The present invention relates to a rectifying element.
A rectenna (rectifying antenna) element is an element that receives a high frequency such as a microwave and converts the high frequency into DC power. The rectenna element is expected to be applied to a power regeneration technology and a wireless power supply technology. As the wireless power supply technology, an application to a space solar power satellite (SPS) concept is expected, in which the power is sent from a huge solar power station provided in the outer space to the ground using a radio wave in a microwave range (3 to 30 GHz).
A rectifying element is used for the rectenna element. Patent Literature 1 discloses a rectenna element using a PN junction diode as the rectifying element.
Patent Literature 1: Japanese Translation of PCT Application No. 2008-516455
Non Patent Literature 1: B. Berland, “Photovoltaic Technologies Beyond the Horizon: Optical Rectenna Solar Cell Final Report 1 Aug. 2001-30 Sep. 2002”, National Renewable Energy Laboratory, February 2003.
Non Patent Literature 2: I. Hotovy et al, “Characterization of NiO thin films deposited by reactive sputtering”, Vacuum, volume 50, number 1-2 pp. 41-44 (1998).
Non Patent Literature 3: R. Srnanek et al., “A Raman study of NiOx films for gas sensors applications”, ASDAM 2000. The Third International Euro Conference on Advanced Semiconductor Devices and Microsystems, pp. 303-306.
By the way, in recent years, as a method of realizing a next-generation highly efficient solar cell, an optical rectenna that receives light and converts the light into a DC current has drawn attention (Non Patent Literature 1). Since the light has a higher frequency (for example, 150 THz or more) than the microwave, a rectifying element having a higher-speed switching characteristic is required. Therefore, in Non Patent Literature 1, a metal-insulator-metal (MIM) type tunnel diode, which is expected to have a high-speed switching characteristic, is used.
However, the conventional MIM type tunnel diode has not obtained a sufficient asymmetric I-V characteristic and sufficient rectification properties.
Meanwhile, a Schottky diode realizes the rectification properties by a Schottky barrier. The
Schottky diode is a majority carrier device, which is different from the PN junction diode, and thus has a fast switching speed and can perform switching at a high frequency. However, in a normal Schottky diode, a majority carrier outside a depletion layer needs to move toward the Schottky junction in order to perform switching from a reverse direction to a forward direction. Since this movement takes a time, even a Schottky diode for high frequency made of GaAs is said to have a frequency response of 5 THz.
The present invention has been made in view of foregoing, and an objective is to provide a rectifying element that realizes a high-speed switching characteristic and sufficient rectification properties.
In order to solve the above problems and to attain the objective, a rectifying element according to the present invention includes: a first electrode having a first work function; a second electrode having a second work function larger than the first work function; and a semiconductor layer having a third work function that is a value between the first work function and the second work function, and joined to the first electrode and the second electrode.
In the rectifying element according to the present invention, the semiconductor layer is set to have a thickness with which the rectifying element becomes fully depleted in a state where a bias voltage is not applied between the first electrode and the second electrode, in the above invention.
In the rectifying element according to the present invention, a carrier of the semiconductor layer is a hole, in the above invention.
In the rectifying element according to the present invention, the semiconductor layer is made of a metal oxide, in the above invention.
In the rectifying element according to the present invention, the metal oxide is an NiOx (x=1 to 1.5), in the above invention.
In the rectifying element according to the present invention, the metal oxide is an NiOx (x=1 to 1.5), the first electrode is made of Al, and the second electrode is made of Ni, in the above invention.
In the rectifying element according to the present invention, a hole concentration in the NiOx is in a 10−2 cm−3 range to a 1017 cm−3 range, in the above invention.
In the rectifying element according to the present invention, the metal oxide is an NiOx (x=1 to 1.5), the first electrode is made of Ni, the second electrode is made of Pt, in the above invention.
In the rectifying element according to the present invention, a hole concentration in the NiOx is in a 1017 cm−3 range or more, in the above invention.
In the rectifying element according to the present invention, the metal oxide is generated in a manner that a metal serving as a raw material of the metal oxide is irradiated with an ultraviolet ray and oxidized, in the above invention.
According to the present invention, an effect to realize a rectifying element having a high-speed switching characteristic and sufficient rectification properties is exhibited.
Hereinafter, embodiments of a rectifying element according to the present invention will be described in detail with reference to the drawings. Note that the invention is not limited by the embodiments.
As illustrated in the energy band structure EB1 of
As illustrated in the energy band structure EB2 of
As illustrated in the energy band structure EB3 of
Next, as illustrated in
As a result, the rectifying element 10 functions as a Schottky diode, and sufficient rectification properties can be obtained. Further, the rectifying element 10 has a higher-speed switching characteristic than the PN junction diode. Further, if the thickness of the semiconductor layer 3 is set to a thickness with which the semiconductor layer 3 becomes completely depleted in a state where no bias voltage is applied between the first electrode 1 and the second electrode 2 (a 0 bias state), occurrence of a parasitic capacity due to movement of the majority carrier outside the depletion layer toward the Schottky junction can be prevented. As a result, a further higher-speed switching characteristic can be obtained.
Hereinafter, the present embodiment will be described in more detail with a specific example of the first electrode 1, the second electrode 2, and the semiconductor layer 3.
As the semiconductor layer 3, for example, it is favorable to use a thin film made of a metal oxide. That is, to realize a semiconductor layer having a thickness with which it becomes completely depleted at the 0 bias, the semiconductor layer is favorably a thin film. However, when trying to produce a semiconductor thin film of an ultrathin film on a metal electrode with a crystal semiconductor material, such a semiconductor having an ultrathin film is difficult to have crystal growth. Further, even if trying to produce the ultrathin film by deposition, the quality of the ultrathin film is easily deteriorated, such as an increase in a defect like a pinhole. Therefore, the metal electrodes may be short circuited.
In contrast, when a thin film made of a metal oxide is used as the semiconductor layer, the semiconductor layer can be formed by oxidation of a metal surface, and thus the production is easy. As the metal oxide, a zinc oxide, an indium oxide, a tin oxide, a nickel oxide, or the like can be used. Especially, when the nickel oxide is used, a semiconductor layer having an ultrathin film, which has good quality and has less defects such as pinholes, can be favorably realized. Note that, when the semiconductor layer is an N type semiconductor, the first electrode forms an ohmic junction with the semiconductor layer, and thus becomes an ohmic electrode, and the second electrode forms a Schottky junction with the semiconductor layer, and thus becomes a Schottky electrode. The zinc oxide, the indium oxide, and the tin oxide may become an N type semiconductor.
A case of using a nickel oxide as the semiconductor layer will be described. Hereinafter, the nickel oxide is appropriately expressed as NiOx (x=1 to 1.5). It is known that an oxygen-excessive NiOx exhibits a P type conductive type. However, it has been difficult to control a hole concentration of the NiOx to control the P type conductivity.
However, as a result of diligent study, the inventors of the present invention have found out to be able to control the oxygen content of the NiOx (that is, the value of x) to adjust the hole concentration, thereby to realize an NiOx having the work function qφ3 of a value between the work function qφ1 of the first electrode 1 and the work function qφ2 of the second electrode 2.
Further, the inventors of the present invention have found out that it is favorable to irradiate an ultraviolet ray to generate a nickel oxide as a method of producing the NiOx. That is, when the NiOx is produced by thermal oxidation, nickel is not oxidized by the thermal oxidation at a low temperature of 500° C. or less. Also, in the thermal oxidation at a higher temperature than 500° C., a NiOx having high resistance is produced. Therefore, when the thermal oxidation is used, it is difficult to control the hole concentration of the NiOx. Meanwhile, according to the findings of the inventors of the present invention, when oxidation is performed at a treatment temperature of 300° C. or less while the ultraviolet irradiation is performed, a generated NiOx exhibits P type conduction. Further, the inventors have found out that it is possible to control the conductivity of the NiOx (or the hole concentration or the work function) according to conditions such as the treatment temperature, an oxidizing species (oxygen, or water vapor), pressure of the oxidizing species, and the like.
Next, favorable materials of the first electrode and the second electrode will be described.
Meanwhile, as described above, a high-quality NiOx is obtained by oxidation of Ni, and contact between Ni and NiOx is good. Further, the work function of Ni is 5.1 eV. Therefore, for example, the material of the first electrode 1 is Al illustrated by the reference sign M1 in
To control the work function of the NiOx to become the value between 4.1 eV and 5.1 eV, an energy difference (EF−EV) between the Fermi level and the lower end of the valence band of the NiOx may just be 0.2 to 1.3 eV, considering that the electron affinity qχ3 of the NiOx is 1.7 to 1.8 eV. To realize this work function, the hole concentration of the NiOx thin film favorably falls in the 10−2 cm−3 range to the 1017 cm−3 range.
Hereinafter, the hole concentration will be more specifically described. A hole concentration p of the NiOx is expressed by the following formula (1):
NV is effective density of states of a valence band, k is the Boltzmann constant, and T is an absolute temperature. Further, NV is expressed by the following formula (2):
m* is an effective mass of a hole, m0 is a rest mass of an electron, and h is the Planck's constant.
As an example, when NV˜3.7×1020 cm−3 in the vicinity of the room temperature (T˜300 K), a favorable hole concentration is 7×10−2 to 1.7×1017 cm−3, which is in the 10−2 cm−3 range to the 1017 cm−3 range.
In the above description, the material of the first electrode 1 is Al, the material of the second electrode 2 is Ni, and the semiconductor layer 3 is the NiOx thin film. However, the present embodiment is not limited to the above example. For example, the material of the first electrode 1 is Ni, the material of the second electrode 2 is Pt illustrated by the reference sign M3 of
To control the work function of the NiOx to become the value between 5.1 eV and 5.7 eV, the hole concentration in the NiOx may just be the 1017 cm−3 range (for example, 1.7×1017 cm−3) or more. An upper limit of the hole concentration can be set up to about a high concentration with which the semiconductor degenerates.
Next, a favorable thickness of the semiconductor layer 3 will be described. As described above, if the thickness of the semiconductor layer 3 is set to a thickness that causes complete depletion in the 0 bias state, a higher-speed switching characteristic can be obtained, and thus it is favorable. The complete depletion can be more easily realized as the thickness of the semiconductor layer 3 is thinner. Meanwhile, to decrease the capacity of the rectifying element 10, a thicker thickness of the semiconductor layer 3 is favorable. Therefore, the thickness of the semiconductor layer 3 is most favorably a maximum thickness that causes the complete depletion.
A depletion width WD of the Schottky diode in the 0 bias state is expressed by the following formula (3):
εS is a dielectric constant of the semiconductor layer 3, q is an elementary charge, NA is the hole concentration (corresponding to p of the formula (1)).
A case in which the material of the first electrode 1 is Al, the material of the second electrode 2 is Ni, and the semiconductor layer 3 is the NiOx thin film will be described as an example. Consider a case in which the work function of the NiOx is set to 4.6 eV that is a mean value between the work function (4.1 eV) of Al and the work function (5.1 eV) of Ni. In this case, from the formula (1), the hole concentration is 1.6×107 cm−3. When the electron affinity qχ3 of the NiOx is 1.8 eV, and a relative dielectric constant of the NiOx is 12, WD becomes 6,400 μm (0.64 cm) from the formula (3). Therefore, if the thickness of the NiOx thin film is made 0.64 cm or less, the completed depletion is realized in the 0 bias state, and thus it is favorable. Note that, when the thickness is about 0.64 cm, it is not too thin. Therefore, a good-quality NiOx thin film without pinholes can be easily produced, and thus it is favorable. When the thickness of the NiOx thin film is 0.64 cm, the capacity per unit area is 1.7×10−12 F/cm2. In this case, even if the size of the surface area of the rectifying element 10 is a 1 μm square, the capacity of the element is 1.7×10−20 F, and is sufficiently small to realize a high-speed switching characteristic. Note that, when the electron affinity qχ3 of the NiOx is 1.7 eV, WD is 0.094 cm. In this case, the good-quality NiOx thin film can be also easily produced.
Note that the maximum thickness that realizes the complete depletion becomes thicker as the hole concentration of the NiO thin film is lower. Therefore, it is favorable in terms of easiness of realization of the complete depletion, easiness of formation of the semiconductor layer, and good quality.
Next, a method of producing a favorable NiOx thin film will be further described. As described above, according to the findings of the inventors of the present invention, when the oxidation is performed at the treatment temperature of 300° C. or less while the ultraviolet radiation is performed, the generated NiOx exhibits P type conduction. Further, the electrical conductivity (or the hole concentration or the work function) of the NiOx can be controlled according to the conditions such as the treatment temperature, the oxidizing species (oxygen or water vapor), the pressure of the oxidizing species, and the like. Accordingly, the semiconductor layer 3 made of the NiOx having an appropriately controlled work function can be favorably realized.
Therefore, the rectifying element 10 can be produced such that an NiOx thin film, the work function or the hole concentration of which has been appropriately controlled by the above method, is formed on a surface of an Ni electrode obtained by vacuum deposition, and an Al electrode or a Pt electrode obtained by vacuum deposition is joined to sandwich the NiOx thin film.
Further, for example, the rectifying element 10 can be produced as follows. First, a Pt/Ti electrode (each thickness is 20 nm) and an Ni thin film (the thickness is 100 nm) are formed in order on an Si substrate with an SiO2 film formed on a surface, by electron beam lithography, electron beam deposition, and lift-off. Next, the Ni thin film is oxidized to have a predetermined thickness, and an NiOx thin film that forms an ohmic junction with the Ni thin film is formed. Further, an Al/Pt electrode (respective thicknesses are 100 nm and 20 nm) are formed on the NiOx thin film by electron beam lithography, electron beam deposition, and lift-off. Accordingly, the rectifying element 10 can be produced. Here, Ti in the Pt/Ti electrode is formed between Pt and the SiO2 film to enhance adhesion. Pt in the Al/Pt electrode is used for an extraction electrode using a characteristic that Pt is less likely to be oxidized. Further, if the Pt/Ti electrode and the Al/Pt electrode are formed into comb shapes having a comb-teeth width of about 300 nm and a comb-teeth length of about 100 μm, and intersecting with each other, the rectifying element 10 is formed in a portion where these comb-shaped electrodes intersect with each other. Further, the Al/Pt electrode of a surface side is extended and is formed into a shape of an optical antenna, whereby the optical rectenna having the configuration exemplarily disclosed in Non Patent Literature 1, and the like, can be formed.
Note that, in the above-described embodiment, the semiconductor layer is a P type semiconductor. However, the semiconductor layer may be an N type semiconductor. In this case, the semiconductor layer forms an ohmic junction with the first electrode, and forms a Schottky junction with the second electrode.
Further, the present invention is not limited by the above-described embodiments. A configuration of an appropriate combination of the above-described configuration elements is also included in the present invention. Further, more effects and modifications can be easily led by a person skilled in the art. Therefore, broader embodiments of the present invention are not limited by the above-described embodiments, and various changes can be made.
As described above, the rectifying element according to the present invention is favorably used for a rectenna element.
Filing Document | Filing Date | Country | Kind |
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PCT/JP13/67564 | 6/26/2013 | WO | 00 |