Patent Description:
A Schottky barrier diode, in which a Schottky electrode formed of Pt is connected to a Ga<NUM>O<NUM> single crystal, is known (see, e.g., Non-Patent Literature <NUM>). The turn-on voltage (forward voltage) of the Schottky barrier diode described in Non-Patent Literature <NUM> is <NUM> V.

Also, a Schottky barrier diode, in which a Schottky electrode having a Ni/Au stacked structure is connected onto a Ga<NUM>O<NUM> single crystal, is known (see, e.g., Non-Patent Literature <NUM>).

Also, a Schottky barrier diode, which has a Schottky electrode containing one selected from the group consisting of Au, Pd, Pt, Ni, Mo, W, Ta, Nb, Cr, Ag, In and Al, is known (see, e.g., Patent Literature <NUM>).

Also, a trench MOS Schottky barrier diode using Si as a semiconductor layer and a trench MOS Schottky barrier diode using SiC as a semiconductor layer are known (e.g., Non-Patent Literatures <NUM> and <NUM>).

<NPL>, discloses a Schottky barrier diode, where the anode electrode comprises Au and Cu.

<CIT> discloses A semiconductor film, a sheet like object, and a semiconductor device are provided that have inhibited semiconductor properties, particularly leakage current, and excellent withstand voltage and heat dissipation. A crystalline semiconductor film or a sheet like object includes a corundum structured oxide semiconductor as a major component, wherein the film has a film thickness of <NUM> micron or more. Particularly, the semiconductor film or the object includes a semiconductor component of oxide of one or more selected from gallium, indium, and aluminum as a major component. A semiconductor device has a semiconductor structure including the semiconductor film or the object.

<CIT> discloses a crystalline multilayer structure having good semiconductor properties. In particular, the crystalline multilayer structure has good electrical properties as follows: the dontrollability of conductivity is good; and vertical conduction is possible. A crystalline multilayer structure includes a metal layer containing a uniaxially oriented metal as a major component and a semiconductor layer disposed directly on the metal layer or with another layer therebetween and containing a crystalline oxide semiconductor as a major component. The crystalline oxide semiconductor contains one or more metals selected from gallium, indium, and aluminum and is uniaxially oriented.

In general, it is necessary to change the turn-on voltage of Schottky barrier diode according to the intended use thereof. Schottky barrier diode having a Ga<NUM>O<NUM>-based semiconductor layer is also required to have a turn-on voltage in a range different from the known Schottky barrier diodes, particularly, to have a low turn-on voltage so that forward loss can be kept low.

Thus, it is an object of the invention to provide a Schottky barrier diode which is formed using a Ga<NUM>O<NUM>-based semiconductor and has a lower turn-on voltage than the known Schottky barrier diodes.

To achieve the above-mentioned object, the present invention provides a Schottky barrier diode according to the independent claims. The dependent claims define embodiments of the invention.

According to the invention, it is possible to provide a Schottky barrier diode which is formed using a Ga<NUM>O<NUM>-based semiconductor and has a lower turn-on voltage than the known Schottky barrier diodes.

<FIG> is a vertical cross-sectional view showing a Schottky barrier diode <NUM> in the first embodiment. The Schottky barrier diode <NUM> is a vertical Schottky barrier diode and has a semiconductor layer <NUM>, an anode electrode <NUM> formed on one surface of the semiconductor layer <NUM>, and a cathode electrode <NUM> formed on another surface of the semiconductor layer <NUM>.

The semiconductor layer <NUM> is a plate-shaped member formed of a Ga<NUM>O<NUM>-based single crystal and is typically a Ga<NUM>O<NUM>-based substrate. The semiconductor layer <NUM> may be undoped (with no intentional doping) or may contain a dopant such as Si or Sn. A carrier concentration of the semiconductor layer <NUM> is preferably, e.g., not less than 1x10<NUM> cm-<NUM> and not more than 1x10<NUM> cm-<NUM>.

The Ga<NUM>O<NUM>-based single crystal here means a Ga<NUM>O<NUM> single crystal or, in embodiments not falling within the scope of the present invention, is a Ga<NUM>O<NUM> single crystal doped with an element such as Al or In, and may be, e.g., a (GaxAlyIn(<NUM>-x-y))<NUM>O<NUM> (<NUM><x≤<NUM>, <NUM>≤y<<NUM>, <NUM><x+y≤<NUM>) single crystal which is a Ga<NUM>O<NUM> single crystal doped with Al and In. The band gap is widened by adding Al and is narrowed by adding In. The Ga<NUM>O<NUM> single crystal mentioned above has, e.g., a β-crystal structure.

A thickness of the semiconductor layer <NUM> is preferably not less than <NUM> nm so that the Schottky barrier diode <NUM> can have sufficient withstand voltage characteristics. Withstand voltage of the Schottky barrier diode <NUM> is determined by the thickness and carrier concentration of the semiconductor layer <NUM>. There is no specific upper limit for the thickness of the semiconductor layer <NUM>. However, since electrical resistance in the thickness direction increases with an increase in the thickness, the semiconductor layer <NUM> is preferably as thin as possible provided that the required withstand voltage characteristics are obtained.

The semiconductor layer <NUM> may alternatively have a multilayer structure composed of two or more Ga<NUM>O<NUM>-based single crystal layers. In this case, the semiconductor layer <NUM> is composed of, e.g., a Ga<NUM>O<NUM>-based single crystal substrate and a Ga<NUM>O<NUM>-based single crystal film epitaxially grown thereon. In case that the anode electrode <NUM> is connected to the Ga<NUM>O<NUM>-based single crystal film and the cathode electrode <NUM> is connected to the Ga<NUM>O<NUM>-based single crystal substrate, for example, the carrier concentration of the Ga<NUM>O<NUM>-based single crystal film is set to not less than 1x10<NUM> cm-<NUM> and not more than 1x10<NUM> cm-<NUM> and the carrier concentration of the Ga<NUM>O<NUM>-based single crystal substrate is set to not less than 1x10<NUM> cm-<NUM> and not more than 4x10<NUM> cm-<NUM>.

The anode electrode <NUM> is configured so that a portion in contact with the semiconductor layer <NUM> is formed of Mo (molybdenum) or W (tungsten). In detail, the anode electrode <NUM> when having a single layer structure is entirely formed of Mo or W, and the anode electrode <NUM> when having a multilayer structure is configured so that a layer in contact with the semiconductor layer <NUM> is formed of Mo or W. In both cases, a Schottky barrier is formed at an interface between the Mo or W portion of the anode electrode <NUM> and the semiconductor layer <NUM>, and a Schottky junction is formed between the anode electrode <NUM> and the semiconductor layer <NUM>.

When the portion of the anode electrode <NUM> in contact with the semiconductor layer <NUM> is formed of Mo, the turn-on voltage of the Schottky barrier diode <NUM> is not less than <NUM>V and not more than <NUM>V. Meanwhile, when the portion of the anode electrode <NUM> in contact with the semiconductor layer <NUM> is formed of W, the turn-on voltage of the Schottky barrier diode <NUM> is also not less than <NUM>V and not more than <NUM>V.

The thickness of the Mo or W portion of the anode electrode <NUM> is preferably not less than <NUM> nm. When the thickness is less than <NUM> nm, pinholes may be formed and good rectifying properties may not be obtained. In contrast, when the Mo or W portion of the anode electrode <NUM> has a thickness of not less than <NUM> nm, good rectifying properties are obtained. In addition, when the anode electrode <NUM> has a single layer structure, differential on-resistance after the current value started to rise is reduced.

There is no upper limit for the thickness of the Mo or W portion of the anode electrode <NUM> in terms of performance of element.

When the anode electrode <NUM> has a stacked structure, e.g., an Au layer is stacked on a layer formed of Mo or W. The Au layer is used to reduce wiring resistance of the electrode itself. The thicker Au layer is better for reducing the wiring resistance, but the thickness of the Au layer is preferably not more than <NUM> µm in view of the manufacturing cost.

The cathode electrode <NUM> is configured so that a portion in contact with the semiconductor layer <NUM> is formed of a metal such as Ti forming an ohmic junction with Ga<NUM>O<NUM>-based single crystal and thus forms an ohmic junction with the semiconductor layer <NUM>. That is, the cathode electrode <NUM> when having a single layer structure is entirely formed of Ti, etc., and the cathode electrode <NUM> when having a multilayer structure is configured so that a layer in contact with the semiconductor layer <NUM> is formed of Ti, etc. Examples of the multilayer structure of the cathode electrode <NUM> include Ti/Au and Ti/Al.

In the Schottky barrier diode <NUM>, an energy barrier at an interface between the anode electrode <NUM> and the semiconductor layer <NUM> as viewed from the semiconductor layer <NUM> is lowered by applying forward voltage between the anode electrode <NUM> and the cathode electrode <NUM> (positive potential on the anode electrode <NUM> side), allowing a current to flow from the anode electrode <NUM> to the cathode electrode <NUM>. On the other hand, when reverse voltage is applied between the anode electrode <NUM> and the cathode electrode <NUM> (negative potential on the anode electrode <NUM> side), the current does not flow due to the Schottky barrier.

An example of a method for manufacturing the Schottky barrier diode <NUM> will be described below.

Firstly, a bulk crystal of a Ga<NUM>O<NUM>-based single crystal grown by a melt-growth technique such as the FZ (Floating Zone) method or the EFG (Edge Defined Film Fed Growth) method is sliced and the surface thereof is then polished, thereby forming a Ga<NUM>O<NUM>-based substrate as the semiconductor layer <NUM>.

Next, the front and back surfaces of the semiconductor layer <NUM> are pre-treated with a sulfuric acid/hydrogen peroxide mixture (e.g., with a volume ratio of sulfuric acid: hydrogen peroxide: water = <NUM>:<NUM>:<NUM>). In case that a treatment solution other than the sulfuric acid/hydrogen peroxide mixture, such as hydrochloric acid, nitric acid, sulfuric acid, hydrofluoric acid or buffered hydrofluoric acid, is used, treatment with the sulfuric acid/hydrogen peroxide mixture is performed after treatment with such solution. If treatment with the sulfuric acid/hydrogen peroxide mixture is not performed at the end of the pre-treatment, it could cause the turn-on voltage of the Schottky barrier diode <NUM> to be fixed to about <NUM>V to <NUM>V, regardless of the material of the anode electrode <NUM>.

Next, the anode electrode <NUM> and the cathode electrode <NUM> are respectively formed on the front and back surfaces of the semiconductor layer <NUM> by vacuum deposition, etc. The anode electrode <NUM> may be patterned into a predetermined shape such as circle by photo etching, etc..

<FIG> is a vertical cross-sectional view showing a trench MOS Schottky barrier diode <NUM> in the second embodiment. The trench MOS Schottky barrier diode <NUM> is a vertical Schottky barrier diode having a trench MOS region.

The trench MOS Schottky barrier diode <NUM> has a first semiconductor layer <NUM>, a second semiconductor layer <NUM> stacked thereon, an anode electrode <NUM> connected to the first semiconductor layer <NUM>, and a cathode electrode <NUM> connected to the second semiconductor layer <NUM>.

The first semiconductor layer <NUM> has trenches <NUM> opening on a surface <NUM> opposite to the second semiconductor layer <NUM>. Inner surfaces of the trenches <NUM> are covered with insulating films <NUM>, and trench MOS barriers <NUM> are buried in the trenches <NUM> so as to be covered with the insulating films <NUM>. The anode electrode <NUM> is in contact with the trench MOS barriers <NUM>.

The trench MOS Schottky barrier diode <NUM> has a field-plate structure to prevent insulation breakdown from occurring at an edge portion of the electrode and to improve withstand voltage. A dielectric film <NUM> formed of a dielectric material such as SiO<NUM> is provided on the surface <NUM> of the first semiconductor layer <NUM> so as to be located around the anode electrode <NUM>, and the edge of the anode electrode <NUM> rides over the dielectric film <NUM>.

In the trench MOS Schottky barrier diode <NUM>, an energy barrier at an interface between the anode electrode <NUM> and the first semiconductor layer <NUM> as viewed from the first semiconductor layer <NUM> is lowered by applying forward voltage between the anode electrode <NUM> and the cathode electrode <NUM> (positive potential on the anode electrode <NUM> side), allowing a current to flow from the anode electrode <NUM> to the cathode electrode <NUM>.

On the other hand, when reverse voltage is applied between the anode electrode <NUM> and the cathode electrode <NUM> (negative potential on the anode electrode <NUM> side), the current does not flow due to the Schottky barrier. When reverse voltage is applied between the anode electrode <NUM> and the cathode electrode <NUM>, a depletion layer spreads from an interface between the anode electrode <NUM> and the first semiconductor layer <NUM> and from an interface between the insulating films <NUM> and the first semiconductor layer <NUM>.

In general, the upper limit of reverse leakage current in Schottky barrier diode is <NUM> µA. In the embodiment, reverse voltage when a leakage current of <NUM> µA flows is defined as withstand voltage.

According to data of dependence of reverse leakage current on electric field strength at Schottky interface in Schottky diode having a SiC semiconductor layer described in, e.g., "<NPL>, electric field strength immediately under Schottky electrode is about <NUM> MV/cm when a current density of reverse leakage current is <NUM> A/cm<NUM>. <NUM> A/cm<NUM> here is a current density immediately under the Schottky electrode when a current of <NUM> µA flows through the Schottky electrode having a size of <NUM> mm × <NUM> mm.

Thus, even when breakdown field strength of the semiconductor material itself is several MV/cm, a leakage current of more than <NUM> µA flows when the electric field strength immediately under the Schottky electrode exceeds <NUM> MV/cm.

In order to obtain withstand voltage of <NUM>V in, e.g., a known Schottky diode not having a special structure to reduce electric field strength immediately under Schottky electrode, a donor concentration in a semiconductor layer needs to be reduced to the order of <NUM><NUM> cm-<NUM> and also the semiconductor layer needs to be very thick so that the electric field strength immediately under the Schottky electrode is kept not more than <NUM> MV/cm. This causes a significant increase in conduction loss, and it is thus difficult to make a Schottky barrier diode having a high withstand voltage and low loss.

The trench MOS Schottky barrier diode <NUM> in the second embodiment has a trench MOS structure and thus can have a high withstand voltage without an increase in resistance of the semiconductor layer. In other words, the trench MOS Schottky barrier diode <NUM> is a Schottky barrier diode having a high withstand voltage and low loss.

Junction barrier Schottky (JBS) diode is known as a Schottky barrier diode having a high withstand voltage and low loss. However, Ga<NUM>O<NUM> is not suitable as a material for the JBS diode requiring a p-type region since it is difficult to manufacture p-type Ga<NUM>O<NUM>.

The second semiconductor layer <NUM> is formed of an n-type Ga<NUM>O<NUM>-based single crystal containing a Group IV element, such as Si or Sn, as a donor. A donor concentration of the second semiconductor layer <NUM> is, e.g., not less than <NUM>. 0x10<NUM> and not more than <NUM>. 0x10<NUM> cm-<NUM>. A thickness Ts of the second semiconductor layer <NUM> is, e.g., <NUM> to <NUM> µm. The second semiconductor layer <NUM> is, e.g., a Ga<NUM>O<NUM>-based single crystal substrate.

The first semiconductor layer <NUM> is formed of an n-type Ga<NUM>O<NUM>-based single crystal containing a Group IV element, such as Si or Sn, as a donor. A donor concentration of the first semiconductor layer <NUM> is lower than the donor concentration of the second semiconductor layer <NUM>. The first semiconductor layer <NUM> is, e.g., an epitaxial layer epitaxially grown on the second semiconductor layer <NUM> which is a Ga<NUM>O<NUM>-based single crystal substrate.

A high-donor-concentration layer containing a high concentration of donor may be additionally formed between the first semiconductor layer <NUM> and the second semiconductor layer <NUM>. In other words, the first semiconductor layer <NUM> and the second semiconductor layer <NUM> may be stacked via the high-donor-concentration layer. The high-donor-concentration layer is used when, e.g., the first semiconductor layer <NUM> is epitaxially grown on the second semiconductor layer <NUM> as a substrate. At the early growth stage of the first semiconductor layer <NUM>, the amount of dopant incorporated thereinto is unstable and an acceptor impurity is diffused from the second semiconductor layer <NUM> as a substrate. Thus, in some cases, resistance increases in a region of the first semiconductor layer <NUM> close to the interface with the second semiconductor layer <NUM> when the first semiconductor layer <NUM> is grown directly on the second semiconductor layer <NUM>. The high-donor-concentration layer is used to avoid such problems. The concentration in the high-donor-concentration layer is set to be, e.g., higher than the concentration in the first semiconductor layer <NUM>, more preferably, higher than the concentration in the second semiconductor layer <NUM>.

As the donor concentration in the first semiconductor layer <NUM> increases, electrical field strength in each part of the trench MOS Schottky barrier diode <NUM> increases. The donor concentration in the first semiconductor layer <NUM> is preferably not more than about <NUM>. 0x10<NUM> cm-<NUM> to lower the maximum electric field strength in a region of the first semiconductor layer <NUM> immediately under the anode electrode <NUM>, the maximum electric field strength in the first semiconductor layer <NUM> and the maximum electric field strength in the insulating film <NUM>. On the other hand, as the donor concentration decreases, resistance of the first semiconductor layer <NUM> increases and the forward loss increases. Therefore, to obtain withstand voltage of, e.g., not more than <NUM>V, the donor concentration is preferably not less than <NUM>. 0x10<NUM> cm-<NUM>. To obtain higher withstand voltage, the donor concentration may be reduced to, e.g., about <NUM>. 0x10<NUM> cm-<NUM>.

As a thickness Te of the first semiconductor layer <NUM> increases, the maximum electric field strength in the first semiconductor layer <NUM> and the maximum electric field strength in the insulating film <NUM> decrease. By adjusting the thickness Te of the first semiconductor layer <NUM> to not less than about <NUM> µm, it is possible to effectively reduce the maximum electric field strength in the first semiconductor layer <NUM> and the maximum electric field strength in the insulating film <NUM>. In view of reduction in these maximum electric field strengths and downsizing of the trench MOS Schottky barrier diode <NUM>, the thickness Te of the first semiconductor layer <NUM> is preferably about not less than <NUM> µm and not more than <NUM> µm.

Electrical field strength in each part of the trench MOS Schottky barrier diode <NUM> changes depending on a depth Dt of the trench <NUM>. The depth Dt of the trench <NUM> is preferably about not less than <NUM> µm and not more than <NUM> µm to lower the maximum electric field strength in a region of the first semiconductor layer <NUM> immediately under the anode electrode <NUM>, the maximum electric field strength in the first semiconductor layer <NUM> and the maximum electric field strength in the insulating film <NUM>.

When a width Wt of the trench <NUM> is narrower, the conduction loss can be more reduced but it is more difficult to manufacture, causing a decrease in production yield. Therefore, the width Wt is preferably not less than <NUM> µm and not more than <NUM> µm.

As a width Wm of a mesa-shaped portion between adjacent trenches <NUM> on the first semiconductor layer <NUM> decreases, the maximum electric field strength in a region of the first semiconductor layer <NUM> immediately under the anode electrode <NUM> decreases. The width Wm of the mesa-shaped portion is preferably not more than <NUM> µm to lower the maximum electric field strength in a region of the first semiconductor layer <NUM> immediately under the anode electrode <NUM>. At the same time, the width Wm of the mesa-shaped portion is preferably not less than <NUM> µm since it is more difficult to make the trenches <NUM> when the width of the mesa-shaped portion is smaller.

Since the maximum electric field strength in the insulating film <NUM> decreases as permittivity of the insulating film <NUM> increases, the insulating film <NUM> is preferably formed of a high-permittivity material. For example, Al<NUM>O<NUM> (relative permittivity of about <NUM>) and HfO<NUM> (relative permittivity of about <NUM>) can be used as a material of the insulating film <NUM>, and it is particularly preferable to use HfO<NUM> which has high permittivity.

Meanwhile, as a thickness Ti of the insulating film <NUM> increases, the maximum electric field strength in the first semiconductor layer <NUM> decreases but the maximum electric field strength in the insulating film <NUM> and the maximum electric field strength in a region immediately under the anode electrode <NUM> increase. In view of ease of manufacturing, the thickness of the insulating film <NUM> is preferably smaller, and is more preferably not more than <NUM> nm. It is, however, obvious that a certain thickness is required so that a current virtually does not flow directly between the trench MOS barrier <NUM> and the second semiconductor layer <NUM>.

A length LFP of overlap between the anode electrode <NUM> and the dielectric film <NUM> is preferably not less than <NUM> µm so that the effect of the field-plate structure to improve withstand voltage is sufficiently exerted.

The anode electrode <NUM> is configured so that a portion in contact with the first semiconductor layer <NUM> is formed of Mo or W, and the anode electrode <NUM> is in Schottky contact with the first semiconductor layer <NUM>.

The material of the trench MOS barrier <NUM> is not specifically limited as long as it is electrically conductive, and it is possible to use, e.g., polycrystalline Si doped at a high concentration and a metal such as Ni or Au. However, when the trench MOS barriers <NUM> and the anode electrode <NUM> are formed integrally as shown in <FIG>, the surface layer of each trench MOS barrier <NUM> is also formed of Mo or W since the portion of the anode electrode <NUM> in contact with the first semiconductor layer <NUM> is formed of Mo or W.

<FIG> is an enlarged view showing the vicinity of the trench <NUM> when the trench MOS barriers <NUM> and the anode electrode <NUM> are formed integrally. The anode electrode <NUM> has a first layer <NUM>a in contact with the first semiconductor layer <NUM>, and a second layer <NUM>b formed thereon. The trench MOS barrier <NUM> has a first layer 26a in contact with the insulating film <NUM>, and a second layer <NUM>b formed thereon.

The first layer 23a of the anode electrode <NUM> and the first layers <NUM>a of the trench MOS barriers <NUM> are formed as a continuous single film of Mo or W. Likewise, the second layer <NUM>b of the anode electrode <NUM> and the second layers <NUM>b of the trench MOS barriers <NUM> are formed as a continuous single film of a conductive material such as Au.

When the portion of the anode electrode <NUM> in contact with the first semiconductor layer <NUM> (i.e., the first layer 23a) is formed of Mo or W, the turn-on voltage of the trench MOS Schottky barrier diode <NUM> is not less than <NUM>V and not more than <NUM>V. This turn-on voltage is slightly higher than that of the Schottky barrier diode <NUM> in the first embodiment even though the anode electrode material is the same, because the trench MOS structure causes a potential barrier to be formed in the mesa-shaped portion. This is determined by the width Wm of the mesa-shaped portion, such that the smaller the width Wm, the higher the turn-on voltage.

The electric field strength in the trench MOS Schottky barrier diode <NUM> is affected by the width of the mesa-shaped portion between two adjacent trenches <NUM>, the depth Dt of the trench <NUM> and the thickness Ti of the insulating film <NUM>, etc., as described above but is hardly affected by the planar pattern of the trenches <NUM>. Therefore, the planar pattern of the trenches <NUM> on the first semiconductor layer <NUM> is not specifically limited.

The cathode electrode <NUM> is in in ohmic contact with the second semiconductor layer <NUM>. The cathode electrode <NUM> is formed of a metal such as Ti. The cathode electrode <NUM> may have a multilayer structure formed by stacking different metal films, e.g., Ti/Au or Ti/Al. For reliable ohmic contact between the cathode electrode <NUM> and the second semiconductor layer <NUM>, the cathode electrode <NUM> is preferably configured that a layer in contact with the second semiconductor layer <NUM> is formed of Ti.

An example of a method for manufacturing the trench MOS Schottky barrier diode <NUM> will be described below.

<FIG>, <FIG>, <FIG>and <FIG>are vertical cross-sectional views showing a process of manufacturing the trench MOS Schottky barrier diode <NUM> in the second embodiment.

Firstly, as shown in <FIG>, a Ga<NUM>O<NUM>-based single crystal is epitaxially grown on the second semiconductor layer <NUM> such as a Ga<NUM>O<NUM>-based single crystal substrate by the HVPE (Hydride Vapor Phase Epitaxy) method, etc., thereby forming the first semiconductor layer <NUM>.

Next, as shown in <FIG>, the trenches <NUM> are formed on the upper surface of the first semiconductor layer <NUM> by photolithography and dry etching, etc..

In case that dry etching is used to form the trenches <NUM>, the preferable conditions are, e.g., use of BCl<NUM> (<NUM> sccm) as an etching gas, pressure of <NUM> Pa, antenna power of <NUM>W, bias power of <NUM>W, and duration of <NUM> minutes.

Treatment with phosphoric acid is preferably performed after forming the trenches <NUM> to remove roughness or plasma damage on inner surfaces of the trenches. Typically, immersion in phosphoric acid heated to <NUM> to <NUM>°C for <NUM> to <NUM> minutes is preferable.

Next, as shown in <FIG>, the insulating film <NUM> made of HfO<NUM>, etc., is formed on the upper surface of the first semiconductor layer <NUM> by the ALD (Atomic Layer Deposition) method. , etc., so that the inner surfaces of the trenches <NUM> are covered. The conditions for HfO<NUM> film formation are not specifically limited, and the film is formed by, e.g., alternately supplying TDMAH as an Hf raw material for <NUM> seconds and O<NUM> as an oxidizing agent for <NUM> seconds. The substrate temperature at this time is <NUM>°C.

Next, as shown in <FIG>, part of the insulating film <NUM> outside the trenches <NUM> (portions located on the mesa-shaped portions between the trenches <NUM>) is removed by a planarization process such as CMP (Chemical Mechanical Polishing).

Next, as shown in <FIG>, the dielectric film <NUM> is formed on the surface <NUM> of the first semiconductor layer <NUM>. To form the dielectric film <NUM>, for example, a SiO<NUM> film is deposited on the entire surface <NUM> by plasma CVD (Chemical Vapor Deposition) or sputtering and is then patterned by fluorine-based dry etching or wet etching with buffered hydrofluoric acid.

Next, as shown in <FIG>, the cathode electrode <NUM> having a Ti/Au stacked structure, etc., is formed on the bottom surface of the second semiconductor layer <NUM> by electron beam evaporation, etc. After that, heat treatment is performed in a nitrogen atmosphere at <NUM>°C for <NUM> minute. This heat treatment reduces contact resistance between the cathode electrode <NUM> and the second semiconductor layer <NUM>.

Next, as shown in <FIG>, the trench MOS barriers <NUM> and the anode electrode <NUM>, which have a Cu/Au/Ni stacked structure, etc., are formed continuously and integrally by electron beam evaporation, etc..

Before depositing the trench MOS barriers <NUM> and the anode electrode <NUM>, treatment with a sulfuric acid/hydrogen peroxide mixture is performed to remove CMP abrasive, etc. When a treatment solution other than the sulfuric acid/hydrogen peroxide mixture, such as hydrochloric acid, nitric acid, sulfuric acid, hydrofluoric acid or buffered hydrofluoric acid, is used, treatment with the sulfuric acid/hydrogen peroxide mixture is performed after treatment with such solution to prevent the turn-on voltage from being fixed to about <NUM> to <NUM>V.

Next, as shown in <FIG>, the anode electrode <NUM> is patterned into a predetermined shape such as circle by photolithography and wet etching, etc..

In the first and second embodiments, by using Mo or W as the material of the anode electrode which serves as a Schottky electrode, Schottky barrier diode having a semiconductor layer formed of a Ga<NUM>O<NUM>-based single crystal can have a lower turn-on voltage than the known Schottky barrier diodes.

Using Schottky barrier diodes having the same structure as the Schottky barrier diode <NUM> in the first embodiment, change in turn-on voltage when the material of the anode electrode serving as a Schottky electrode is different was examined.

In Example <NUM>, a <NUM> µm-thick Ga<NUM>O<NUM> substrate which was undoped (with no intentionally added donor) and had a donor concentration of about <NUM><NUM> cm-<NUM> was used as the semiconductor layer.

A circular electrode having a diameter of <NUM> µm was formed as the anode electrode by electron beam evaporation. Before depositing the anode electrode, the surface of the semiconductor layer was treated with a sulfuric acid/hydrogen peroxide mixture. Al, Ti, Mo, W, Fe, Cu, Ni, Pt and Pd were used as the anode electrode material.

Then, an electrode having a Ti/Au stacked structure formed by stacking a <NUM> nm-thick Ti film and a <NUM> nm-thick Au film was formed as the cathode electrode on a portion of the semiconductor layer by electron beam evaporation.

<FIG> is a graph showing a relation between a material of the anode electrode and turn-on voltage of the Schottky barrier diode in Example <NUM>.

<FIG> shows that the turn-on voltages of the Schottky barrier diodes, when the anode electrode material is Al, Ti, Mo, W, Fe, Cu, Ni, Pt and Pd, are respectively about <NUM>V, <NUM>V, <NUM>. 35V, <NUM>. 55V, <NUM>. 65V, <NUM>. 85V, <NUM>. 95V and <NUM>V.

Of those materials, Ni and Pt are known as materials of Schottky electrode to be in contact with semiconductor layer formed of Ga<NUM>O<NUM>-based single crystal. Mo and W provide different turn-on voltages from when using Ni and Pt and are thus usable as new Schottky electrode materials.

When the anode electrode is formed of Mo, the turn-on voltage of the Schottky barrier diode, including variation, is not less than <NUM>V and not more than <NUM>V. Meanwhile, when the anode electrode is formed of W, the turn-on voltage of the Schottky barrier diode, including variation, is also not less than <NUM>V and not more than <NUM>.

Although Ag is a material with a lower turn-on voltage than Mo and W, several experiments confirmed that its repeatability and reproducibility of turn-on voltage is very low and Ag is not suitable as an electrode material for Schottky barrier diode.

The trench MOS Schottky barrier diodes <NUM> in the second embodiment were made, and a relation between the width Wm of the mesa-shaped portion and device characteristics was examined. Also, the device characteristics of the trench MOS Schottky barrier diodes <NUM> were compared to those of a normal Schottky barrier diode not having trenches.

The configuration of the trench MOS Schottky barrier diodes <NUM> in Example <NUM> is as follows.

An Sn-doped Ga<NUM>O<NUM> substrate having a thickness of <NUM> µm and a donor concentration of 6x10<NUM> cm-<NUM> was used as the second semiconductor layer <NUM>. An Si-doped Ga<NUM>O<NUM> film having a thickness of <NUM> µm and a donor concentration of 6x10<NUM> cm-<NUM> was used as the first semiconductor layer <NUM>.

The trenches <NUM> had the depth Dt of about <NUM> µm and the width Wt of <NUM> µm, the mesa-shaped portion had the width Wm of <NUM> to <NUM> µm, and the length LFP of overlap between the anode electrode <NUM> and the dielectric film <NUM> was <NUM> µm. A <NUM> nm-thick HfO<NUM> film was used as the insulating film <NUM>.

A Mo/Au/Ni stacked film formed by stacking a <NUM> nm-thick Mo film, a <NUM> µm-thick Au film and a <NUM> nm-thick Ni film was used as the trench MOS barriers <NUM> and the anode electrode <NUM>. The Mo film and the Au film were buried in the trenches <NUM>. The portion to be the anode electrode <NUM> was patterned into a circular shape with a diameter of <NUM> µm. The Ni film as the outermost layer was formed to increase adhesion with photoresist used for the patterning.

A Ti/Au stacked film formed by stacking a <NUM> nm-thick Ti film and a <NUM> nm-thick Au film was used as the cathode electrode <NUM>. The cathode electrode <NUM> was formed on the entire back surface of the Sn-doped Ga<NUM>O<NUM> substrate and annealing was performed at <NUM>°C for <NUM> minute to reduce contact resistance with the Sn-doped Ga<NUM>O<NUM> substrate.

For the purpose of comparison, a sample without trenches (normal Schottky barrier diode) was also made on the same epi-wafer.

<FIG> shows forward characteristics of the trench MOS Schottky barrier diodes <NUM> in Example <NUM> and of the normal Schottky barrier diode in Comparative Example.

In the drawing, "trench SBD" denotes the trench MOS Schottky barrier diode <NUM> and "SBD" denotes the normal Schottky barrier diode as Comparative Example in which trenches are not formed. In addition, each of "<NUM> µm", "<NUM> µm", "<NUM> µm" and "<NUM> µm" indicates the width Wm of the mesa-shaped portion of the trench MOS Schottky barrier diode <NUM>.

<FIG> shows that on-resistance in the trench MOS Schottky barrier diode <NUM> increases with a decrease in the width Wm of the mesa-shaped portions. This is because the area of the trench <NUM> not serving as the current path was increased relative to the area of the mesa-shaped portions serving as the current path in a region under the anode electrode <NUM>, hence, it is a reasonable result.

On the other hand, the turn-on voltage of the trench MOS Schottky barrier diode <NUM> hardly depends on the width Wm of the mesa-shaped portions and was about <NUM>V in each case. The turn-on voltage of the trench MOS Schottky barrier diode <NUM> in Example <NUM>, including variation, is not less than <NUM>V and not more than <NUM>.

In addition, since W as an anode electrode material for the Schottky barrier diode <NUM> has properties close to those of Mo as described above, W when used in place of Mo in the trench MOS Schottky barrier diode <NUM> also exhibits properties close to those of Mo, and thus, the turn-on voltage including variation is not less than <NUM>V and not more than <NUM>.

In trench MOS Schottky barrier diode as is the trench MOS Schottky barrier diode <NUM> in the second embodiment, reverse leakage is effectively reduced when the turn-on voltage is not less than <NUM>V. This means that when using Mo or W as the anode electrode material, it is possible to lower the turn-on voltage while effectively reducing the reverse leakage.

<FIG> also shows that the trench MOS Schottky barrier diode <NUM> has a higher on-resistance than the normal Schottky barrier diode. This is due to the fact that the electric current path is narrowed by providing the trench MOS structure, hence, it is also a reasonable result.

<FIG> shows reverse characteristics of the trench MOS Schottky barrier diodes <NUM> in Example <NUM> and of the normal Schottky barrier diode in Comparative Example.

Based on <FIG>, the leakage current in the trench MOS Schottky barrier diode <NUM> is several orders of magnitude less than that in the normal Schottky barrier diode not having trenches, which confirmed that the trench MOS structure has the effect of increasing the withstand voltage. It was also found that the narrower the width Wm of the mesa-shaped portions, the smaller the reverse leakage current.

<FIG> shows forward characteristics of the trench MOS Schottky barrier diode <NUM> in Example <NUM> and of commercially available SiC Schottky barrier diodes in Comparative Example. The width Wm of the mesa-shaped portions in the trench MOS Schottky barrier diode <NUM> pertaining to <FIG> (described later) is <NUM> µm.

"SBD1", "SBD2" and "SBD3" in the drawing denote three different types of commercially available SiC Schottky barrier diodes.

Based on <FIG>, it was confirmed that the trench MOS Schottky barrier diode <NUM> using Mo for the anode electrode has a lower turn-on voltage than the commercially available SiC Schottky barrier diodes and operates with low loss.

<FIG> shows reverse characteristics of the trench MOS Schottky barrier diode <NUM> in Example <NUM> and of the commercially available SiC Schottky barrier diodes in Comparative Example.

Based on <FIG>, the reverse leakage current in the trench MOS Schottky barrier diode <NUM> is reduced to the same level as that in the commercially available SiC Schottky barrier diodes.

The results shown in <FIG> demonstrate the first-ever proof of performance of Ga<NUM>O<NUM> Schottky barrier diode which surpasses performance of SiC Schottky barrier diode.

Although the embodiments and Examples of the invention have been described, the invention is not intended to be limited to the embodiments and Examples.

In addition, the invention according to claims is not to be limited to the embodiments and Examples described above. Further, it should be noted that all combinations of the features described in the embodiments and Examples are not necessary to solve the problem of the invention.

The scope of the invention of defined exclusively by the appended claims.

Claim 1:
A Schottky barrier diode (<NUM>), comprising:
a semiconductor layer (<NUM>) comprising a Ga<NUM>O<NUM> single crystal;
an anode electrode (<NUM>) that forms a Schottky junction with a surface of the semiconductor layer (<NUM>) and is configured so that a portion in contact with the surface of the semiconductor layer (<NUM>) comprises Mo or W; and
a cathode electrode (<NUM>),
characterised in that, before forming the anode electrode (<NUM>), the surface of the semiconductor layer (<NUM>) is treated with a sulfuric acid/hydrogen peroxide mixture, thereby resulting in a turn-on voltage of the Schottky barrier diode of not less than <NUM> V and not more than <NUM> V.