WO2018225855A1 - 半導体層、発振素子及び半導体層の製造方法 - Google Patents
半導体層、発振素子及び半導体層の製造方法 Download PDFInfo
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- WO2018225855A1 WO2018225855A1 PCT/JP2018/022013 JP2018022013W WO2018225855A1 WO 2018225855 A1 WO2018225855 A1 WO 2018225855A1 JP 2018022013 W JP2018022013 W JP 2018022013W WO 2018225855 A1 WO2018225855 A1 WO 2018225855A1
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- oxide film
- aluminum oxide
- semiconductor layer
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- 239000004065 semiconductor Substances 0.000 title claims abstract description 283
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- TWNQGVIAIRXVLR-UHFFFAOYSA-N oxo(oxoalumanyloxy)alumane Chemical compound O=[Al]O[Al]=O TWNQGVIAIRXVLR-UHFFFAOYSA-N 0.000 claims abstract description 148
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Definitions
- the present invention relates to a semiconductor layer, an oscillation element, and a method for manufacturing the semiconductor layer.
- the semiconductor field high power, high withstand voltage, high temperature operation and high frequency are required.
- high withstand voltage is important, and for this reason, a wide band gap semiconductor having a larger band gap than conventional Si-based semiconductors is desired.
- the band gap of Si is 1.12 eV
- the band gap of SiC which is attracting attention as a wide band gap semiconductor, is 2.20 to 3.02 eV
- the band gap of GaN is 3.39 eV
- the band gap is Development of larger wide bandgap semiconductors is ongoing.
- the present inventor has made a detailed study on an oscillation element using a semiconductor layer based on aluminum oxide and a method for manufacturing the oscillation element.
- diamond having a band gap of 5.47 eV is attracting attention as a wide band gap semiconductor material.
- diamond is not a semiconductor, it is necessary to form donor levels and acceptor levels by ion implantation.
- diamond ion implantation requires high temperature and pressure and cannot be performed easily.
- the band gap of aluminum oxide is 8.8 eV, and it would be attractive if it could be a wide band gap semiconductor material, but it has been difficult to form donor levels and acceptor levels in the band gap so far. It was. In particular, it was difficult to form acceptor levels. For this reason, although aluminum oxide has obtained high reliability as an extremely excellent insulator, it has been difficult to form a p-type semiconductor and a pn junction using aluminum oxide.
- the present inventor has succeeded in producing a semiconductor layer having a Schottky junction using aluminum oxide as a base material. Further, the present inventors have found that in this semiconductor layer, the depletion layer thickness is made extremely thin, a tunnel current is passed, and a voltage is applied in the reverse bias direction, whereby oscillation is seen in the current. It was confirmed that current oscillation appears at a predetermined high current density or less.
- an inverter that uses direct current, such as a battery
- using an oscillation element invented by the present inventor for example, by applying a wide bias voltage including a zero bias voltage from the forward bias voltage side to the reverse bias voltage side. It is desirable to oscillate current.
- One embodiment of the present invention has been newly conceived based on further research by the present inventor, and an object thereof is to realize a semiconductor layer, an oscillation element, and a method for manufacturing the semiconductor layer, which are superior in performance to those of the prior art.
- a semiconductor layer according to one embodiment of the present invention includes an n-type semiconductor in which an aluminum oxide film contains an excessive amount of aluminum and donor levels are formed, and the aluminum oxide film contains excess oxygen. It includes a pn junction in which a p-type semiconductor in which an acceptor level is formed is joined.
- the semiconductor layer according to another embodiment of the present invention includes a p-type semiconductor in which an acceptor level is formed by excessively containing oxygen in an aluminum oxide film.
- metal aluminum is brought into contact with one surface of an aluminum oxide film, and a probe is brought into contact with the other surface of the aluminum oxide film.
- oxygen gas a voltage that causes dielectric breakdown of the aluminum oxide film is applied between the metal aluminum as an anode and the probe as a cathode to melt the aluminum oxide film, and during the melting,
- a molten salt electrolysis reaction in the aluminum oxide film and cooling an n-type semiconductor layer is formed on the metal aluminum side of the aluminum oxide film, and a p-type semiconductor layer is formed on the probe side of the aluminum oxide film, respectively.
- the n-type semiconductor layer and the p-type semiconductor layer are joined.
- metal aluminum is brought into contact with one surface of an aluminum oxide film, and a probe is brought into contact with the other surface of the aluminum oxide film, so that the air, gas, or vacuum is used.
- the aluminum oxide film is melted by applying a voltage that causes dielectric breakdown of the aluminum oxide film between the metal aluminum as the cathode and the probe as the anode, and the aluminum oxide film is melted during the melting.
- a molten salt electrolysis reaction is generated in the film and cooled to generate a p-type semiconductor layer on the metal aluminum side of the aluminum oxide film and an n-type semiconductor layer on the probe side of the aluminum oxide film, In addition, the n-type semiconductor layer and the p-type semiconductor layer are joined.
- the donor concentration of the n-type semiconductor or the acceptor concentration of the p-type semiconductor can be controlled by adjusting the amount of electricity that causes the molten salt electrolysis reaction. It is preferable.
- the probe When the molten salt electrolysis reaction is caused in the aluminum oxide film, the probe is preferably moved while being in contact with the aluminum oxide film.
- the applied voltage is (1) continuously changed, (2) discontinuously changed, and (3) polarity is changed in one direction or both directions. Alternatively, it is preferable to change the combination of (1) to (3).
- An oscillation element includes an n-type semiconductor in which a donor level is formed by excessively containing aluminum in an aluminum oxide film, and an acceptor level in which oxygen is excessively contained in the aluminum oxide film. It includes a pn junction joined to a p-type semiconductor in which a position is formed.
- metal aluminum is brought into contact with one surface of an aluminum oxide film and a probe is brought into contact with the other surface of the aluminum oxide film, and the atmosphere, oxygen-containing gas, or In oxygen gas, a voltage that causes dielectric breakdown of the aluminum oxide film is applied between the metal aluminum as an anode and the probe as a cathode to melt the aluminum oxide film, and during the melting,
- a molten salt electrolysis reaction in the aluminum oxide film and cooling an n-type semiconductor layer is formed on the metal aluminum side of the aluminum oxide film, and a p-type semiconductor layer is formed on the probe side of the aluminum oxide film, respectively.
- a thickness of a depletion layer formed by joining the n-type semiconductor layer and the p-type semiconductor layer It is 1nm or less.
- metal aluminum is brought into contact with one surface of an aluminum oxide film, and a probe is brought into contact with the other surface of the aluminum oxide film.
- the aluminum oxide film is melted by applying a voltage that causes dielectric breakdown of the aluminum oxide film between the metal aluminum as the cathode and the probe as the anode, and the aluminum oxide film is melted during the melting.
- a molten salt electrolysis reaction is generated in the film and cooled to generate a p-type semiconductor layer on the metal aluminum side of the aluminum oxide film and an n-type semiconductor layer on the probe side of the aluminum oxide film,
- the n-type semiconductor layer and the p-type semiconductor layer are bonded together, and the thickness of the depletion layer formed by the bonding is 1 nm. It is below.
- the probe When the molten salt electrolysis reaction is caused in the aluminum oxide film, the probe is preferably moved while being in contact with the aluminum oxide film.
- FIG. 10 is a schematic diagram showing a reaction in the semiconductor layer when the temperature of the semiconductor layer in FIG. 9 drops slightly below the melting point. It is a schematic diagram which shows the structure of a semiconductor layer when the semiconductor layer of FIG. 10 falls to room temperature. It is a schematic diagram showing a configuration of a pn junction diode formed by spark using metal aluminum as an anode and a probe as a cathode. It is a schematic diagram which shows the structure of the pn junction contained in the semiconductor layer of FIG. It is a schematic diagram showing a reaction in a semiconductor layer when metal aluminum is used as a cathode and a probe is used as an anode when it is melted at a high temperature by spark.
- FIG. 16 is a schematic diagram illustrating a configuration of a semiconductor layer when the temperature of the semiconductor layer in FIG. 15 is lowered to room temperature. It is a schematic diagram showing a configuration of a pn junction diode formed by sparking with metallic aluminum as a cathode and a probe as an anode. It is a figure explaining the method to produce
- the film for forming the semiconductor layer by the spark method can be any metal compound that has a higher ionic bond than a covalent bond and is an insulator or a substance having extremely low conductivity.
- metal oxides such as aluminum oxide and titanium oxide, metal hydroxides such as aluminum hydroxide, metal nitrides such as aluminum nitride, and the like can be used.
- a compound that contains water molecules in aluminum oxide, such as boehmite can be used.
- the metal ion species in the film for forming the semiconductor layer by the spark method may not be the same as the metal of the substrate.
- a film formed by sputtering zirconium oxide on metal aluminum or a film formed by chemical conversion treatment on the surface of metal aluminum is also possible.
- an aluminum alloy can be used as the metallic aluminum. That is, in addition to 4N or higher purity aluminum and pure aluminum (1000 series), Al—Mn alloy (3000 series), Al—Si alloy (4000 series), Al—Mg alloy (5000 series), Al— Any of a Cu—Mg alloy (2000 series), an Al—Mg—Si alloy (6000 series), and an Al—Zn—Mg alloy (7000 series) can be used.
- the material of the film is a metal oxide
- many of them are transparent oxides. Even if the semiconductor is formed by the spark method, if the band gap is large, the energy is not absorbed in the visible light region, so that it becomes a transparent oxide semiconductor.
- a substance having good conductivity such as platinum, stainless steel, copper, or carbon can be used as a probe material for bringing the above-mentioned film material into contact and causing sparking.
- a material having high heat resistance is preferred because the temperature is increased by the spark.
- Platinum is an excellent material but expensive. It is possible to use a material in which the outermost surface is platinum-plated on a Si core material.
- Embodiment 1 of the present invention will be described below with reference to FIGS.
- the same portions are denoted by the same reference numerals, and those having the same reference numerals in the drawings will not be described again as appropriate.
- the dimensions, materials, shapes, relative arrangements, processing methods, and the like of the configurations described in each embodiment are merely examples, and the technical scope of the present invention should not be construed as being limited by these descriptions.
- the drawings are schematic, and the ratio and shape of dimensions may be different from actual ones.
- the semiconductor layer according to the first embodiment will be described.
- the semiconductor layer according to Embodiment 1 is formed by a method of sparking an aluminum oxide film. An example is shown below.
- Example and equipment As shown in FIG. 1, a sample in which the surface of a metal aluminum 103 is covered with an aluminum oxide film 102 is prepared.
- the natural oxide film there are dispersed current passing points having a diameter of about 100 nm. For this reason, when a high voltage is applied to the sample in which the probe 101 is in contact with the natural oxide film, a short-circuit current flows through the current passing point, and no spark is generated even when a high applied voltage is applied. Cannot be formed. Therefore, an aluminum oxide film 102 is coated on the surface of the metal aluminum 103 in advance. Examples of the method for coating the aluminum oxide film 102 include sputtering, anodic oxidation, atmospheric heating, and boehmite treatment.
- the coated aluminum oxide film 102 contains water and may not be pure aluminum oxide.
- molecular formula of boehmite is Al 2 O 3 ⁇ H 2 O , containing one molecule of water.
- the film thickness of the aluminum oxide film 102 is 5 to 100 nm.
- a manual prober 20 was prepared, and a probe 101 having the configuration shown in FIG. 1 was attached.
- the probe tip 101a of a platinum wire (H material) having a diameter of 0.2 mm was shaved so that the diameter of the contact surface of the probe tip 101a with the sample was 0.02 mm.
- the base of the probe is wound in a coil shape, so that the tip of the probe can be brought into contact with the sample at a low pressure by a spring action.
- the metal aluminum side is made to be the anode (plus).
- the metal aluminum side is the cathode (minus)
- the polarity of the DC stabilized power source may be reversed.
- the positive side of the switch box 13 (with built-in reed relay) is connected to the metal aluminum side attached to the manual prober 20 via the current limiting resistor 15, and the negative side of the switch box 13 is connected to the current side.
- the probe was connected to the probe through a measurement shunt resistor 14. By switching the switch 22, the metal aluminum side and the probe are connected to the IV measuring instrument 23, and the IV characteristics of the sample are measured. The voltage applied to the sample and the current flowing through the sample were measured using a high voltage / floating input oscilloscope 12.
- 1085 materials (12 mm ⁇ 30 mm, thickness 20 ⁇ m) were used as metallic aluminum, and the sample surface was subjected to boehmite treatment.
- the boehmite treatment was performed by immersing the sample in 95 ° C. pure water for 30 s, washing with water, and drying. Since the boehmite film is insulative, a part of the sample is rubbed to remove the boehmite film so that it can be energized.
- a sample was set on the manual prober 20 shown in FIG. 2 using the probe shown in FIG. 1, and the prober tip position was adjusted by the manipulator 21 so that the tip contacted the boehmite-treated film. A force of about 0.01 N was applied to the boehmite film at the probe tip.
- the current limiting resistor 15 in FIG. 2 was set to 100 ⁇
- the shunt resistor 14 was set to 100 ⁇
- the output of the current direct current stabilized power supply was set to 36V
- the switch of the switch box 13 was closed. Sparking occurred between the probe tip and metal aluminum.
- a semiconductor layer having a thickness of about 30 nm was formed at the portion where the probe tip was in contact. The voltage applied to the sample at this time and the current flowing through the sample were measured with an oscilloscope 12. The result is shown in FIG.
- the timing when the switch was closed was -0.28 ⁇ s, but since the trigger was actually applied when the current value exceeded 0.05 A, the time when the trigger was applied was set to 0 ⁇ s. Since a general-purpose DC power supply was used, the power supply was in a standby state between ⁇ 0.28 and 0 ⁇ s. Therefore, during this period, the output did not reach 36 V set as an output, and a voltage of about 20 V was output. At a voltage of 20V, there was no spark and almost no current flowed.
- IV measurement result The IV measurement of the semiconductor layer thus obtained was performed. The result is shown in FIG. A voltage range of ⁇ 0.6 to 1.0 V was scanned at a speed of 0.1 V / s. A nearly linear relationship was obtained between ⁇ 0.55 and + 0.2V. In this voltage range, it is considered that a tunnel current or a current due to metallization of the electronic state of the film flowed between the metal aluminum and the probe. A large current flowed out of the linear relationship at ⁇ 0.55 V or less and +0.2 V to +0.55 V. Also, almost no current flowed between +0.55 and + 1.0V.
- the current limiting resistor 15 shown in FIG. 2 is changed from 100 ⁇ to 1 k ⁇ , the probe tip is moved, the probe tip is brought into contact with the boehmite treatment film at another location of the same sample, and the semiconductor layer is sparked by the same method as described above. Formed.
- the voltage and current of the sample at this time behaved almost the same as in FIG. 3, but the electrolysis time of the molten salt decreased to about 30 ns and the current decreased to about 0.03 A.
- the amount of electricity required for the reaction of molten salt electrolysis was about 1/100 of the above example.
- FIG. 5 shows the IV characteristics of the semiconductor layer generated here.
- molten salt is used in the following broad sense. That is, the movement (electrophoresis) of aluminum ions and oxygen ions by electrolysis is possible even in a solid-liquid mixture in which the solid state is mixed even if it is not a complete molten salt. Therefore, even in a solid-liquid mixed state, the expression is “molten salt”.
- the carrier concentration of the semiconductor layer increases, and the thickness of the depletion layer formed at the pn junction decreases, and the tunnel current or the electron of the film Current flows due to the metallization of the state, the carrier concentration of the semiconductor layer decreases when the amount of electricity applied to the molten salt electrolysis by spark is small, and the thickness of the depletion layer formed at the pn junction increases. is there.
- the above description shows a method of forming a semiconductor layer by sparking an aluminum oxide film with the metal aluminum side as the anode (plus side) and the probe side as the cathode (minus side), and as a result, the metal of the aluminum oxide film It was shown that the aluminum side becomes an n-type semiconductor and the probe side of the aluminum oxide film becomes a p-type semiconductor layer.
- the semiconductor layer can also be formed by sparking the aluminum oxide film with the metal aluminum side as the cathode (minus side) and the probe side as the anode (plus side).
- the metal aluminum side of the aluminum oxide film is a p-type semiconductor
- the probe side of the aluminum oxide film is an n-type semiconductor layer.
- the semiconductor layer according to an embodiment of the present invention is formed by dielectric breakdown of the aluminum oxide film by spark.
- cross-sectional TEM (transmission electron microscope) imaging and EDS analysis (elemental analysis) of the semiconductor layer were performed.
- TEM imaging a cross section of the semiconductor layer was thinned with FIB (focused ion beam) to prepare a sample.
- AA is a line of natural oxide film of metal aluminum
- the lower side of the line is the structure of metal aluminum
- the upper side of the line is the filler used for preparing the TEM observation sample.
- the central portion of the portion surrounded by the dotted line is a semiconductor layer.
- Metal aluminum has a large depression, and the upper layer along the depression is a semiconductor layer.
- the thickness is 5 to 100 nm, and the upper side of the maximum thickness portion is the position of the probe tip.
- FIG. 7 shows the result of EDS analysis of the thickest part of the semiconductor layer surrounded by the dotted line in FIG. 6 and its periphery.
- the semiconductor layer At the center of the BF (bright field image) is the semiconductor layer, and the top of the Mt. Fuji type portion is the probe tip position.
- the lower side of the semiconductor layer is metallic aluminum, and the upper side is a filler used when preparing the sample.
- the other 5 sheets are EDS analysis results of Al, O, Pt, C, and Ga.
- the rightmost diagram is a schematic diagram showing the material structure of the semiconductor layer by enlarging the square frame portion of each EDS analysis result.
- Al has a high concentration on the lower metal aluminum side
- O has a high concentration on the upper filler side (atmosphere side before making the analysis sample).
- the thick Al portion is approximately 15 nm thick and exists in a strip shape along the Al metal surface. This portion is considered to be a portion where Al 3+ is excessively present due to a deficiency of O 2 ⁇ of Al 2 O 3 non-stoichiometrically (N ++ ).
- N ++ deficiency of O 2 ⁇ of Al 2 O 3 non-stoichiometrically
- the portion where the O is deep has a thickness of about 15 nm and exists in a band shape along the surface of the semiconductor layer film. This part is considered to be a part (P ++ ) in which Al 3+ of Al 2 O 3 is deficient in a non-stoichiometric manner and O 2 ⁇ is excessively present.
- the left side of FIG. 8 is obtained by adding a probe tip (Pt) to the schematic configuration diagram of the semiconductor layer of FIG. FIG. 8 on the right side of FIG. 8 shows that there are three types of semiconductor layer configurations due to differences in current paths passing through the semiconductor layers.
- the cross section of aa including the thick part (about 70 nm) of the semiconductor layer has a stoichiometric or near stoichiometric composition between N ++ (Al3 + excess part) and P ++ (O2 - excess part). Of Al 2 O 3 is considered to exist. Since Al 2 O 3 has no new energy level, it is not conductive.
- the cross section of ab is a portion where N ++ and P ++ are close to each other, and it is considered that a depletion layer is formed in this portion and a pn junction is formed as described later. Furthermore, in the cross section of ac, it is considered that N ++ and P ++ cross and are mixed. The Al 3+ excess portion and the O 2 ⁇ excess portion cause ionic bonds, and it is considered that Al 2 O 3 having a stoichiometric or near stoichiometric composition exists. Therefore, this cross section is not conductive like the aa cross section.
- the semiconductor layer formed by the spark method is an aluminum oxide film, but Al is segregated on the metal aluminum side, O is segregated on the semiconductor layer surface side, and the pn junction is It was found to exist in a very limited part.
- the cross-sectional area of the ab is small, but it always exists in a crossed portion of P ++ and N ++ , so that it is a highly reproducible manufacturing method.
- the structure of the semiconductor layer obtained by one embodiment of the present invention is considered to be amorphous. This is because it is considered that the aluminum oxide layer is difficult to crystallize because the time from the melting of the aluminum oxide layer by the spark to the solidification by cooling is extremely short.
- Covalent semiconductors such as Si semiconductors have directionality in the size of the bonds, so a carrier conduction path cannot be obtained without crystallinity.
- semiconductors with strong ion bonding properties such as aluminum oxide are not bonded. Since there is no directionality in the size, a carrier conduction path can be secured even with amorphous. As a result, semiconductor characteristics can be obtained even in an amorphous state.
- FIG. 9 shows a state in which the aluminum oxide film that is an insulator is melted by the spark.
- a spark current flows for a short time.
- the aluminum oxide film becomes a molten salt at a temperature higher than the melting point and becomes a molten salt.
- the melting point of the aluminum oxide film is 2072 ° C.
- Molten salt electrolysis occurs according to the formulas (1) and (2) at a temperature equal to or higher than the melting point, and Al 3+ and O 2 ⁇ accumulate as shown in the formula (3), resulting in a thick semiconductor layer.
- Al 3+ ions are excessive on the metal aluminum side of the semiconductor layer, but a new donor level is formed in the semiconductor layer to maintain electrical neutrality.
- O 2 ⁇ ions are excessive on the probe side, a new acceptor level is formed in the semiconductor layer in order to maintain electrical neutrality.
- a semiconductor layer having the structure shown in FIG. 11 is formed.
- the metal aluminum side of the semiconductor layer becomes an n-type semiconductor with excessive Al concentration
- the probe side (Pt side) becomes a p-type semiconductor with excessive O concentration. Both become pn junctions, and a depletion layer is formed at the joint.
- the thickness of the depletion layer is determined by the newly generated carrier concentration of the donor level and the carrier concentration of the acceptor level, but the maximum value of the carrier concentration is extremely high (10 27 based on the cross-sectional EDS analysis result of the semiconductor layer). ⁇ 10 29 / m 3 ), the thickness of the depletion layer is 1 nm or less, and a tunnel current easily flows.
- the carrier concentration is extremely high, the electronic state of the film is metalized and high conductivity is obtained.
- FIG. 12 shows an example of a pn junction diode formed by spark using metal aluminum as an anode and a probe as a cathode.
- FIG. Aa has an Al 2 O 3 layer in the middle, but the carrier concentration in this part is low, and it is considered that there is little or no conductivity.
- ab is a state in which the above-described pn junction is formed.
- ac it is considered that Al 3+ excess part and O 2 ⁇ excess part are mixed, and these are reacted to form ionic bonds to produce Al 2 O 3 .
- the carrier concentration in this part is low, and it is considered that there is little or no conductivity. It is assumed that the actual semiconductor element operates in the part ab.
- FIG. 14 shows a state in which the aluminum oxide film is thick on the metal aluminum as far as it can be sparked, and the aluminum oxide film as an insulator is melted by the spark.
- a spark current flows for a short time.
- the aluminum oxide film becomes a molten salt at a temperature higher than the melting point.
- Molten salt electrolysis occurs according to the equations (4) and (5) at a temperature equal to or higher than the melting point, and Al 3+ and O 2 ⁇ are consumed as shown in the equation (6) to become metallic aluminum and oxygen, and the semiconductor layer becomes thin.
- a semiconductor layer having the structure shown in FIG. 16 is formed.
- the metal aluminum side of the semiconductor layer becomes a p-type semiconductor with an excessive O (oxygen) concentration
- the probe side (Pt side) becomes an n-type semiconductor with an excessive Al concentration. Both become pn junctions, and a depletion layer is formed at the joint.
- the thickness of the depletion layer is determined by the newly generated carrier concentration of the donor level and the carrier concentration of the acceptor level, the maximum value of the carrier concentration of the semiconductor layer is extremely high (10 27 to 10 29 / m 2. 3 ), the thickness of the depletion layer is 1 nm or less, and a tunnel current easily flows.
- the carrier concentration is extremely high, the electronic state of the film is metalized and high conductivity is obtained.
- FIG. 17 shows an example of a pn junction diode formed in a spark using metallic aluminum as a cathode and a probe as an anode. The description of the semiconductor layer cross section is omitted.
- Embodiment 2 Scaling-up of semiconductor layers by scanning method
- N AO ⁇ A v / M (10), where Avogadro's number is A v (6.022 ⁇ 10 23 pieces / mol). It is.
- n it / (6F c ) (13)
- ⁇ N AO A v n / V m (14) It is.
- FIG. 18 shows a semiconductor layer (semiconductor layer 202 being produced and produced) formed by scanning a probe with the aluminum oxide film surface 201 on the metal aluminum and using the metal aluminum side as an anode according to the reaction formula (7).
- the conceptual diagram of the semiconductor layer 203) is shown. Although the actual contact area with the aluminum oxide film surface 201 is smaller than the area of the probe tip, it is assumed here that the contact is made in a rectangular region of u 1 (m) ⁇ u 2 (m).
- a spark current i (A) between the metal aluminum and the probe While flowing a spark current i (A) between the metal aluminum and the probe, the probe is scanned in the direction of the arrow at a constant speed v (m / s) to generate a semiconductor layer.
- the thickness of the semiconductor layer increases due to the spark and grows to h (m).
- a rectangular semiconductor layer having a width u 2 (m) and a thickness h (m) is formed.
- the spark current i is considered to be stabilized at a substantially constant value while fluctuating because the scanning is performed with the applied voltage being constant in actual operation.
- the width of the probe tip contact surface perpendicular to the scanning direction (u 2 ) is 10 ⁇ m
- the scanning speed (v) is 14.6 ⁇ m
- the thickness (h) of the generated semiconductor layer is 50 nm.
- u 2 1.0 ⁇ 10 -5 ( m)
- v 14.6 ⁇ 10 -6 (m / s)
- h 5.0 ⁇ 10 -8 to (m) (21) formula, i ⁇ 1.7 ⁇ 10 - 7 (A), and a spark current value of 0.17 ⁇ A is required.
- FIG. 19 shows a semiconductor layer (the semiconductor layer 302 being decomposed and decomposed) formed by contacting the probe with the surface 301 of the aluminum oxide film on the metal aluminum and scanning the metal aluminum side as a cathode according to the reaction formula (8).
- the conceptual diagram of the semiconductor layer 303) is shown. In this case, since the semiconductor layer is formed by the decomposition of the aluminum oxide film, the aluminum oxide film is depressed by scanning. The semiconductor layer is formed from the remaining aluminum oxide film. Therefore, it is necessary to select the production conditions so that the thickness of the aluminum oxide film on the metal aluminum is thicker than the height of the recess formed by scanning.
- the detailed description of FIG. 19 is the same if all the “generation” in the detailed description of FIG.
- FIG. 1 An example of the scanning pattern is shown in FIG.
- the semiconductor layer is formed in an area of a (m) ⁇ b (m) by scanning with the probe in contact with the aluminum oxide film.
- the region U formed by the semiconductor layer at a certain moment is the above-described u 1 ⁇ u 2 where the tip of the probe contacts the aluminum oxide film and the spark current flows through the entire portion.
- the probe tip is scanned in the a ⁇ b region while flowing a spark current i as indicated by an arrow at a speed v.
- the probe In the first scanning, the probe is moved to the start point in FIG. 20, the tip of the probe is brought into contact with the sample, a spark voltage V is applied, i is turned on, and immediately moved from left to right along the uppermost line m 1 . Is the action. V is set to zero at the right end, the probe is once separated from the sample, i is turned OFF, the line is moved to the lower line at line feed r 1 and moved to the left end of the second line.
- Second scanning is an operation of moving from left to right immediately along the second line m 2 ON the i by applying a voltage V to the probe tip is again brought into contact with the sample at the left end of the second line.
- V is set to zero, the probe is moved away from the sample, i is turned off, and the line moves to the lower line at line feed r 2 to the left end of the third line.
- the carrier concentration has a donor density that increases with the concentration of the excessively injected Al 3+ ions and an acceptor density that increases with the concentration of the excessively injected O 2 ⁇ ions.
- the calculation is made by taking the donor density, that is, the Al 3+ ion concentration as an example.
- the Al 3+ ion concentration C Al (pieces / m 3 ) is a value obtained by multiplying the generation or decomposition concentration ⁇ N Al (pieces / m 3 ) by the efficiency ⁇ Al by the spark of the aluminum oxide film, the equation (16) Is as follows.
- This device can scan with a constant voltage applied between the probe and the sample, but cannot scan with constant current.
- the maximum current can be set by setting the measurement current range.
- Probe tip width U 2 is 1 ⁇ 10 6 m (1.0 ⁇ m), semiconductor layer formation height h is 2 ⁇ 10 ⁇ 8 m (20 nm), scan speed v is 1.46 ⁇ 10 ⁇ 5 m / s (14.6 ⁇ m / s) )
- ⁇ Al the carrier generation efficiency
- i 1.4 ⁇ 10 ⁇ 8 (A)
- a semiconductor layer can be formed although C Al is slightly lowered.
- the semiconductor layer shown so far has produced a p-type semiconductor and an n-type semiconductor simultaneously by a spark method, and formed a pn junction. In actuality, these methods are not necessarily suitable for mass production in order to make semiconductor elements such as diodes and transistors and oscillation elements.
- this semiconductor element it is desirable to be able to form a thin film by sputtering, which is an existing semiconductor manufacturing technique, and to form an element structure by photolithography and etching. Therefore, a method for obtaining the semiconductor layer of the present invention by sputtering will be described.
- the outermost surface of the semiconductor layer formed by the spark method is a p-type semiconductor layer in which O 2 ⁇ is excessive.
- a part other than this part (surface) is masked and a target material is sputtered to form a semiconductor element.
- the sputtering is performed until the n-type semiconductor layer is extracted, or the anode and cathode are switched by reversing the current direction during sparking.
- the surface can be an n-type semiconductor layer, which can be used as a target material.
- the above-described scanning can be performed to increase the area of the target material.
- an n-type semiconductor thin film is formed on an aluminum substrate (also serving as a cathode current collector) by sputtering using an Al 3 + -excess semiconductor layer as a target material, Next, it is possible to form a p-type semiconductor thin film by sputtering on the n-type semiconductor thin film using a semiconductor layer containing excess O 2 ⁇ as a target material, and attach a cathode terminal to the p-type semiconductor layer.
- semiconductor elements such as MOS-FETs can be manufactured by repeating sputtering, photolithography and etching using other materials.
- FIG. 21 is a cross-sectional view schematically showing a main part structure of the oscillation element of the present embodiment.
- the structure of the oscillation element is the same as that of the pn junction diode formed by the spark method shown in FIG.
- the structure of the depletion layer is shown in FIG.
- O is deficient from Al 2 O 3 and Al is excessive, and Al 3+ cations and electrons for maintaining electrical neutrality are present.
- Al is depleted from Al 2 O 3 , O becomes excessive, and there are holes for maintaining electrical neutrality with an anion of O 2 ⁇ .
- the electrons of the n-type semiconductor and the holes of the p-type semiconductor are bonded and disappeared, and a portion where neither electrons nor holes are present is generated, but this portion is a depletion layer.
- An oscillation element can be formed by making the depletion layer extremely thin.
- the thickness needs to be 1 nm or less.
- the size of one molecule of Al 2 O 3 is calculated.
- the Avogadro number is 6.022 ⁇ 10 23 (1 / mol)
- the molecular weight of Al 2 O 3 is 101.96 (g / mol)
- the density is 4.0 ⁇ 10 3 (kg / m 3 ). 4.23 ⁇ 10 ⁇ 29 (m 3 ).
- this provisionally calculates the length d m of approximately one side as a cube 3.5 ⁇ 10 -10 (m) i.e. d m becomes 0.35 nm.
- Conditions for oscillation are considered to depletion of thickness x dep is not more than 3 times the d m. Therefore, the thickness x dep of the depletion layer is desirably 1 nm or less.
- Thinning the empty layer can be realized by increasing the carrier concentration of p-type semiconductor carriers (holes), n-type semiconductor carriers, or both p-type and n-type semiconductors.
- the acceptor concentration N A, the donor concentration N D, the depletion layer thickness X n of the p-side of the depletion layer thickness X p and n-side can be calculated by the following equation.
- the carrier concentration is high, the error is considered to be large, and is only a guide and is not a strict calculation formula.
- V bi built-in potential (V)
- n i intrinsic semiconductor carrier concentration (m ⁇ 3 )
- X p depletion layer width (m) in p-type region
- X n depletion layer width in n-type region (m)
- k B Boltzmann constant (1.38 ⁇ 10 -23 (J / K))
- T temperature (K)
- q elementary charge 1.602 ⁇ 10 -19 (C)
- ⁇ r relative permittivity
- ⁇ 0 Vacuum dielectric constant (8.854 ⁇ 10 ⁇ 12 (F / m)).
- the thickness x dep of the entire depletion layer is expressed as follows.
- N 2.6 ⁇ 10 27 (m ⁇ 3 ).
- the carrier concentration is increased by an ion implantation method, the high carrier concentration often indicates 1 ⁇ 10 27 (m ⁇ 3 ) or more. Therefore, it can be said that the value of N is also a high carrier concentration.
- FIGS. 24 (a) and 24 (b) Examples of IV characteristics during oscillation are shown in FIGS. 24 (a) and 24 (b).
- the negative region of the bias voltage indicates the forward bias side
- the positive region indicates the reverse bias side.
- Current oscillation occurred when the bias voltage was between -0.1V and + 0.1V in (a) and between -0.2 and + 0.17V in (b).
- the oscillation waveform in (a) is shown in FIG.
- the oscillation current was generated with an amplitude of about -0.4 to +0.4 ⁇ A. Even at a bias voltage of 0 V, an oscillation current was generated with a similar amplitude.
- the frequency was about 3.4kHz.
- FIG. 26 shows the electric field in the depletion layer when the carrier concentration is relatively low.
- the first E a is attracted by the force of f a due to the electric field applied to the portion of the a depletion layer in which the cation and anion surfaces face each other.
- the distance between the cation and the anion is considered to be about 0.35 nm between the central portions.
- the second E b is attracted by the force of f b due to the electric field applied to the electron surface (n-type semiconductor side) and the hole surface (p-type semiconductor side) existing outside the depletion layer.
- the distance is about several nm. Since E a and E b have opposite directions and E a >> E b , f a >> f b , which is substantially the electrostatic force between the innermost cations.
- FIG. 27 shows the case where the carrier concentration is very high and the distance between the depletion layers is about 0.35 nm.
- E a E b
- the force acting between the innermost ions (cation and anion) is equal to the force acting between the innermost electrons and holes. That is, the innermost cation and anion are released from the electrostatic force, and only attractive force acts between them.
- This attractive force is considered to be a force such as van der Waals force.
- the innermost cation and anion oscillate according to Newton's equation of motion. This oscillation occurs even when the bias voltage is 0V.
- Example 1 First, an aluminum plate (1085 material, 12 ⁇ 30 mm, thickness 0.2 mm) was prepared. This sample was immersed in pure water heated to 95 ° C. to form a boehmite film having a thickness of about 20 nm on the aluminum plate. Next, a manual prober 20 was prepared. The probe used was a platinum wire (H material) with a diameter of 0.2 mm, with a tip of 0.02 mm in diameter, and the base of the platinum wire wound in a coil shape. An apparatus 10 shown in FIG. 2 was prepared and connected.
- H material platinum wire
- An apparatus 10 shown in FIG. 2 was prepared and connected.
- the current limiting resistor 15 was 100 ⁇
- the shunt resistor 14 was 100 ⁇
- the output of the current direct current stabilized power supply was 36V
- the switch of the switch box 13 was closed after contacting the tip of the probe in the air with the sample. Sparking occurred between the probe tip and the aluminum plate.
- a semiconductor layer having a thickness of about 30 nm was formed at the portion where the probe tip was in contact.
- the voltage applied to the sample at this time and the change in current flowing through the sample were measured with an oscilloscope 12.
- 36 V was applied to the sample for a moment, but the voltage immediately dropped to about 10 V.
- the current was about 0.1A. This state continued for 0.3 ⁇ s.
- Example 2 Similar to Example 1, an aluminum plate (1085 material, 20 ⁇ 60 mm, thickness 0.18 mm) was prepared. This sample was immersed in pure water heated to 95 ° C. to form a boehmite film having a thickness of about 20 nm on the aluminum plate.
- the current limiting resistor 15 shown in FIG. 2 was changed from 100 ⁇ to 1 k ⁇ , the probe tip was brought into contact with the boehmite-treated film, and a semiconductor layer was formed by sparking in the same manner as described above.
- the voltage and current of the sample at this time behaved in the same manner as in FIG. 3, but the electrolysis time of the molten salt decreased to about 30 ns and the current decreased to about 0.03 A.
- the quantity of electricity related to the reaction of molten salt electrolysis was about 1/100 of the above example.
- FIG. 5 shows the IV characteristics of the semiconductor layer generated here.
- a negative current flowed at a voltage of -0.6V or less, but no current flowed at a voltage higher than that, indicating rectification characteristics.
- a pn junction was formed in the semiconductor layer, and a pn junction diode in which the aluminum plate side of the semiconductor layer was an n-type semiconductor and the probe side was a p-type semiconductor was formed.
- Example 3 First, the following samples were prepared. Sputtering is performed for about 40 minutes under the conditions of Ar + O 2 gas, total pressure 0.4Pa, using 5N aluminum Al ( ⁇ 76 ⁇ 6mm thickness) made by Furuuchi Chemical as a substrate with an aluminum plate (24 ⁇ 24mm, thickness 0.1mm) An aluminum oxide film of about 30 nm was applied on the aluminum plate. As a sputtering apparatus, SPC-350 type made by Nidec Anelva was used.
- a semiconductor layer was formed by a scanning method using JSPM-5200.
- a probe having a tip width of 1.0 ⁇ m was prepared, and scanning was performed at a scanning speed of 14.6 ⁇ m / s and a current value of 10 nA to form an element having a size of 25 ⁇ 25 ⁇ m.
- the IV probe was measured by bringing the same probe as in Example 1 into contact with the surface of the element fabricated by sputtering. A linear relationship was obtained between -0.5 and + 0.5V. In this voltage range, it is considered that a tunnel current or a current due to metallization of the electronic state of the film flowed between the aluminum plate and the probe. Almost no current flowed below -0.5V and above + 0.5V. Although there was no rectifying action, IV characteristics specific to this semiconductor device were obtained when the carrier concentration was high.
- Example 4 The following samples were prepared. Sputtering is performed for about 40 minutes under the conditions of Ar + O 2 gas, total pressure 0.4Pa, using 5N aluminum Al ( ⁇ 76 ⁇ 6mm thickness) made by Furuuchi Chemical as a substrate with an aluminum plate (24 ⁇ 24mm, thickness 0.1mm) An aluminum oxide thin film of about 30 nm was applied on the aluminum plate. As a sputtering apparatus, SPC-350 type made by Nidec Anelva was used.
- the current limiting resistor 15 was 100 ⁇
- the shunt resistor 14 was 100 ⁇
- the output of the current direct current stabilized power supply was 60V
- the switch of the switch box 13 was closed after contacting the tip of the probe in the air with the sample. Sparking occurred between the probe tip and the aluminum plate.
- a semiconductor layer having a thickness of about 50 nm was formed at the portion where the probe tip was in contact.
- the voltage applied to the sample at this time and the change in current flowing through the sample were measured with an oscilloscope 12. Immediately after energization, 60 V was applied to the sample for a moment, but the voltage immediately dropped to about 10 V. The current was about 0.2A. This state continued for 0.3 ⁇ s.
- FIG. 1 An example of IV characteristics is shown in FIG.
- the negative region of the bias voltage indicates the forward bias side
- the positive region indicates the reverse bias side.
- Current oscillation occurred when the bias voltage was between -0.1V and + 0.1V.
- the oscillation waveform is shown in FIG.
- the oscillation current was generated with an amplitude of about -0.4 to +0.4 ⁇ A. Even at a bias voltage of 0 V, an oscillation current was generated with a similar amplitude.
- the frequency was about 3.4kHz.
- the carrier concentration of each of the p-type semiconductor layer and the n-type semiconductor layer is adjusted to ⁇ 10 27 / m 3 and a pn junction is formed, a diode is obtained.
- a semiconductor element such as a transistor or a thyristor can be obtained by three-dimensionally combining a p-type semiconductor layer and an n-type semiconductor layer with appropriately adjusted carrier concentrations and an insulator such as stoichiometric aluminum oxide. These can be expected to be used as wide band gap power semiconductors. Furthermore, if the characteristics of the transparent oxide semiconductor are combined, it can be applied to a constituent material of a solar cell and a display panel.
- each carrier concentration of the p-type semiconductor layer and the n-type semiconductor layer is set to 10 28 to 10 29 / m 3 and a pn junction is formed, it can be used for an inverter that becomes an oscillation element and converts a direct current into an alternating current.
- Combining the characteristics of a transparent oxide semiconductor and a light emitting diode in the ultraviolet region with this oscillating element makes it a frequency variable type inverter and can expand the applicable range of the inverter.
- the electronic state of the semiconductor layer is metallized at such a high carrier concentration, extremely good conductivity can be obtained and it can be expected as a high performance conductor.
- it combines the characteristics of a transparent oxide semiconductor, it can be used as a transparent conductor.
- the p-type semiconductor layer and the n-type semiconductor layer according to the present invention are extremely high oxidizing agents or reducing agents, they are promising as new chemical substances. If the characteristics of transparent oxide semiconductors are combined, there is a possibility that they can be used for fuel cell electrode materials and photosynthetic electrodes. Furthermore, it can be expected to be used as a semiconductor target material.
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Abstract
Description
半導体層をスパーク法により形成するための膜は、共有結合性よりもイオン結合性の大きい金属化合物であり、かつ絶縁体又は導電性の極めて小さい物質であればいずれも用いることができる。例えば、酸化アルミニウムや酸化チタン等の金属酸化物、水酸化アルミニウム等の金属水酸化物、窒化アルミニウム等の金属窒化物等が使用可能である。更に、ベーマイトのように酸化アルミニウムに水分子を含むような化合物を使用することができる。
本発明の実施形態1について、図1~図17に基づいて説明すれば、以下の通りである。なお、各実施形態では、同一の部分には同一の符号を付し、図面で同一の符号を付したものは、再度の説明を適宜省略する。また、各実施形態に記載されている構成の寸法、材質、形状、相対配置、加工法等はあくまで一例に過ぎず、これらの記載によって本発明の技術的範囲が限定解釈されるべきではない。更に、図面は模式的なものであり、寸法の比率、形状は実際のものとは異なる場合もある。
本実施形態1に係る半導体層について説明する。本実施形態1に係る半導体層は、酸化アルミニウム膜をスパークさせる方法で形成する。その一例を次に示す。
図1に示すように、金属アルミニウム103の表面に酸化アルミニウム膜102を被覆した試料を用意する。自然酸化皮膜には直径100nm程度の電流通過点が分散して存在する。このため、自然酸化皮膜にプローブ101を接触させた試料に高電圧を印加すると、電流通過点を通して短絡電流が流れてしまい、高印加電圧をかけてもスパークが生じず、その結果、半導体層を形成することができない。そこで、金属アルミニウム103の表面に予め酸化アルミニウム膜102を被覆しておく。酸化アルミニウム膜102の被覆方法は、スパッタ法、アノード酸化法、大気加熱法、ベーマイト処理法などがある。なお、被覆した酸化アルミニウム膜102中は、水を含み、純粋な酸化アルミニウムではない場合もある。例えば、ベーマイトの分子式は、Al2O3・H2Oであり、水1分子を含む。酸化アルミニウム膜102の膜厚は、5~100nmとする。
図2の電流制限抵抗15を100Ω、シャント抵抗14を100Ω、電流直流安定化電源の出力を36Vとし、スイッチボックス13のスイッチを閉じた。プローブ先端と金属アルミニウム間でスパークした。プローブ先端を接触させた部分には厚さ30nm程度の半導体層が形成された。この時の試料に印加される電圧と試料に流れる電流をオシロスコープ12で測定した。その結果を図3に示す。
このようにして得られた半導体層のI-V測定を行った。その結果を図4に示す。-0.6~1.0Vの電圧範囲を0.1V/sの速度で走査した。-0.55~+0.2Vの間でほぼ直線関係が得られた。この電圧範囲においては金属アルミニウムとプローブとの間にトンネル電流又は皮膜の電子状態が金属化したことによる電流が流れたと考えられる。-0.55V以下と+0.2V~+0.55Vで直線関係から外れて大電流が流れた。また、+0.55~+1.0Vの間、電流はほとんど流れなかった。このように広い電圧範囲で直線関係から外れる理由は現時点では分からないが、半導体層のキャリア挙動が不安定な状況にあると考えられる。なお、仮に金属アルミニウムとプローブとが短絡していたとするならばI-V特性の全電圧領域で直線関係になるはずである。全電圧領域で直線関係にならない以上、-0.55V~+0.2Vにおける直線関係は形成した半導体層固有の特性であるといえる。
(半導体層の構成元素)
上述のように、本発明の一実施形態に係る半導体層は、スパークにより酸化アルミニウム膜を絶縁破壊させたことにより形成する。スパークにより形成した半導体層の構成を確認するため、半導体層の断面TEM(透過型電子顕微鏡)撮影とEDS分析(元素分析)を行った。TEM撮影では半導体層の断面をFIB(集束イオンビーム)で薄膜にして試料とした。
(金属アルミニウムをプラス、プローブをマイナスにしてスパークした場合)
このような極めて特殊な元素構成を有する半導体層になる理由について推定した。
アノード反応(Al金属側) Al→Al3++3e (1)
カソード反応(プローブ側) O2+4e→2O2- (2)
全反応 4Al+3O2→4Al3++6O2- (3)
スパークが終了すると温度が下がるが、温度下降速度は半導体層内で完全に均一ではなく、固液が混在して部分的に凝固してAl2O3を形成する。溶融状態が残っている部分はまだイオン化していることにより、又は固体のAl2O3は室温では絶縁体であるが、融点近くの高温では電子伝導性が有ることにより、更にはこれらの両方の理由により、図10に示すように上記アノード反応、カソード反応が引き続き起きる。半導体層の金属アルミニウム側にはAl3+イオンが過剰になるが、電気的中性を保つため半導体層内に新たなドナー準位ができる。一方、プローブ側にO2-イオンが過剰になるが、電気的中性を保つため半導体層内に新たなアクセプタ準位ができる。
次に、スパーク時、金属アルミニウムをマイナス、プローブをプラスにした場合について示す。
カソード反応(金属アルミニウム側) Al3++3e→Al (4)
アノード反応(プローブ側) 2O2-→O2+4e (5)
全反応 4Al3++6O2-→4Al+3O2 (6)
スパークが終了すると温度が下がるが、温度下降速度は半導体層内で完全に均一ではなく、固液が混在して部分的に凝固してAl2O3を形成する。溶融状態が残っている部分はまだイオン化していることにより、又は固体のAl2O3は室温では絶縁体であるが、融点近くの高温では電子伝導性が有ることにより、更にはこれらの両方の理由により、図15に示すように上記カソード反応、アノード反応が引き続き起きる。半導体層の金属アルミニウム側にはO2-イオンが過剰になるが、電気的中性を保つため半導体層内に新たなアクセプタ電位ができる。一方、プローブ側にAl3+イオンが過剰になるが、電気的中性を保つため半導体層内に新たなドナー準位ができる。
(スキャニング法による半導体層のスケールアップ化)
本発明の実施形態2について、図18~図20に基づいて説明すれば、以下の通りである。
金属アルミニウム上に生成した酸化アルミニウム膜にプローブを接触させる際、プローブの位置を固定して金属アルミニウムとプローブとの間にスパーク電流を流すと、生成する半導体層又は分解する酸化アルミニウム膜は大きくても直径1~2μm程度でこれを大きくすることは難しい。半導体層のスケールアップ化を実現するために、スパーク電流を流しながら酸化アルミニウム膜に接触させたプローブを移動させることにより半導体層の面積や体積を更に大きくすることができると考えた。本方法を「スキャニング法」と称することにし、生成する半導体層又は分解する酸化アルミニウム膜とスパーク電気量との関係とその具体的方法について示す。
(スパーク時における電気化学反応)
金属アルミニウム上に生成した酸化アルミニウム膜にプローブを接触させて、金属アルミニウムとプローブ間にスパーク電流を流した場合、1μs以下の極短時間、酸化アルミニウム膜は溶融し、金属アルミニウム側をアノード(プラス)にした場合は(7)式の溶融塩反応が、金属アルミニウム側をカソード(マイナス)にした場合は(8)式の溶融塩反応がそれぞれ生じると考えられる。
金属アルミニウム側をアノードにした場合 4Al+3O2→4Al3++6O2- (7)
金属アルミニウム側をカソードにした場合 4Al3++6O2-→4Al+3O2 (8)
この反応は1molのAl2O3(実際は2Al3++3O2-)生成((7)式)又は分解((8)式)に対し6F(ファラデー)の電気量が消費される。Al2O3の分子量をM(101.96g/mol)、密度ρ(4.0×106g/m3)とすると、1mol当たりのAl2O3の体積は、M/ρ(m3/mol)である。1Fで生成又は分解するAl2O3の体積は、M/6ρ(m3/F)である。M、ρを代入すると、1Cで生成又は分解するAl2O3の体積は、
M/(6ρFc)/=4.4×10-11(m3/C)
となる。ただしFcはファラデー定数(1F=96500C)である。
Vm=MQ/(6ρFc)=4.4×10-11・Q (9)
となる。
である。半導体形成に関る電気量Q(C)は、電流をi(A)、通電時間をt(s)とすると、Q=it (11)
であり、n molのAl2O3を生成する電気量をQn(C)、ファラデー定数をFc(c/mol)とすると、Qn=6Fcnより、
n=Qn/(6Fc) (12)
となる。(11)式より、
n=it/(6Fc) (13)
であり、また、n molのAl2O3の生成数、分解数をΔNAOとすると、
ΔNAO=Avn/Vm (14)
である。(13)式より、
ΔNAO=Avit/(6FcVm) (15)
となる。Al2O3 1mol に対してAl3+は2mol発生するので、n molのAlの生成数、分解数をΔNAlとすると、
ΔNAl=Avit/(3FcVm) (16)
である。Al2O3 1molに対してO2-は3mol発生するので、n molのOの生成数、分解数をΔNOとすると、
ΔNO=Avit/(2FcVm) (17)
となる。
次に、プローブの先端位置を酸化アルミニウム膜表面201上で移動させる方法のひとつとしてスキャニングさせる方法について述べる。図18は、金属アルミニウム上の酸化アルミニウム膜表面201にプローブを接触させ、金属アルミニウム側をアノードにして(7)式の反応式によりスキャニングで形成した半導体層(生成中の半導体層202及び生成した半導体層203)の概念図を示す。プローブ先端の面積よりも実際に酸化アルミニウム膜表面201と接触する面積は小さくなるが、ここではu1(m)×u2(m)の長方形領域で接触すると仮定する。金属アルミニウムとプローブ間にスパーク電流i(A)を流しながら、プローブを定速度v(m/s)で矢印の方向にスキャニングさせて半導体層を生成させる。スパークにより半導体層の厚さは増加してh(m)まで成長する。スキャニングを連続させることにより幅u2(m)、厚さh(m)の直方体状の半導体層が形成されていく。スパーク電流iは、実際の操作では印加電圧を一定にしてスキャニングするので変動しながら凡そ一定値に安定すると考えられるが、ここでは便宜的に定電流とした。
Q=u1i/v (18)
となる。tの時間で生成する半導体層の体積VmはVm=u1u2hであるので、
h=Vm/(u1u2) (19)
となる。(19)式に(9)式及び(18)式を代入すると、
h=4.4×10-11・i/(u2v) (20)
となる。これより、
i=2.3×1010・u2vh (21)
となる。
図19は、金属アルミニウム上の酸化アルミニウム膜表面301にプローブを接触させ、金属アルミニウム側をカソードにして(8)式の反応式によりスキャニングで形成した半導体層(分解中の半導体層302及び分解した半導体層303)の概念図を示す。この場合、酸化アルミニウム膜の分解によって半導体層が形成されるので、スキャニングにより酸化アルミニウム膜が窪む状態になる。半導体層は残った酸化アルミニウム膜より形成される。したがって、金属アルミニウム上の酸化アルミニウム膜の厚さはスキャニングにより窪む高さよりも厚くなるよう作製条件を選ぶ必要がある。図19の詳細説明については、上述の図18の詳細説明における「生成」を「分解」と読み替えれば全て同じであるのでここでは省略する。
スキャニングパターンの例を図20に示す。半導体層を生成又は分解するため、酸化アルミニウム膜にプローブを接触したままスキャニングすることによりa(m)×b(m)の領域に半導体層を形成させる。ある瞬間において半導体層が形成させる領域Uはプローブの先端が酸化アルミニウム膜に接触してスパーク電流がこの部分全体に流れる部分で上述のu1×u2である。プローブ先端をa×b領域内を速度vで矢印のようにスパーク電流iを流しながらスキャンさせる。
ここでスパーク電流により形成したい半導体層のキャリア濃度に対して設定するスキャニング条件との関係を求める。キャリア濃度は過剰に注入されるAl3+イオン濃度に応じて増加するドナー準位のドナー密度と、過剰に注入されるO2-イオン濃度に応じて増加するアクセプタ準位のアクセプタ密度があるが、ここではドナー密度、すなわちAl3+イオン濃度を例にとって計算する。
CAl=ηAlΔNAl=ηAlAvit/(3FcVm) (22)
ここで、Vm=u1u2h、t=u1/vであるので、これらを(22)式に代入すると、
CAl=ηAl・Av /(3Fcu2h)・i/v
よって、スキャニングにおけるプローブの走査速度v(m/s)、通電電流i(A)、酸化アルミニウム膜の生成又は分解厚さh(m)、プローブの走査方向に直角の幅u2(m)、キャリア生成効率ηAlと、Al3+イオン濃度CAl(個/m3)は(23)式で表される。
CAl=2.08×1018・i/(u2hv)・ηAl (23)
(スキャニング装置の仕様)
日本電子製走査型プローブ顕微鏡JSPM-5200に導電性プローブを取り付けで半導体層が形成できるか装置の仕様を調べた。この結果スキャニング法による半導体層形成に関る装置の仕様は表1に示す通りであった。
(高キャリア濃度半導体層の形成)
JSPM-5200を用いてスキャニング法による半導体層の形成について計算した。一例として、(7)式に従った電流方向で、ベースとなる酸化アルミニウム膜を自然酸化皮膜とし、CAlを高濃度1×1028(個/m3)にする場合の電流値iを(23)式にて計算した。プローブ先端幅U2を1×106m(1.0μm)、半導体層形成高さhを2×10-8m(20nm)、スキャン速度vを1.46×10-5m/s(14.6μm/s)、キャリア生成効率ηAlを0.1とすると、i=1.4×10-8(A)、すなわち、14nAとなった。最大電流10nAのレンジを用いれば、CAlは若干低くなるものの半導体層を形成できる。
(7)式に従った電流方向で、CAlを低濃度1×1025(個/m3)にする場合について検討した。濃度は上記高濃度の場合よりも3桁低いので、電流値を3桁低くするか、速度を3桁上げるか、又は両者の設定を変えることが必要だが、いずれもこの装置の仕様範囲を超える設定となる。よって、本装置では低キャリア濃度半導体層の形成はできない。高キャリア濃度に限定される。
上述のように1回のスキャニングの範囲が25μm2に限られるため、更に範囲を広げるためには複数のスキャニングを行う必要がある。ただし、1回の測定時間30分の場合でも複数回だと長時間が必要である。更なる面積の拡大については別途検討することにする。
これまでに示した半導体層はスパーク法によりp型半導体とn型半導体を同時に生成し、pn接合を形成していた。実際にダイオードやトランジスタ等の半導体素子や発振素子にするためにはこれらの方法は必ずしも量産に適した方法とはいえない。
(発振素子)
本発明の実施形態3ついて、図21~図29に基づいて説明すれば、以下の通りである。
まず、本発明の一態様に係る半導体層について詳細に説明する前に、当該半導体層を備えた発振素子の概要について述べる。本発明者は、特許文献1にて、上記発振素子について詳細に説明している。発振素子についての以下の説明は、特許文献1の開示内容の一部であり、より詳細な内容については特許文献1を参照されたい。
次に、発明者の考える発振メカニズムについて述べる。本発振素子の電流発振のメカニズムはあくまでも仮説であり、メカニズムの全容を解明するには、今後より深い研究を行う必要があることに留意すべきである。
はじめに、アルミニウム板(1085材、12×30mm、厚さ0.2mm )を用意した。この試料を95℃に加熱した純水に浸せきして、厚さ約20nmのベーマイト皮膜をアルミニウム板上に形成した。次にマニュアルプローバ20を用意した。プローブには、直径0.2mmの白金線(H材)の先端を削り直径0.02mmにし、白金線の根本はコイル状に巻いたものを用いた。図2に示す装置10を用意し結線した。
通電終了後プローブはそのままの状態を維持し、図2における結線をI-V測定に切り換え、得られた半導体層のI-V測定を行った。その結果を図4に示す。-0.6~1.0Vの電圧範囲を0.1V/sの速度で走査した。-0.55~+0.2Vの間でほぼ直線関係が得られた。この電圧範囲においてはアルミニウム板とプローブとの間にトンネル電流又は皮膜の電子状態の金属化による電流が流れたと考えられる。-0.55V以下と+0.2V~+0.55Vで直線関係からはずれて大電流が流れた。また+0.55~+1.0Vの間、電流はほとんど流れなかった。このように広い電圧範囲で直線関係からはずれる理由は現時点では分からないが、半導体層のキャリア挙動が不安定な状況にあると考えられる。なお、仮にアルミニウム板とプローブとが短絡していたとするならばI-V特性の全電圧領域で直線関係になるはずであるが、そうではないので直線関係は形成した半導体層固有の特性である。
実施例1と同様に、アルミニウム板(1085材、20×60mm、厚さ0.18mm)を用意した。この試料を95℃に加熱した純水に浸せきして、厚さ約20nmのベーマイト皮膜をアルミニウム板上に形成した。
ここで生成した半導体層のI-V特性を図5に示す。電圧-0.6V以下で負の電流が流れたがそれ以上の電圧では電流は流れなく、整流特性が示された。半導体層の中にpn接合が形成され、半導体層のアルミニウム板側がn型半導体、プローブ側がp型半導体のpn接合ダイオードが形成された。
はじめに次の試料を用意した。アルミニウム板(24×24mm、厚さ0.1mm)を基板とし、フルウチ化学製5NアルミニウムAl(φ76×6mm厚)をターゲット材としてAr+O2ガス、全圧0.4Paの条件で約40分間スパッタを行い、アルミニウム板上に約30nmの酸化アルミニウム膜を付与した。スパッタ装置として、日電アネルバ製SPC-350型を用いた。
スパッタで作製した素子の表面に実施例1と同じプローブを接触させてI-V特性を測定した。-0.5~+0.5Vの間で直線関係が得られた。この電圧範囲においてはアルミニウム板とプローブとの間にトンネル電流又は皮膜の電子状態の金属化による電流が流れたと考えられる。-0.5V以下と+0.5V以上ではほとんど電流は流れなかった。整流作用はなかったが、キャリア濃度が高い場合の本半導体素子固有のI-V特性が得られた。
キャリア濃度を低濃度(例えば1×1025(m-3))にする場合について検討した。濃度は上記高濃度の場合よりも3桁低いので、電流値を3桁低くするか、速度を3桁上げるか、又は両者の設定を変えることが必要だが、いずれもこの装置の仕様範囲を超える設定となる。よって本装置では低キャリア濃度半導体層の形成はできなかった。高キャリア濃度に限定される。
次の試料を用意した。アルミニウム板(24×24mm、厚さ0.1mm)を基板とし、フルウチ化学製5NアルミニウムAl(φ76×6mm厚)をターゲット材としてAr+O2ガス、全圧0.4Paの条件で約40分間スパッタを行い、アルミニウム板上に約30nmの酸化アルミニウム薄膜を付与した。スパッタ装置として、日電アネルバ製SPC-350型を用いた。
I-V特性の例を図24に示す。この図はバイアス電圧(横軸)の負の領域が順バイアス側、正の領域が逆バイアス側をそれぞれ示す。バイアス電圧が-0.1V~+0.1Vの間で電流発振の現象が生じた。発振波形を図25に示す。発振電流は凡そ-0.4~+0.4μAの振幅で生じた。バイアス電圧0Vにおいても同様な大きさの振幅で発振電流が生じた。周波数は約3.4kHzであった。
本発明に係る、半導体層として生成したアルミニウム酸化物は、化学量論的物質であるAl2O3(O/Al=1.5)ではなく、Al欠乏又はO過剰(O/Al>1.5)のp型半導体、及び、Al過剰又はO欠乏(O/Al<1.5)のn型半導体である。p型半導体層及びn型半導体層の各キャリア濃度を~1027/m3に調整し、pn接合すればダイオードとなる。キャリア濃度を適宜調整したp型半導体層及びn型半導体層と化学量論的な酸化アルミニウム等の絶縁体とを立体的に組み合わせればトランジスタやサイリスタ等の半導体素子になる。これらは、ワイドバンドギャップのパワー半導体としての利用が期待できる。更に、透明酸化物半導体の特性を組み合わせれば太陽電池の構成材料、ディスプレーパネルへ適応できる。
11 直流安定化電源
12 オシロスコープ
13 スイッチボックス
14 シャント抵抗
15 電流制限抵抗
20 マニュアルプローバ
21 マニュピレータ
22 スイッチ
23 I-V測定器
101 プローブ
101a プローブ先端
102 酸化アルミニウム膜
103 金属アルミニウム
201、301 酸化アルミニウム膜表面
202、203、302、303 半導体層
Claims (11)
- 酸化アルミニウム膜にアルミニウムを過剰に含有させることによりドナー準位を形成したn型半導体と、酸化アルミニウム膜に酸素を過剰に含有させることによりアクセプタ準位を形成したp型半導体とを接合したpn接合を含むことを特徴とする半導体層。
- 酸化アルミニウム膜に酸素を過剰に含有させることによりアクセプタ準位を形成したp型半導体を含むことを特徴とする半導体層。
- 酸化アルミニウム膜の一方の面に金属アルミニウムを、前記酸化アルミニウム膜の他方の面にプローブを接触させ、大気中、酸素含有ガス中又は酸素ガス中で、アノードとしての前記金属アルミニウムとカソードとしての前記プローブとの間に、前記酸化アルミニウム膜の絶縁破壊が生じる電圧を印加して前記酸化アルミニウム膜を溶融させ、
前記溶融の間に、前記酸化アルミニウム膜に溶融塩電解反応を生じさせ、冷却させることにより、前記酸化アルミニウム膜の前記金属アルミニウム側にn型半導体層を、前記酸化アルミニウム膜の前記プローブ側にp型半導体層を、それぞれ生成し、且つ、前記n型半導体層と前記p型半導体層とを接合することを特徴とする半導体層の製造方法。 - 酸化アルミニウム膜の一方の面に金属アルミニウムを、前記酸化アルミニウム膜の他方の面にプローブを接触させ、大気中、ガス中又は真空中で、カソードとしての前記金属アルミニウムとアノードとしての前記プローブとの間に、前記酸化アルミニウム膜の絶縁破壊が生じる電圧を印加して前記酸化アルミニウム膜を溶融させ、
前記溶融の間に、前記酸化アルミニウム膜に溶融塩電解反応を生じさせ、冷却させることにより、前記酸化アルミニウム膜の前記金属アルミニウム側にp型半導体層を、前記酸化アルミニウム膜の前記プローブ側にn型半導体層を、それぞれ生成し、且つ、前記n型半導体層と前記p型半導体層とを接合することを特徴とする半導体層の製造方法。 - 前記酸化アルミニウム膜に溶融塩電解反応を生じさせる際、前記溶融塩電解反応を起こす通電電気量を調整することにより、前記n型半導体のドナー濃度又は前記p型半導体のアクセプタ濃度を制御可能にしたことを特徴とする請求項3又は4に記載の半導体層の製造方法。
- 前記酸化アルミニウム膜に溶融塩電解反応を生じさせる際、前記プローブを前記酸化アルミニウム膜に接触させながら移動させることを特徴とする請求項3~5のいずれか1項に記載の半導体層の製造方法。
- 前記プローブを前記酸化アルミニウム膜に接触させながら移動させる際、前記印加電圧を、(1)連続的に変化させる、(2)不連続に変化させる、(3)極性を一方向又は両方向に変化させる、又は、前記(1)~(3)を組み合わせて変化させることを特徴とする請求項3~6のいずれか1項に記載の半導体層の製造方法。
- 酸化アルミニウム膜にアルミニウムを過剰に含有させることによりドナー準位を形成したn型半導体と、酸化アルミニウム膜に酸素を過剰に含有させることによりアクセプタ準位を形成したp型半導体とを接合したpn接合を含むことを特徴とする発振素子。
- 酸化アルミニウム膜の一方の面に金属アルミニウムを、前記酸化アルミニウム膜の他方の面にプローブを接触させ、大気中、酸素含有ガス中又は酸素ガス中で、アノードとしての前記金属アルミニウムとカソードとしての前記プローブとの間に、前記酸化アルミニウム膜の絶縁破壊が生じる電圧を印加して前記酸化アルミニウム膜を溶融させ、
前記溶融の間に、前記酸化アルミニウム膜に溶融塩電解反応を生じさせ、冷却させることにより、前記酸化アルミニウム膜の前記金属アルミニウム側にn型半導体層を、前記酸化アルミニウム膜の前記プローブ側にp型半導体層を、それぞれ生成し、且つ、前記n型半導体層と前記p型半導体層とを接合し、
前記接合により形成される空乏層の厚さは、1nm以下であることを特徴とする発振素子の製造方法。 - 酸化アルミニウム膜の一方の面に金属アルミニウムを、前記酸化アルミニウム膜の他方の面にプローブを接触させ、大気中、ガス中又は真空中で、カソードとしての前記金属アルミニウムとアノードとしての前記プローブとの間に、前記酸化アルミニウム膜の絶縁破壊が生じる電圧を印加して前記酸化アルミニウム膜を溶融させ、
前記溶融の間に、前記酸化アルミニウム膜に溶融塩電解反応を生じさせ、冷却させることにより、前記酸化アルミニウム膜の前記金属アルミニウム側にp型半導体層を、前記酸化アルミニウム膜の前記プローブ側にn型半導体層を、それぞれ生成し、且つ、前記n型半導体層と前記p型半導体層とを接合し、
前記接合により形成される空乏層の厚さは、1nm以下であることを特徴とする発振素子の製造方法。 - 前記酸化アルミニウム膜に溶融塩電解反応を生じさせる際、前記プローブを前記酸化アルミニウム膜に接触させながら移動させることを特徴とする請求項9又は10に記載の発振素子の製造方法。
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