Patent Application
20090050899
The invention relates to a high-output diamond semiconductor element, in particular, typically to a diamond Schottky barrier diode, a diamond pn diode, a diamond thyristor, a diamond transistor and a diamond field effect transistor.
In conventional technologies, diamond is large in a band gap (5.5 eV), high in avalanche breakdown electric field (10 MV/cm), high in saturated carrier mobility (4000 cm2/Vs) and high in the thermal conductivity (20 W/cmK), and is expected as a practically workable element under a high temperature or a radiation exposure environment. So far, in order to develop an electronic element that makes use of these features, a structure and a preparation method of a diamond diode have been proposed.
At the same time, with regard to diamond sensitive to a surface modification, surface inactivation is necessary in order to make use of high breakdown electric field. In other power devices, the surface inactivation technology has been gradually developed (see non-patent literature 1). However, with regard to diamond, an effective measure has not been developed.
In general, in a high-voltage operating diode, in order to inhibit an electric field from concentrating in a margin of an electrode, a guard ring structure using a pn junction, a field plate structure or a combined structure thereof (see non-patent literature 2) has been used. With regard to diamond, p-type and n-type dopings have been realized and a pn junction has been realized as well. However, since the n-type doping is very difficult and a value of leakage current at an interface of a formed pn junction is large (see non-patent literatures 3 and 4), an electric field relaxation technology that realizes a low leakage current under a high voltage in a margin of an electrode has not been obtained yet.
Non-patent literature 1: C. I. Harris et al. “SiC power device passivation using porous SiC” Appl. Phys. Lett. 66 (1995) 1501
Non-patent literature 2: K. Kinoshita et al. “Guard Ring Assisted RESURF”, Proc. 14th ISPSD (2002) p. 253
Non-patent literature 3: S. Koizumi et al. “Formation of diamond p-n junction and its optical emission characteristics”, Diam. Relat. Mater. 11 (2002) p. 307
Non-patent literature 4: T. Makino et al. “Electrical and optical characterization of (001)-oriented homoepitaxial diamond p-n junction”, Diam. Relat. Mater. 15 (2005) p. 513
Although diamond has been said to be high in the insulation resistance, the insulation resistance such high as 10 MV/cm has not been utilized effectively. In a Schottky barrier diode, in particular, when a high voltage is applied, it is considered that a leakage current is likely to appear on a surface, whereby the insulation breakdown earlier than a physical property limit is caused.
According to the invention, a surface of diamond is inactivated to provide a high-output diamond semiconductor element that operates up to a high voltage under a low leakage current.
Furthermore, in diamond, the dielectric breakdown of the diamond per se is larger than the dielectric strength of an oxide film. Accordingly, in a conventional voltage resistant structure using SiO2, the dielectric breakdown of the oxide film precedes thereby being incapable of making use of performance of diamond.
In this connection, according to the invention, a high specific permittivity material is formed on a diamond p− drift layer that is a selected region on p− type diamond to enable to inhibit an electric field from concentrating in a margin of a Schottky electrode, whereby a high-output diamond semiconductor element that operates up to a high voltage at a low leakage current even under a high electric field is provided.
In order to achieve the foregoing objects, the inventors have made intensive studies and found that, when an oxide and a nitride are formed around a Schottky electrode, a diamond surface is inativated and reverse leakage characteristics of diamond are improved, whereby a high-output diamond semiconductor element that may operate under a high voltage at a low leakage current can be obtained.
According to the invention, in a high-output diamond semiconductor element including a Schottky electrode as a cathode, a diamond P− drift layer, a diamond p+ ohmic layer, and an ohmic electrode as an anode, an oxygen-terminated diamond surface exposed around the Schottky electrode is inactivated by forming a particular insulating film to shield a surface current path, whereby a high-output diamond semiconductor element that is capable of operating at a low leakage current under a high voltage is realized.
That is, the invention provides, as a first embodiment, the following (1) to (5).
(1) A high-output diamond semiconductor element, comprising:
a Schottky electrode as a cathode,
a diamond P− drift layer,
a diamond p+ ohmic layer,
an ohmic electrode as an anode, and
an insulating film layer disposed to surround a circumference of the Schottky electrode.
(2) The high-output diamond semiconductor element according to (1), wherein an insulating material forming the insulating film layer is a nitride or an oxide.
(3) The high-output diamond semiconductor element according to (2), wherein the insulating material is Si3N4, SiO2 or Al2O3.
(4) The high-output diamond semiconductor element according to (1), wherein a surface of the diamond joined to the Schottky electrode is oxygen-terminated diamond.
(5) The high-output diamond semiconductor element according to (1), which is a Schottky barrier diode.
Still furthermore, in order to achieve the foregoing objects, the inventors have made intensive studies and found that, when a dielectric layer made of dielectric material is disposed on a part of a junction surface of the Schottky electrode and diamond p− drift layer, and a field plate made of a conductor is disposed on an external surface of the dielectric layer to surround a circumference of the Schottky electrode, a high-output diamond semiconductor element that alleviates an electric field in the proximity of a cathode electrode can be obtained.
According to the invention, in a high-output diamond semiconductor element including a Schottky electrode as a cathode, a diamond P− drift layer, a diamond p+ ohmic layer, and an ohmic electrode as an anode, a dielectric layer is disposed on a part of a junction surface of the Schottky electrode and diamond p− drift layer, and furthermore, a field plate made of a conductor is disposed on an external surface of the dielectric layer to surround a circumference of the Schottky electrode, whereby a high-output diamond semiconductor element that alleviates an electric field in the proximity of a cathode electrode is realized.
That is, the invention provides, as a second embodiment, the following (6) to (10).
(6) A high-output diamond semiconductor element, comprising:
a Schottky electrode as a cathode,
a diamond P− drift layer,
a diamond p+ ohmic layer,
an ohmic electrode as an anode,
a dielectric layer disposed on a part of a junction surface of the Schottky electrode and the diamond p− drift layer, and
a field plate comprising a conductor, said field plate being disposed on an external surface of the dielectric layer to surround a circumference of the Schottky electrode.
(7) The high-output diamond semiconductor element according to (6), wherein a dielectric material forming the dielectric layer is a dielectric material having a higher dielectric constant than that of the diamond.
(8) The high-output diamond semiconductor element according to (7), wherein the dielectric material is Si3N4, Al2O3 or SrTiO3.
(9) The high-output diamond semiconductor element according to (6), wherein a surface of the diamond joined to the Schottky electrode is oxygen-terminated diamond.
(10) The high-output diamond semiconductor element according to (6), which is a Schottky barrier diode.
According to the invention, since the reverse leakage current is reduced, a leakage current at the time of applying a high electric field to a high-output diamond semiconductor element is decreased, an operable voltage is increased, as well as an long-term reliability is improved. Furthermore, since local concentration of an electric field is reduced, a leakage current at the time of applying a high electric field to the high-output diamond semiconductor element is reduced and an operable voltage is increased.
The first embodiment of the present invention is explained below.
As an insulating material used in an insulating film that is an inactivating material formed around a Schottky electrode of the invention, a nitride or an oxide may be preferably used, and Si3N4, SiO2 or Al2O3 may, for example, be used. An insulating film as an inactivating material is disposed on a surface of a diamond p− drift layer (Schottky electrode side), and the insulating film may be disposed to surround a circumference of the Schottky electrode in accordance with an ion sputtering method, a PLD method, an RF sputtering method or the like. A thickness of the insulating film is not particularly restricted and is, for example, in the range of 1000 Å to 2 μm, and a distance from an adjacent electrode is desirably 10 μm or more.
Although the insulating film may have any shape, it usually has an island shape surrounding a circumference of the Schottky electrode (see
An insulating material used as a surface inactivation material in the invention is preferably a nitride or an oxide, and is for instance, Al2O3, Si3N4 or SiO2. It is preferable that at least the material per se has the dielectric strength of 1 MV/cm or more.
The surface inactivation material may be formed in accordance with any conventional method. For example, a wet method using a solvent, a vapor deposition method or a plasma CVD method may be used.
In the invention, the Schottky electrode is a Schottky electrode having a well-known shape for use in power electronics and means a Schottky electrode that executes a well-known operation. A material of the Schottky electrode, so long as it is a metal, is not particularly limited. For instance, Ti, Mo, Ta, Pt, Au and the like may be used.
A Schottky electrode is formed as a pattern electrode constituted of a plurality of electrodes which are formed and scattered to have an island shape on a surface of a 15 diamond semiconductor on a substrate.
The preparation method of a diamond semiconductor used in the invention is not particularly limited. For example, on p- or p−-type diamond, a layer of a nitride or an oxide is formed to have a thickness in the range of 0.1 to 10 μm preferably in accordance with an ion beam sputtering method, a PLD method, an RF sputtering method or a CVD method.
In the invention, any types of diamond may be used. Crystal structures such as (001), (111) and (110) may be mentioned. As the diamond surface, carbon-terminated diamond, hydrogen-terminated diamond, oxygen-terminated diamond and the like may be mentioned.
However, regarding at least the diamond joined to the Schottky electrode, a diamond having an oxygen-terminated diamond on the surface thereof is particularly suitable.
In the invention, as to preparation of an ohmic electrode as well, coventional materials and conventional processes may be used in any procedure.
In the next place, the second embodiment of the present invention is explained below.
A material that is used in a field plate of the invention is a conductive material and Pt, a Pt—Ru alloy, a Pt—Ir alloy and the like may be used. A field plate is disposed on an external surface of a dielectric layer and also on a circumferential surface of the Schottky electrode in accordance with an ion sputtering method, a PLD method, an RF method or the like. Namely, the field plate is disposed on an external surface of a dielectric layer to surround a circumference of the Schottky electrode. A thickness of a dielectric layer is substantially ¼ to ¾ a thickness of the Schottky electrode and a sum total of the thickness of the dielectric layer and the thickness of the field plate is desirably substantially same as the thickness of the Schottky electrode.
Although the field plate may have any shape, it usually has a circular island shape surrounding a circumference of the Schottky electrode (see
A dielectric (dielectric material) used in an electric field relaxation dielectric layer according to the invention is, for instance, SiO2, Si3N4, Al2O3 or SrTiO3 and one having a higher specific permittivity than at least that of diamond is selected.
The specific permittivity is 3.9 to 4.1 for SiO2, 7 to 8 for Si3N4, 8.7 to 10 for Al2O3, and 200 to 250 for SrTiO3.
Any conventional method may be used to form a dielectric layer. For example, a method using a solvent, a vapor deposition method and a CVD method may be used.
In the invention, the Schottky electrode is a Schottky electrode having a well-known shape for use in power electronics and means a Schottky electrode that executes a well-known operation. As a Schottky electrode material, a Pt—Ru alloy, a Pt—Ir alloy and the like may be used.
A Schottky electrode is formed as a pattern electrode constituted of a plurality of electrodes which are formed and scattered to have a island shape on a surface of a diamond semiconductor on a substrate.
The preparation method of a diamond semiconductor used in the invention is not particularly limited. Preferably, on p- or p−-type diamond, a nitrogen-doped diamond region is formed in accordance with an ion beam sputtering method, a PLD method, a RF sputtering method or a CVD method.
In the invention, any types of diamond may be used. Crystal structures such as (001), (111) and (110) may be mentioned. As the diamond surface, carbon-terminated diamond, hydrogen-terminated diamond, oxygen-terminated diamond and the like may be mentioned.
However, regarding at least the diamond joined to the Schottky electrode, a diamond having an oxygen-terminated diamond on the surface thereof is particularly suitable.
In the invention, as to preparation of an ohmic electrode as well, coventional materials and conventional processes may be used in any procedure.
In the followings, the invention will be more detailed with reference to examples. However, the invention is not limited to these examples.
In the beginning, on oxygen-terminated diamond where a p− film of 1.5 μm was deposited on a p+ film, by the use of an electron beam drawing unit, a Schottky electrode pattern having a diameter of 30 μm was prepared, followed by forming a Ru thin film by the use of an RF sputtering unit with a Ru target under conditions of RF output of 200 W and Ar gas flow rate of 10 sccm for 3 min (500 Å). In the next place, similarly, by the use of an electron beam drawing unit, a pattern for protecting a Schottky electrode with a resist was depicted, and, around a Schottky electrode, Al2O3 was formed by the use of an RF sputtering unit with a Al2O3 target under conditions of RF output of 200 W and Ar gas flow rate of 10 sccm for 70 min (1,000 Å). An ohmic electrode was formed in such a manner that a p− film was partially cut to reach a p+ layer by means of the ICP etching, and Ti, Pt and Au were sequentially deposited thereon, followed by annealing at 420° C. for 30 min in an RTA furnace. Consequently, a diamond semiconductor element provided with an insulating film layer around a Schottky electrode was obtained.
In the beginning, on oxygen-terminated diamond where a p− film of 1.5 μm was deposited on a p+ film, by the use of an electron beam drawing unit, a Schottky electrode pattern having a diameter of 30 μm was prepared, followed by forming a Ru thin film by the use of an RF sputtering unit with a Ru target under conditions of RF output of 200 W and Ar gas flow rate of 10 sccm for 3 min (500 Å). An ohmic electrode was formed in such a manner that a p− film was partially cut to reach a p+ layer by means of the ICP etching, and Ti, Pt and Au were sequentially deposited thereon, followed by annealing at 420° C. for 30 min in an RTA furnace. Consequently, a diamond semiconductor element provided with an insulating film layer around a Schottky electrode was obtained.
Of a high-output diamond semiconductor element obtained in example 1, voltage-current characteristics were measured. The result is shown in
In view of these results, it is found that disposition of an insulating film around a Schottky electrode in order for to inactivation is effective to suppress the leakage current and to homogenize the performance.
As to Si3N4 and SiO2 as well, except that Si3N4 and SiO2 targets were respectively used in place of an Al2O3 target, diamond semiconductor elements provided with an insulating film layer around a Schottky electrode were prepared in the same manner as in Example 1.
As a structure of Schottky diode, one obtained in such a manner that, a high concentration ohmic layer where boron had been doped at such a high concentration as 1020 or more was disposed on a Ib (0001) substrate by means of a CVD method, and as a drift layer of 10 μm, a p− low boron concentration layer was disposed thereon at a boron concentration of 5×1015. Herein, Pt (φ˜2.5 eV) was disposed on a p− drift layer as a Schottky electrode. Further, as an ohmic electrode, a part obtained by cutting a Ib substrate, and laminating Ti of 300 Å, Pt of 300 Å and Au of 1000 Å, respectively on a p+ film, followed by annealing at 420° C. for 30 min was used.
A pattern was drawn by the use of a mask using an EB resist or a photoresist, metal for forming a Schottky electrode was deposited, and a lift-off process where the resist is dissolved in a resist peeling liquid to remove an unnecessary portion was executed, to thereby obtain a Schottky electrode having a diameter of 30 μm and a thickness of 5,000 Å. Then, according to the similar patterning, Al2O3 was deposited at substantially 1.5 μm by the use of a sputtering device so as to have a diameter twice or more (60 μm or more) the diameter of the Schottky electrode. Thereafter, by the use of a known patterning technique, metal was deposited as a field plate to have a diameter of 45 μm and a thickness of 2,000 Å.
A size of an electrode was set at φ30 μm. An initial breakdown voltage became 880 V, when, with an insulation breakdown due to an avalanche breakdown as a main parameter, a parallel plate model was considered and a simulation was carried out by assuming an electric field of substantially 4.3 MV/cm at the maximum. When an electrode termination is applied on this diamond semiconductor element as shown below, an electric field concentration in diamond is relaxed to thereby largely improve the breakdown voltage.
In the beginning, a pattern was drawn by the use of a mask using an EB resist or a photoresist and SiO2 was deposited at 0.75 μm according to a known method. Subsequently, a pattern of a Schottky electrode was drawn by the use of an EB resist so as to overlap with the SiO2 pattern, and a hole having a diameter of 30 μm was formed for a Schottky electrode by the use of a wet etching method. Similarly, by the use of EB lithography, a pattern (diameter: 60 μm) larger than the hole for the Schottky electrode was drawn, and metal for forming the Schottky electrode was deposited thereto at a thickness of 10,000 Å. A lift-off process where the resist is dissolved in a resist peeling liquid to remove an unnecessary portion was executed, whereby a Schottky electrode having a diameter of contact surface between diamond and a Schottky metal of 30 μm and a diameter including a field plate of 60 μm was obtained.
Of the high-output diamond semiconductor element obtained in example 3, reverse breakdown voltage characteristics in a structure where a low dielectric constant insulating film (SiO2)-utilizing voltage resistance structure was used in a marginal electric field relaxation layer and a structure where a high dielectric constant insulating film-utilizing voltage resistance structure was used in a marginal electric field relaxation layer were compared (experimental results using Al2O3) and the results are shown in
Except that SrTiO3 was used in place of Al2O3 in a process carried out in example 3, an insulating film was prepared in the same manner as Example 3. Experimental results thereof are shown in
In
In this case, 4 points are considered as the places where an electric field concentrates. It is preferable that values of electric field intensities at points A, B, C each are desirably small and close to each other. At a point D, although tolerance ranges differ depending on insulating materials, the value of electric field intensity is desirably as small as possible. In the case of this diode, when a voltage of substantially 1500 V is applied, the minimum value of substantially from 2.2 to 2.3 MV/cm is expected.
As to SiO2 and Al2O3, values in structures optimized in
As described above, it is found that a high-output diamond semiconductor element of the invention may be diverted to a diamond Schottky barrier diode, a diamond pn diode, a diamond thyristor, a diamond transistor and a diamond field effect transistor, and is very high in the industrial applicability.
While the present invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the scope thereof.
This application is based on Japanese patent application No. 2007-217412 filed Aug. 23, 2007 and Japanese patent application No. 2007-251370 filed Sep. 27, 2007, the entire contents thereof being hereby incorporated by reference.
Further, all references cited herein are incorporated in their entireties.
| Number | Date | Country | Kind |
|---|---|---|---|
| P. 2007-217412 | Aug 2007 | JP | national |
| P. 2007-251370 | Sep 2007 | JP | national |
