal of 100 kHz and an amplitude of 10 V (DC bias component = 0) is given. Figure 5(b) shows the result of similarly mapping the height of the probe at this time, which is the measurement result by normal AFM for the same sample. This result reflects the fine unevenness of the sample surface.

[0032] Here, as the binder, silsesquioxane (manufactured by Toagosei Co., Ltd.: trade name OX-SQ SI-20) was used. In Figure 5(a), in a wide region with a substantially constant concentration, there are minute regions where dC / dV is on the positive side (higher concentration) and minute regions where dC / dV is on the negative side (lower concentration: closer to white) dotting around. As described above, the wide region with a substantially constant concentration corresponds to the binder, the minute region with a higher concentration corresponds to the n-type fine particles 201, and the minute region with a lower concentration corresponds to the p-type fine particles 202. The average particle diameters of the n-type fine particles 201 and the p-type fine particles 202 are about 10 to 500 nm as described in Patent Document 1, and no significant difference is recognized between the n-type and the p-type. On the other hand, in the result of AFM (Figure 5(b)), it is difficult to distinguish between the n-type fine particles 201 and the p-type fine particles 202.

[0033] Therefore, from the result of Figure 5(a), the numbers of the n-type fine particles 201 and the p-type fine particles 202 in this image can be counted. Actually, this measurement is performed at more locations, and by counting fine particles in a statistically significant number (200 or more), the p-type abundance ratio, i.e., the number of p-type fine particles / (the number of n-type fine particles + the number of p-type fine particles) (hereinafter referred to as p / (n + p)), can be measured. As a result, as will be described later, by repeatedly performing the above p-type concentration process on the powder with a p / (n + p) of about 0.38 before the p-type concentration process, a maximum p / (n + p) of about 0.85 was obtained.

[0034] However, as described in Patent Document 4, even when powder with a ratio of 0.38 (when the abundance ratio of n-type fine particles is higher) is used, a semiconductor layer that actually functions as a p-type semiconductor layer was obtained. However, it is preferable to sinter powder with a higher p-type abundance ratio to form a p-type layer in a light-emitting element or a light-receiving element. The p-type abundance ratio is preferably large (close to 1), but in order to approach 1, the time required for the p-type concentration process becomes long, or the number of times of performing this increases. In practice, 0.8 or more is particularly preferable, and as will be described later, if it is 0.52 or more, the effect can be obtained.

[0035] Also, from the above principle, it is clear that the above p-type concentration process (powder purification method) is also effective for powder mainly composed of ZnO obtained by other manufacturing methods. In this case, at least the nitrogen concentration is in the same range as above, 10 １６ cm －３ ~4×10 ２０ cm －３ Even if the average particle size is in a wider range than the powder described in Patent Document 1, for example, 50 nm to 1 μm, it is clear that the same effect is obtained. That is, by the above purification method, powder having a nitrogen concentration, an average particle size within this range, and p / (n + p) of 0.52 or more can be obtained.

[0036] Next, the results of actually manufacturing a semiconductor device (light-emitting element) using the powder that has undergone the p-type concentration process as described above will be described.

[0037] This light-emitting element 10 has substantially the same structure as that described in Patent Document 2. FIG. 6 is a cross-sectional view showing its overall structure, and this figure is substantially the same as FIG. 1 in Patent Document 2.

[0038] In this light-emitting element 10, a ZnO-based transparent conductive film (conductive layer) 12 is formed on a transparent substrate 11, and an n-type ZnO-based thin film (n-type layer) 13 is sequentially formed thereon. On this n-type ZnO-based thin film 13, a fine particle layer 14, which will become a p-type layer, is formed by firing a powder consisting of fine particles 141, which are Zn fine particles as described above, using a binder 142. Here, while Patent Document 2 states that the fine particle layer 14 is composed of a mixture of multiple types of p-type fine particles with different emission wavelengths, here it is composed of fine particles 141 that are either n-type or p-type fine particles as described above. Also, while Patent Document 2 states that the thickness of the fine particle layer 14 is equal to the diameter of the fine particles, the fine particle layer 14 in Figure 6 is set to be thicker than this. However, in practice, this thickness is set appropriately considering the light emission intensity, leakage current, etc.

[0039] In the left side of Figure 6, the n-type ZnO-based thin film 13 is partially removed, and the n-side electrode 15 is formed on top of it. Also, the p-side electrode 16 is formed on top of the fine particle layer 14. When current is passed between the n-side electrode 15 and the p-side electrode 16 as shown in the figure, the fine particle layer 14 mainly emits light, causing the light-emitting element 10 to emit light. At this time, as indicated by the arrow in Figure 6, it is possible to extract this light emission to the lower side through the substrate 11.

[0040] As the transparent substrate 11, for example, a glass substrate can be used, and a ZnO-based transparent conductive film 12 and an n-type ZnO-based thin film 13 are sequentially deposited on the substrate 11. As the ZnO-based transparent conductive film 12, for example, a Ga-doped ZnO film can be used. As the n-type ZnO-based thin film 13, for example, ZnO doped with Ga or Al can be used. However, the amount of doping is smaller than that of the ZnO-based transparent conductive film 12. As a method for forming the n-type ZnO-based thin film 13, magnetron sputtering or CVD can be used. With these methods, an n-type ZnO-based thin film 13 can be obtained as a thin film of a semiconductor that is uniformly n-type.

[0041] The fine particle layer 14 is a layer that becomes the p-type light-emitting layer in the light-emitting element 10, and is formed by firing a powder composed of the fine particles 141 described above at a low temperature using a binder. The n-side electrode 15 and the p-side electrode 16 are both made of metals with low electrical resistance, and as materials, for example, gold, aluminum, or a laminated structure containing these can be used. As shown in Figure 6, terminals to which voltage is applied are connected to these, so their thickness is set appropriately to allow for this connection.

[0042] In the structure shown in Figure 6, a pn junction is formed between the n-type ZnO thin film 13 and the fine particle layer 14. Since it is easier to obtain n-type ZnO than p-type, the above-described film formation method yields a uniformly n-type n-type ZnO thin film 13. On the other hand, as described above, since the powder constituting the fine particle layer 14 contains a mixture of p-type and n-type fine particles, it is difficult to make the fine particle layer 14 have the same crystallinity as the n-type ZnO thin film 13. However, to obtain good properties, it is preferable to have a high proportion of p-type fine particles.

[0043] The method for forming the fine particle layer 14 (p-type fine particle layer formation process: fine particle layer formation process) is the same as the method for forming the fine particle layer 14 in Patent Document 3. That is, first, the powder in which the proportion of p-type fine particles 202 has been increased by the p-type concentration process is mixed with a binder 142, such as SOG (Spin On Glass) liquid and a solvent (such as an organic solvent like alcohol), in a predetermined mixing ratio, and a liquid (coating solution) is produced by mixing using a ball mill or the like in this liquid. This coating solution is applied to the entire surface of the n-type ZnO-based thin film 13 with a uniform film thickness using methods such as spin coating and dip coating. Alternatively, if a printing method or inkjet method is used, it is also possible to pattern and apply only to the desired area. The type and viscosity of the solvent and binder can be appropriately selected depending on the coating method.

[0044] Subsequently, by firing, the dispersed fine particles 141 are sintered to form a fine particle layer 14. This firing can be carried out, for example, in air at a temperature range of 200 to 300°C. In order to ensure p-type conductivity after sintering, it is preferable to keep this temperature below 300°C, but if SOG is used as the binder 142, sintering can be carried out at this temperature range. Furthermore, the SOG component is transparent to visible light, and the light component absorbed by SOG can be reduced. In addition, for example, a resin substrate with low heat resistance can be used as the substrate 11. As the binder 142, a photocurable binder that hardens with ultraviolet light can be used, or the firing can be carried out by locally increasing only the surface temperature by irradiating the surface with a flash lamp or pulsed laser light. The thickness of the fine particle layer 14 before firing can be adjusted by the viscosity of the solvent and the coating conditions, and the thickness of the fine particle layer 14 after firing can be adjusted accordingly.

[0045] As the above-mentioned fine particle layer 14 (fine particles 141), two types were manufactured: one using powder that did not undergo the p-type concentration process (Comparative Example 1) and one using powder that underwent three p-type concentration processes (Example 1), and their luminescence characteristics were compared. Here, as the p-type concentration process, a suspension was prepared by first mixing 22 mg of the powder obtained by the powder extraction process with 16.7 ml of ultrapure water and ultrasonically stirring, as shown in Figure 2. A voltage of 670 V was applied between the two electrodes for 30 min as one batch.

[0046] As described above, the proportion of p-type fine particles 202 can be particularly increased by repeatedly performing the p-type concentration process. Here, as the first p-type concentration process, the voltage under the above conditions was applied to the suspension in 21 batches, and the region on the electrode (negative electrode) 102 side in Figure 2 was extracted. As the second p-type concentration process, the same voltage was applied to the suspension using the extracted powder in 10 batches, and the region at the bottom of the U-shaped tube 100 in Figure 2 was extracted. As the third p-type concentration process, the same voltage was applied to one batch, and the bottom portion was extracted in the same way as the second time.

[0047] Figure 7 shows the results of measuring the relationship between the forward voltage (V) and the light output (μW) of each light-emitting element. From these results, it can be confirmed that Example 1 yields approximately three times the light output of Comparative Example 1. As mentioned above, the measured p / (n+p) values ​​were approximately 0.38 for Comparative Example 1 and approximately 0.85 for Example 1. This difference in light output reflects this difference in p / (n+p).

[0048] Similar measurements were performed by forming a fine particle layer 14 using powder that had undergone the p-type concentration process only once (Example 2) and powder that had undergone it twice (Example 3). The p / (n+p) ratio was measured for each example, and the relationship between the light output and the p-type abundance (p / (n+p)) is shown in Figure 8. From these results, a p / (n+p) value of 0.52 was obtained for Example 2, 0.66 for Example 3, and 0.85 for Example 1. In other words, by sequentially performing the above p-type concentration process, the p / (n+p) ratio can be increased to 0.52 or higher, and the light output is approximately proportional to the p / (n+p) ratio. That is, applying the above p-type concentration process to powder to form a fine particle layer 14 is extremely effective in obtaining high light output.

[0049] Although Figure 6 and the above-described embodiments relate to light-emitting elements, it is clear that this powder can also be used in light-receiving elements such as solar cells. Figure 9 shows the structure of such a light-receiving element (photoelectric conversion element: semiconductor device) 20, and the structure itself is the same as that of Figure 1 in Patent Document 3, except for the reference numerals.

[0050] In this photodetector 20, a ZnO-based transparent conductive film (conductive layer) 22 is formed on a transparent substrate 21, and an n-type ZnO-based thin film 23 is sequentially formed on top of it. A fine particle layer 24 is formed on top of this n-type ZnO-based thin film 23. On the left side of Figure 9, the n-type ZnO-based thin film 23 and the like are partially removed, and an n-side electrode 25 is formed on top of it. A p-side electrode 26 is also formed on top of the fine particle layer 24. In this photodetector 20, an electromotive force is generated at the pn junction by light incident from below through the substrate 21, and this output is taken out via the n-side electrode 25 and the p-side electrode 26.

[0051] The substrate 21, ZnO-based transparent conductive film (conductive layer) 22, n-type ZnO-based thin film (n-type layer) 23, n-side electrode 25, and p-side electrode 26 are the same as those of the substrate 11, ZnO-based transparent conductive film (conductive layer) 12, n-type ZnO-based thin film (n-type layer) 13, n-side electrode 15, and p-side electrode 16, respectively.

[0052] The fine particle layer 24 functions as a p-type layer for forming a pn junction in the light-receiving element 10, and, similar to the fine particle layer 14, is formed by a similar formation method (fine particle layer formation process) using a powder consisting of fine particles 141 with a higher concentration of p-type fine particles as described above, and a binder 142.

[0053] In this case as well, it is clear that increasing the proportion of p-type microparticles in the microparticles 141 constituting the microparticle layer 24 will result in a higher photoelectric conversion efficiency and a reduction in leakage current (reverse current). Therefore, the leakage current can be reduced without using the structure described in Patent Document 4.

[0054] Furthermore, the p-type layer formed using the powder that has undergone the p-type concentration process as described above can be used in various semiconductor devices in combination with an n-type layer formed by other methods. In this case, a large-area p-type layer can also be obtained as a sintered body by coating and firing a mixture of this powder and a binder (liquid). In this case, although the above example was a light-emitting element and a photodetector, elements with other functions can be obtained in the same way. As described above, in this case, the p-type layer, which was previously difficult to form, can be easily formed using powder, making it possible to manufacture high-performance semiconductor devices at low cost.

[0055] Alternatively, an n-type layer using a similar sintered body made from powder in which the proportion of n-type fine particles has been increased by a specific conductivity type (n-type) concentration process may be used instead of the n-type ZnO-based thin films 13 and 23. In this case, an n-type layer can be formed by performing an n-type fine particle layer formation process (fine particle layer formation process) similar to the p-type fine particle layer formation process using powder in which the proportion of n-type fine particles 201 has been similarly increased. In this case, the powder targeted in the p-type concentration process and the n-type concentration process can be a common powder, and both the n-type layer and the p-type layer constituting the pn junction can be patterned and formed as fine particle layers, making this semiconductor device particularly inexpensive.

[0056] The p-type layer used here exhibits p-type conductivity similar to conventional p-type semiconductor layers formed by, for example, epitaxial growth, and the same applies to the n-type layer. Therefore, conventional semiconductor devices can be manufactured in the same way using p-type and n-type layers made of ZnO nanoparticles in this manner. In this case, for example, when forming these layers, any substrate capable of low-temperature firing can be used, and many types of substrates can be used compared to the strict limitations on substrates in the case of epitaxial growth. In this case, if both the p-type and n-type layers are made of the nanoparticle layers described above, the combination and order of stacking these layers are also arbitrary.

[0057] On the other hand, unlike the p-type layer, high-quality n-type layers can be easily formed by other growth methods (epitaxial growth, sputtering, etc.). Therefore, by combining a p-type layer formed as a fine particle layer as described above with such a high-quality n-type layer, high-performance semiconductor devices such as the light-emitting element 10 and the photodetector 20 can be obtained. In other words, the p-type layer, which is a fine particle layer mainly composed of zinc oxide as described above, is particularly effective in semiconductor devices. In this case as well, the combination of stacking n-type and p-type layers can be set as appropriate. Furthermore, although the above example described photoelectric conversion elements (photodetectors, light-emitting elements) as semiconductor devices, other types of semiconductor devices such as switching elements can also be manufactured. In this case, it is clear that the above-described p-type layer (n-type layer) is equally effective for TFTs and the like, where the p-type layer is formed on a large substrate.

[0058] Furthermore, the powder can be used in other forms and for other applications, not just in semiconductor devices. For example, Patent Document 5 describes a photocatalyst and antibacterial agent in which p-type ZnO fine particles are dispersed in a dispersant. When being p-type is advantageous in these properties, it is effective to use a powder in which the proportion of p-type fine particles is increased as described above. Conversely, in applications where being n-type is advantageous, a powder in which the proportion of n-type ZnO fine particles is increased can also be used in a similar manner.

[0059] Furthermore, in the above example, the above-mentioned specific conductivity type concentration step was applied to powder obtained by the manufacturing method (powder extraction step) described in Patent Document 1. However, it is clear that the above-mentioned specific conductivity type concentration step is effective when it is desired to increase the proportion of one conductivity type in powder obtained by any manufacturing method in which p-type ZnO fine particles and n-type ZnO fine particles are mixed. In other words, the content of the powder extraction step is arbitrary as long as a powder containing a mixture of p-type ZnO fine particles and n-type ZnO fine particles is obtained. As mentioned above, when forming p-type ZnO fine particles, n-type fine particles are also likely to be formed simultaneously due to crystal defects, etc. For this reason, the above-mentioned powder extraction step (purification method) is equally effective for powders manufactured by methods other than those described in Patent Document 1.

[0060] 10 Light-emitting element (semiconductor device) 11, 21 Substrate 12, 22 ZnO-based transparent conductive film (conductive layer) 13, 23 n-type ZnO-based thin film (n-type layer) 14, 24 Microparticle layer 15, 25 n-side electrode 16, 26 p-side electrode 20 Photodetector (photoelectric conversion element: semiconductor device) 100 U-shaped tube (container) 101 Electrode (positive electrode) 102 Electrode (negative electrode) 103 Partition 104 Outlet 141 Microparticles 142 Binder 201 n-type microparticles 202 p-type microparticles

Claims

1. A method for purifying a powder, in which a powder mainly composed of zinc oxide and containing a mixture of p-type and n-type fine particles, is used to increase the relative abundance of either the p-type fine particles or the n-type fine particles, characterized by applying a direct current electric field to a medium in which the fine particles are dispersed by mixing the powder, thereby creating a region in the medium where the relative abundance of the p-type fine particles or the n-type fine particles is locally increased, and extracting the fine particles from that region.

2. The method for purifying powder according to claim 1, characterized in that the DC electric field is formed by a plurality of electrodes.

3. A method for purifying powder according to claim 1 or 2, characterized by increasing the proportion of p-type fine particles.

4. The method for purifying a powder according to claim 1 or 2, characterized in that the medium is water.

5. A method for producing a powder containing p-type fine particles mainly composed of zinc oxide, comprising: a powder extraction step of obtaining a powder by heating and evaporating zinc using an electrical discharge in a chamber with a mixed gas containing oxygen gas and nitrogen gas as the atmospheric gas, thereby extracting fine particles having an average particle diameter of 50 nm to 500 nm deposited on the inner wall of the chamber; and a specific conductive type concentration step of increasing the proportion of the p-type fine particles in the powder using the powder purification method described in claim 1 or 2.

6. A powder characterized in that, by applying the powder purification method described in claim 1 or 2, the proportion of one of the p-type fine particles and the n-type fine particles is increased compared to before the application of the purification method.

7. A powder characterized by being produced by the method for producing a powder described in claim 5.

8. The powder according to claim 6, characterized in that the ratio of the number of p-type fine particles to the sum of the number of p-type fine particles and n-type fine particles is 0.52 or more.

9. The powder according to claim 7, characterized in that the ratio of the number of p-type fine particles to the sum of the number of p-type fine particles and n-type fine particles is 0.52 or more.

10. Both have an average particle size of 50 nm to 1 μm, and the nitrogen concentration in the crystal is 10 １６ cm －３ ~4 x 10 ２０ cm －３ A powder comprising a mixture of p-type zinc oxide fine particles and n-type zinc oxide fine particles, characterized in that the ratio of the number of p-type zinc oxide fine particles to the number of n-type zinc oxide fine particles is 0.52 or more.

11. A semiconductor device having a p-type layer provided on a substrate, wherein the p-type layer is composed of fine particles mainly composed of zinc oxide sintered, the fine particles are divided into p-type and n-type fine particles, and the ratio of the number of p-type fine particles to the sum of the number of p-type and n-type fine particles is 0.52 or more.

12. A method for manufacturing a semiconductor device according to claim 11, comprising: a powder extraction step for producing a powder containing a mixture of p-type fine particles and n-type fine particles; a p-type concentration step for applying an electric field using two electrodes to a medium in which the fine particles are dispersed by mixing the powder, and accumulating the p-type fine particles and n-type fine particles on one electrode and the other electrode, respectively, according to the zeta potential of the fine particles in the medium, thereby creating a region in the medium where the proportion of p-type fine particles is locally increased, and extracting the fine particles from the region to obtain a powder in which the proportion of p-type fine particles is increased; and a p-type fine particle layer formation step for forming the p-type layer by sintering the powder obtained in the p-type concentration step.

13. The method for manufacturing a semiconductor device according to claim 12, characterized in that, in the powder extraction step, a mixed gas containing oxygen gas and nitrogen gas is used as the atmospheric gas in the chamber, and zinc is heated and evaporated using an arc discharge in the chamber, thereby removing fine particles deposited on the inner wall of the chamber to obtain the powder.

14. The method for manufacturing a semiconductor device according to claim 13, characterized in that the medium is water.

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