JP5607676B2 - Rectifier element - Google Patents

Description

  The present invention relates to a rectifying element.

  A rectenna (rectifying antenna) element is an element that receives a high frequency wave such as a microwave and converts it into DC power. The rectenna element is expected to be applied to power regeneration technology and wireless power feeding technology. As a wireless power supply technology, for example, a space solar power plant (space solar power plant) that uses a microwave band (3 GHz to 30 GHz) to send power to the ground from a huge solar power plant installed in outer space. Application to satellite (SPS) concept and the like is expected.

  A rectifier element is used as the rectenna element. Patent Document 1 discloses a rectenna element using a PN junction diode as a rectifying element.

Special table 2008-516455 gazette

B. Berland, “Photovoltaic Technologies Beyond the Horizon: Optical Rectenna Solar Cell Final Report 1 August 2001-30 September 2002”, National Renewable Energy Laboratory, February 2003. I. Hotovy et al, “Characterization of NiO thin films deposited by reactive sputtering”, Vacuum, volume 50, number 1-2 pp. 41-44 (1998). R. Srnanek et al., “A Raman study of NiOx films for gas sensors applications”, ASDAM 2000. The Third International EuroConference on Advanced Semiconductor Devices and Microsystems, pp. 303-306.

  By the way, in recent years, as a method for realizing a next-generation high-efficiency solar cell, an optical rectenna that receives light and converts it into a direct current has attracted attention (Non-Patent Document 1). Since light has a higher frequency than microwaves (for example, 150 THz or more), a rectifying element having faster switching characteristics is required. For this reason, Non-Patent Document 1 uses a MIM (Metal-Insulator-Metal) type tunnel diode which is expected to have high-speed switching characteristics.

  However, the conventional MIM type tunnel diode has not obtained sufficient asymmetric IV characteristics, and has not obtained sufficient rectification.

  On the other hand, the Schottky diode realizes rectification by a Schottky barrier. Since the Schottky diode is a majority carrier device unlike the PN junction diode, the switching speed is fast and switching at a high frequency is possible. However, in a normal Schottky diode, majority carriers outside the depletion layer must move toward the Schottky junction in order to switch from the reverse direction to the forward direction. Since this movement takes time, even a high frequency Schottky diode made of GaAs, for example, is said to have a frequency response of 5 THz.

  The present invention has been made in view of the above, and an object of the present invention is to provide a rectifying element that realizes high-speed switching characteristics and sufficient rectification.

  In order to solve the above-described problems and achieve the object, a rectifying element according to the present invention has a first electrode having a first work function and a second work function larger than the first work function. A second electrode, and a semiconductor layer having a third work function having a value between the first work function and the second work function, the semiconductor layer being joined to the first electrode and the second electrode; It is characterized by providing.

  In the rectifying device according to the present invention, in the above invention, the semiconductor layer is set to a thickness that is completely depleted without applying a bias voltage between the first electrode and the second electrode. It is characterized by being.

  In the rectifier according to the present invention, the carrier of the semiconductor layer is a hole in the above invention.

  In the rectifier according to the present invention, the semiconductor layer is made of a metal oxide.

  In the rectifier according to the present invention, the metal oxide is NiOx (x = 1 to 1.5).

  In the rectifying device according to the present invention, in the above invention, the metal oxide is NiOx (x = 1 to 1.5), the first electrode is made of Al, and the second electrode is made of Ni. It is characterized by that.

Moreover, the rectifier according to the present invention is characterized in that, in the above invention, the hole concentration in the NiOx is 10 ⁻² cm ⁻³ to 10 ¹⁷ cm ⁻³ .

  In the rectifier according to the present invention, in the above invention, the metal oxide is NiOx (x = 1 to 1.5), the first electrode is made of Ni, and the second electrode is made of Pt. It is characterized by that.

Moreover, the rectifier according to the present invention is characterized in that, in the above invention, the hole concentration in the NiOx is 10 ¹⁷ cm ⁻³ or more.

  The rectifying device according to the present invention is characterized in that, in the above invention, the metal oxide is generated by oxidizing a metal which is a raw material of the metal oxide by irradiating with ultraviolet rays. .

  According to the present invention, there is an effect that a rectifying element having high-speed switching characteristics and sufficient rectification can be realized.

FIG. 1 is a schematic cross-sectional view of a rectifying element according to an embodiment. FIG. 2 is a diagram showing an energy band structure. FIG. 3 is a diagram showing the relationship between the electronegativity of main metals and the work function. FIG. 4 is a diagram showing a Raman spectrum of the produced NiOx thin film. FIG. 5 is a diagram showing IV characteristics of the manufactured NiOx thin film.

  Embodiments of a rectifying device according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.

  FIG. 1 is a schematic cross-sectional view of a rectifying element according to an embodiment. As shown in FIG. 1, the rectifying element 10 includes a first electrode 1, a second electrode 2, and a semiconductor layer 3 joined to the first electrode 1 and the second electrode 2. The first electrode 1 and the semiconductor layer 3 are bonded at the bonding surface 4, and the second electrode 2 and the semiconductor layer 3 are bonded at the bonding surface 5, respectively.

  FIG. 2 is a diagram showing an energy band structure. 2A shows the first electrode 1, the second electrode 2, and the semiconductor layer 3 before joining, and FIG. 2B shows the first electrode 1, the second electrode 2, and the semiconductor layer 3 after joining (a state where the rectifying element 10 is in a thermal equilibrium state). Energy band structures EB1, EB2, and EB3 are shown.

As shown in the energy band structure EB1 of FIG. 2A, the first electrode 1 has a work function qφ1 with respect to the vacuum level. E _F 1 represents the Fermi level.

As shown in the energy band structure EB2 of FIG. 2A, the second electrode 2 has a work function qφ2 larger than the work function qφ1 of the first electrode 1. E _F 2 represents the Fermi level.

As shown in the energy band structure EB3 of FIG. 2A, the semiconductor layer 3 is a P-type semiconductor and has a work function qφ3 having a value between the work functions qφ1 and qφ2. E _F 3 is the Fermi level, CB 3 is the conduction band, CBM 3 is the lower end of the conduction band, VB 3 is the valence band, VBM 3 is the upper end of the valence band, and qχ 3 is the electron affinity.

Next, as shown in FIG. 2B, when the first electrode 1, the second electrode 2, and the semiconductor layer 3 are joined and brought into a thermal equilibrium state, the Fermi levels E _F 1, E _F 2, and E _F 3 are Match. At this time, due to the magnitude relationship of the work functions qφ1, qφ2, and qφ3, the energy band of the semiconductor layer 3 in the vicinity of the bonding surfaces 4 and 5 is bent, and both of the bonding surfaces 4 of the first electrode 1 and the semiconductor layer 3 are Schottky junction is performed, and the second electrode 2 and the semiconductor layer 3 are in ohmic junction at the junction surface 5. Thus, the bonding surface 4 Schottky barrier Qfai _B is formed.

  As a result, the rectifying element 10 functions as a Schottky diode, and sufficient rectification is obtained. In addition, it has high-speed switching characteristics compared to a PN junction diode. Further, if the thickness of the semiconductor layer 3 is set to a thickness that is completely depleted when no bias voltage is applied between the first electrode 1 and the second electrode 2 (zero bias state), as described above. The generation of parasitic capacitance due to the movement of majority carriers outside the depletion layer toward the Schottky junction is prevented. As a result, even faster switching characteristics can be obtained.

  Hereinafter, the present embodiment will be described in more detail with specific examples of the first electrode 1, the second electrode 2, and the semiconductor layer 3.

  As the semiconductor layer 3, for example, a thin film made of a metal oxide is preferably used. That is, in order to realize a semiconductor layer having a thickness that is completely depleted at 0 bias, the semiconductor layer is preferably a thin film. However, if an ultrathin semiconductor thin film is made of a crystalline semiconductor material on a metal electrode, the crystal of such an ultrathin semiconductor is difficult to grow. In addition, even if an attempt is made by deposition, in the case of an ultra-thin film, there is a risk that the metal electrodes may be short-circuited because the quality tends to deteriorate, for example, the number of defects such as pinholes increases.

  On the other hand, in the case where a thin film made of a metal oxide is used as the semiconductor layer, the metal surface may be formed by oxidation, so that the production is easy. As the metal oxide, for example, zinc oxide, indium oxide, tin oxide, nickel oxide, or the like can be used. In particular, when nickel oxide is used, an ultra-thin semiconductor layer with good quality and few defects such as pinholes can be suitably realized. When the semiconductor layer is an N-type semiconductor, the first electrode is in ohmic contact with the semiconductor layer and thus becomes an ohmic electrode, and the second electrode is in Schottky contact with the semiconductor layer and thus becomes a Schottky electrode. Zinc oxide, indium oxide, and tin oxide can be N-type semiconductors.

  A case where nickel oxide is used for the semiconductor layer will be described. Hereinafter, the nickel oxide is appropriately expressed as NiOx (x = 1 to 1.5). Excessive oxygen NiOx is known to exhibit P-type conductivity. However, it has been difficult to control the P-type conductivity by controlling the hole concentration of NiOx.

  However, as a result of intensive studies, the inventors have adjusted the hole concentration by controlling the oxygen content of NiOx (that is, the value of x), and the work function qφ1 of the first electrode 1 and the work of the second electrode 2 are adjusted. It has been discovered that NiOx having a work function qφ3 with a value between the functions qφ2 can be realized.

  Furthermore, the present inventors have also found that it is preferable to produce nickel oxide by irradiating ultraviolet rays as a method for producing NiOx. That is, when NiOx is manufactured by thermal oxidation, nickel is not oxidized by thermal oxidation at a low temperature of 500 ° C. or lower, and high resistance NiOx is generated by thermal oxidation at a higher temperature. For this reason, when thermal oxidation is used, it is difficult to control the hole concentration of NiOx. On the other hand, according to the discovery of the present inventors, when oxidation is performed at a treatment temperature of 300 ° C. or lower while performing ultraviolet irradiation, the produced NiOx exhibits P-type conduction. Furthermore, it has been found that the conductivity (or hole concentration and work function) of NiOx can be controlled by conditions such as processing temperature, oxidizing species (oxygen, water vapor), and oxidizing species pressure.

  Next, preferred materials for the first electrode and the second electrode will be described. FIG. 3 is a diagram showing the relationship between the electronegativity (Pauling) of main metals and the work function. The selection of the work function of a normal metal capable of forming a film by vacuum deposition is limited, and Al, In, and Ga are as low as about 4.1 eV.

  On the other hand, as described above, high-quality NiOx is obtained by oxidizing Ni, and the contact between Ni and NiOx is good. The work function of Ni is 5.1 eV. Therefore, for example, the material of the first electrode 1 is Al indicated by reference numeral M1 in FIG. 3, the material of the second electrode 2 is Ni indicated by reference numeral M2 in FIG. 3, and the work function is the work function of Al (4. The NiOx thin film controlled to a value between 1 eV) and the work function of Ni (5.1 eV) is used as the semiconductor layer 3 to realize the rectifying device according to the present embodiment.

To make the work function of NiOx between 4.1 eV and 5.1 eV, considering that the electron affinity qχ3 of NiOx is 1.7 eV to 1.8 eV, the Fermi level of NiOx and the lower end of the valence band The energy difference (E _F −E _V ) may be 0.2 eV to 1.3 eV. In order to realize this, the hole concentration of the NiOx thin film is preferably 10 ⁻² cm ⁻³ to 10 ¹⁷ cm ⁻³ .

  Hereinafter, the hole concentration will be described more specifically. The hole concentration p of NiOx is expressed by the following formula (1).

Ｎ_Ｖは価電子帯の有効状態密度、ｋはボルツマン定数、Ｔは絶対温度である。また、Ｎ_Ｖは以下の式（２）で表される。

N _V is the effective density of states in the valence band, k is the Boltzmann constant, T is the absolute temperature. Further, _{N V} is expressed by the following equation (2).

ｍ^＊は正孔（ホール）の有効質量、ｍ_０は電子の静止質量、ｈはプランク定数である。

m ^* is the effective mass of holes, m ₀ is the static mass of electrons, and h is the Planck constant.

As an example, when N _{V is about} 3.7 × 10 ²⁰ cm ⁻³ in the vicinity of room temperature (T to 300 K), a preferable hole concentration is 10 ⁻² cm ⁻³ to 10 ¹⁷ cm from Equation (1). a is where ^-3 is ^{7 × 10 -2 cm -3 ~1.7 ×} 10 17 cm -3.

  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 a NiOx thin film. However, the present embodiment is not limited to this. For example, the material of the first electrode 1 is Ni, the material of the second electrode 2 is Pt indicated by a symbol M3 in FIG. 3, and the work function is Ni work function (5.1 eV) and Pt work function (5.7 eV). The rectifying device according to the present embodiment can also be realized by using the NiOx thin film having a value between) and the semiconductor layer 3.

The work function of NiOx to between 5.1eV and 5.7eV, the positive hole concentration in NiOx ^as long 10 17 ^{cm -3} units (e.g. 1.7 × ¹⁰ 17 ^{cm -3)} or Good. The upper limit of the hole concentration can be set to such a high concentration that the semiconductor degenerates.

  Next, a preferable thickness of the semiconductor layer 3 will be described. As described above, it is preferable to set the thickness of the semiconductor layer 3 to a thickness that is fully depleted in the zero bias state, because higher-speed switching characteristics can be obtained. The thinner the semiconductor layer 3 is, the easier it is to realize full depletion. On the other hand, in order to reduce the capacity of the rectifying element 10, it is preferable that the semiconductor layer 3 is thick. Therefore, it is most preferable that the thickness of the semiconductor layer 3 is the maximum thickness that is fully depleted.

Depletion width W _D of the Schottky diode at 0 bias state is expressed by the following equation (3).

ε_Ｓは半導体層３の誘電率、ｑは素電荷、Ｎ_Ａは正孔濃度（式（１）のｐに相当）である。

epsilon _S is the dielectric constant of the semiconductor layer 3, q is the elementary charge, the _{N A} is a hole concentration (corresponding to p in equation (1)).

An example will be described 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 a NiOx thin film. Consider a case where the work function of NiOx is set to 4.6 eV, which is an intermediate value between the work function of Al (4.1 eV) and the work function of Ni (5.1 eV). In this case, according to the formula (1), the hole concentration is 1.6 × 10 ⁷ cm ⁻³ . 1.8eV electron affinity qχ3 of NiOx, when the relative dielectric constant of NiOx and 12, the equation (3), _{W D} is the 6400μm (0.64cm). Therefore, it is preferable to set the thickness of the NiOx thin film to 0.64 cm or less because it is fully depleted in a zero bias state. A thickness of about 0.64 cm is preferable because it is easy to produce a good-quality NiOx thin film without pinholes because it is not too thin. When the thickness of the NiOx thin film is 0.64 cm, the capacity per unit area is 1.7 × 10 ⁻¹² F / cm ² . In this case, even if the size of the surface area of the rectifying device 10 is 1 μm square, the capacitance of the device is 1.7 × 10 ⁻²⁰ F, which is sufficiently small to realize high-speed switching characteristics. In the case of a 1.7eV electron affinity qχ3 of NiOx is, _{W D} is a 0.094Cm, easy to prepare a thin film of high quality NiOx Again.

  Note that the lower the hole concentration of the NiO thin film, the larger the maximum depletion thickness, which is preferable from the viewpoint of easy realization of all depletion, ease of forming a semiconductor layer, and good quality.

  Next, a preferable method for producing a NiOx thin film will be further described. As described above, according to the discovery of the present inventors, when oxidation is performed at a processing temperature of 300 ° C. or less while performing ultraviolet irradiation, the produced NiOx exhibits P-type conduction. Furthermore, the conductivity (or hole concentration or work function) of NiOx can be controlled by conditions such as processing temperature, oxidizing species (oxygen, water vapor), and oxidizing species pressure. As a result, the semiconductor layer 3 made of NiOx whose work function is appropriately controlled can be suitably realized.

FIG. 4 is a diagram showing a Raman spectrum of a NiOx thin film prepared by the present inventors. In FIG. 4, oxidation was performed by irradiating the surface of the deposited nickel film with ultraviolet rays in an O ₂ atmosphere. As the ultraviolet light source, a metal halide lamp having a maximum spectral intensity distribution in the vicinity of a wavelength of 380 nm was used. The heat treatment temperature was 200 ° C. or 300 ° C., and the treatment time was 5 minutes. In the Raman spectrum shown in FIG. 4, it is considered that the larger the Raman shift of the peak (the higher the wave number), the greater the oxygen content in the NiOx film and the higher the hole concentration (see Non-Patent Documents 2 and 3). Thus, towards the case of the heat treatment temperature is 300 ° C. (peak center 563cm ^-1) is, in the case of 200 ° C. (peak center 530 cm ^-1) is the hole concentration is considered to be higher than. FIG. 4 shows that the oxygen content and hole concentration in the NiOx thin film can be adjusted by adjusting the treatment temperature.

  FIG. 5 is a diagram showing IV characteristics of the manufactured NiOx thin film. In FIG. 5, the IV characteristic of the NiOx film produced at the processing temperature of 300 ° C. shown in FIG. 4 is measured by the 4-probe method. In addition, the measurement was performed in the state cooled by room temperature and the cooling spray. From FIG. 5, it was confirmed that the manufactured NiOx thin film has a semiconductor characteristic because the resistance (gradient of the IV characteristic) is greatly increased due to a decrease in temperature. That is, it was confirmed that a semiconductor NiOx thin film capable of controlling the hole concentration or the work function can be produced by the above method.

  Therefore, the rectifying element 10 is formed by forming a NiOx thin film whose work function or hole concentration is appropriately controlled by the above method on the surface of the Ni electrode obtained by vacuum vapor deposition, and vacuum vapor deposition so as to sandwich the NiOx thin film. It can be produced by bonding the Al electrode or Pt electrode obtained in (1).

For example, the rectifying element 10 can be produced as follows. First, a Pt / Ti electrode (thickness is 20 nm) and a Ni thin film (thickness: 100 nm) are sequentially formed by electron beam lithography, electron beam evaporation and lift-off on a Si substrate having a SiO ₂ film formed on the surface. Next, the Ni thin film is oxidized to a predetermined thickness to form a NiOx thin film that is in ohmic contact with the Ni thin film. Further, an Al / Pt electrode (thickness: 100 nm / 20 nm) is formed on the NiOx thin film by electron beam lithography, electron beam evaporation and lift-off. Thereby, the rectifying device 10 can be manufactured. Here, Ti in the Pt / Ti electrode is formed between the Pt and the SiO ₂ film to improve adhesion. Pt in the Al / Pt electrode is used as an extraction electrode by utilizing the fact that Pt is difficult to oxidize. Further, if the Pt / Ti electrode and the Al / Pt electrode are formed in a comb-teeth shape having a comb-teeth width of about 300 nm and a comb-teeth length of about 100 μm, for example, the comb-teeth electrodes intersect with each other. A rectifying element 10 is formed. In addition, by extending the Al / Pt electrode on the surface side to form the shape of an optical antenna, an optical rectenna having a configuration exemplified in Non-Patent Document 1 or the like can be formed.

  In the above embodiment, the semiconductor layer is a P-type semiconductor, but may be an N-type semiconductor. In this case, the semiconductor layer has an ohmic junction with the first electrode and a Schottky junction with the second electrode.

  Further, the present invention is not limited by the above embodiment. What was comprised combining each component mentioned above suitably is also contained in this invention. Further effects and modifications can be easily derived by those skilled in the art. Therefore, the broader aspect of the present invention is not limited to the above-described embodiment, and various modifications can be made.

DESCRIPTION OF SYMBOLS 1 1st electrode 2 2nd electrode 3 Semiconductor layer 4 Joining surface 5 Joining surface 10 Rectifier EB1, EB2, EB3 Energy band structure

Claims (7)

A first electrode having a first work function;
A second electrode having a second work function greater than the first work function;
A semiconductor layer having a third work function having a value between the first work function and the second work function, the semiconductor layer being joined to the first electrode and the second electrode;
With
The semiconductor layer is made of NiOx (x = 1 to 1.5) using holes as carriers, which is generated by oxidizing Ni by irradiating ultraviolet rays, and is between the first electrode and the second electrode. A rectifier element that is set to a thickness that is completely depleted when no bias voltage is applied to the rectifier and functions as a fully depleted Schottky diode. The rectifying device according to claim 1 , wherein the first electrode is made of Al, and the second electrode is made of Ni. 3. The rectifying device according to claim 2 , wherein the hole concentration in the NiOx is 10 ⁻² cm ⁻³ to 10 ¹⁷ cm ⁻³ . The rectifying device according to claim 1 , wherein the first electrode is made of Ni, and the second electrode is made of Pt. The rectifying device according to claim 4 , wherein the hole concentration in the NiOx is 10 ¹⁷ cm ⁻³ or more. 6. The rectifying device according to claim 1, wherein the semiconductor layer is formed by oxidizing Ni by irradiating ultraviolet rays while performing heat treatment at 500 ° C. or less. 6. The rectifying device according to claim 1, wherein the semiconductor layer is formed by oxidizing Ni by irradiating ultraviolet rays while performing heat treatment at 300 ° C. or less.

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