Patent Description:
A method is known in which Mg, Be or Zn is doped so as to increase the resistivity of a Ga<NUM>O<NUM> single crystal (see e.g., PTL <NUM>). PTL <NUM> states that the resistivity of the Ga<NUM>O<NUM> single crystal can be increased by adding <NUM> mol% or <NUM> mol% of Mg in growing the Ga<NUM>O<NUM> single crystal by a FZ (Floating Zone) method.

<CIT> discloses a manufacturing method of a Ga<NUM>-xFexO<NUM> crystal which can form a superior, uniform, and large crystal. By a floating zone melting method in which ends of material bars (<NUM>, <NUM>), which are disposed at an upper and a lower position and which are composed of Ga<NUM>-xFexO<NUM>, are heated in a gas atmosphere with halogen lamps (<NUM>, <NUM>) disposed at confocal areas so as to form a floating melting zone between the ends of the material bars (<NUM>, <NUM>) which are disposed at the upper and the lower position and which are composed of Ga<NUM>-xFexO<NUM>, a Ga<NUM>-xFexO<NUM> single crystal having an orthorhombic crystal structure is formed.

<CIT> discloses that in a single crystal manufacturing unit, in a high pressure gas of not lower than <NUM> atmospheres by the floating melt zone method, a single crystal of orthorombic Ga<NUM>-xFexO<NUM> is manufactured. <CIT> solves a problem of providing a single crystal manufacturing unit obtaining a novel single crystal in a high pressure gas atmosphere by a floating melt zone method, and a high pressure single crystal manufacturing unit therewith.

<NPL>] discloses the luminescence of β-Ga<NUM>O<NUM> crystals and powder specimens in the temperature range <NUM>-<NUM>. The measurements have been performed on nominally pure samples and on samples that had been doped with aliovalent cations. In all samples an emission is observed at low temperatures in the ultraviolet (UV) region of the spectrum. We conclude that this emission is intrinsic, since neither its intensity nor its position was influenced by the impurity content or the history of the samples. Measurements of the optical density and the photoconductivity in the energy range where this UV emission can be excited reveal that the UV emission is excited in an interband transition. By analogy with the well-known instrinsic emission of the alkali halides, the intrinsic UV emission in β-Ga<NUM>O<NUM> is attributed to the recombination of an electron (hole) and a self-trapped hole (electron).

<CIT> discloses an ultraviolet sensor including the gallium oxide single-crystal substrate <NUM> which uses an orthogonal plane <NUM>, orthogonal with respect to the growing direction of gallium oxide single crystal or a surface tilted from the orthogonal plane <NUM> by a prescribed angle as a light receiving surface 12r; an ohmic electrode <NUM> formed on a first surface of the gallium oxide single crystal substrate <NUM>; and a Schottky electrode <NUM>, formed on a second surface of the gallium oxide single crystal substrate <NUM> including the light-receiving surface 12r. <CIT> solves the problem of providing an ultraviolet sensor and a method for manufacturing the same, using low-cost inexpensive gallium oxide single-crystal substrate.

As a raw material of Ga<NUM>O<NUM> single crystal, Ga<NUM>O<NUM> powder having a purity of not more than <NUM> mass% is widely used. It is technically possible to produce Ga<NUM>O<NUM> powder having a higher purity but it is not realistic in terms of cost. Ga<NUM>O<NUM> powder having a purity of not more than <NUM> mass% contains a trace amount of Si (donor impurity) as a residual impurity and a Ga<NUM>O<NUM> single crystal grown using such Ga<NUM>O<NUM> powder exhibits n-type conductivity. The concentration of Si included in the Ga<NUM>O<NUM> powder has a distribution in the Ga<NUM>O<NUM> single crystal. For example, the Ga<NUM>O<NUM> single crystal grown using Ga<NUM>O<NUM> powder having a purity of <NUM> mass% as a raw material has a Si concentration of about <NUM>×<NUM><NUM> cm-<NUM> at the most highly concentrated portion.

Therefore, in order to manufacture a high-resistivity Ga<NUM>O<NUM> substrate, the Ga<NUM>O<NUM> single crystal needs to be doped with an acceptor impurity at a concentration of at least <NUM>×<NUM><NUM> cm-<NUM> or more. If cheaper low-purity Ga<NUM>O<NUM> powder is used as a raw material of Ga<NUM>O<NUM> single crystal, it is necessary to dope the acceptor impurity at a higher concentration.

Generally, in doping a high-concentration impurity into a single crystal, a problem may arise that it is difficult to dope an impurity at a target concentration and that the crystal quality of the single crystal decreases due to the doping.

Thus, it is an object of the invention to provide a Ga<NUM>O<NUM>-based single crystal substrate comprising a Ga<NUM>O<NUM>-based single crystal that has a high resistance and while preventing a lowering of crystalline quality.

The present invention is defined in independent claim <NUM>. The dependent claims define embodiments of the invention. A production method, not claimed, for a Ga<NUM>O<NUM>-based single crystal substrate set forth in [<NUM>] to [<NUM>] below is provided so as to achieve the above object.

According to the invention, a Ga<NUM>O<NUM>-based single crystal substrate can be provided which comprises a Ga<NUM>O<NUM>-based single crystal that has a high resistance while preventing a lowering of crystalline quality, as well as a production method therefor.

[<FIG>]
<FIG> shows infrared-heating single crystal manufacturing equipment in an embodiment.

A Ga<NUM>O<NUM>-based single crystal substrate in the present embodiment is formed of a Ga<NUM>O<NUM>-based single crystal which contains Fe as an acceptor impurity in addition to a donor impurity such as Si such that the Fe concentration is higher than the donor impurity concentration. Therefore, the Ga<NUM>O<NUM>-based single crystal substrate in the present embodiment has high electrical resistance.

The principal surface of the Ga<NUM>O<NUM>-based single crystal substrate preferably has a size and a shape which are enough to include a perfect circle of not less than <NUM> in diameter. This size of the Ga<NUM>O<NUM>-based single crystal substrate is suitable for mass production. Typical examples include a square of not less than <NUM> in each side, a perfect circle of not less than <NUM> in diameter, a rectangle having short sides of not less than <NUM>, and an ellipse having a minor axis of not less than <NUM>.

The Ga<NUM>O<NUM>-based single crystal substrate in the present embodiment is cut from a Ga<NUM>O<NUM>-based single crystal containing Fe which is doped as an acceptor impurity.

The Ga<NUM>O<NUM>-based single crystal in the present embodiment is a Ga<NUM>O<NUM> single crystal, or a Ga<NUM>O<NUM> single crystal doped with elements such as Al and In. It 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.

By using Fe as an acceptor impurity, it is possible to dope a sufficient amount of acceptor while inhibiting a reduction in crystal quality, and thereby possible to grow a high-resistivity Ga<NUM>O<NUM>-based single crystal.

The method of growing a Ga<NUM>O<NUM>-based single crystal is not limited to a specific method and is, e.g., a FZ method, an EFG (Edge-defined Film-fed Growth) method or a CZ (Czochralski) method, etc. The reason why the method of growing a Ga<NUM>O<NUM>-based single crystal is not limited is considered that the effect of allowing a high-resistivity Ga<NUM>O<NUM>-based single crystal to grow while inhibiting a reduction in crystal quality is based on a solid solubility limit of Fe in a Ga<NUM>O<NUM>-based single crystal or the level of vapor pressure.

If a Ga<NUM>O<NUM>-based raw material having a purity of <NUM> mass% is used to grow the Ga<NUM>O<NUM>-based single crystal, Fe is added to the Ga<NUM>O<NUM>-based raw material such that the Fe concentration in the grown crystal is not less than <NUM>×<NUM><NUM> cm-<NUM>. To achieve this, Fe is added in an amount of, e.g., not less than <NUM> mol%. Thereby, in the grown Ga<NUM>O<NUM>-based single crystal, the Fe concentration is higher than the concentration of Si which is derived from the Ga<NUM>O<NUM>-based raw material and functions as a donor impurity.

If a Ga<NUM>O<NUM>-based raw material having a purity of <NUM> mass% is used to grow the Ga<NUM>O<NUM>-based single crystal, Fe is added to the Ga<NUM>O<NUM>-based raw material such that the Fe concentration in the grown crystal is not less than 5x10<NUM> cm-<NUM>. To achieve this, Fe is added in an amount of, e.g., not less than <NUM> mol%. Thereby, in the grown Ga<NUM>O<NUM>-based single crystal, the Fe concentration is higher than the concentration of Si which is derived from the Ga<NUM>O<NUM>-based raw material and functions as a donor impurity.

Here, when the Ga<NUM>O<NUM>-based single crystal is, e.g., a Ga<NUM>O<NUM> single crystal, the Ga<NUM>O<NUM>-based raw material is Ga<NUM>O<NUM> powder. Meanwhile, when the Ga<NUM>O<NUM>-based single crystal is a (GaxAlyIn(<NUM>-x-y))<NUM>O<NUM> (<NUM><x≤<NUM>, <NUM>≤y≤<NUM>, <NUM><x+y≤<NUM>) single crystal, a mixture of Ga<NUM>O<NUM> powder, Al<NUM>O<NUM> powder and In<NUM>O<NUM> powder is used.

The Ga<NUM>O<NUM>-based single crystal in the present embodiment is, e.g., a β-Ga<NUM>O<NUM>-based single crystal but may be a Ga<NUM>O<NUM>-based single crystal having another structure such as α-Ga<NUM>O<NUM>-based single crystal. Likewise, the Ga<NUM>O<NUM>-based single crystal substrate is, e.g., a β-Ga<NUM>O<NUM>-based single crystal substrate but may be a Ga<NUM>O<NUM>-based single crystal having another structure such as α-Ga<NUM>O<NUM>-based single crystal substrate.

Next, a method using FZ technique will be described as an example of the production method for a Ga<NUM>O<NUM>-based single crystal substrate.

<FIG> shows infrared-heating single crystal manufacturing equipment in the embodiment. The infrared-heating single crystal manufacturing equipment <NUM> is to grow a Ga<NUM>O<NUM>-based single crystal <NUM> using FZ technique, and has a quartz tube <NUM>, a seed chuck <NUM> for holding a seed crystal <NUM> formed of a Ga<NUM>O<NUM>-based single crystal, a vertically-movable lower rotating shaft <NUM> to transmit rotation to the seed chuck <NUM>, a raw material chuck <NUM> for holding a polycrystalline material <NUM> formed of a Ga<NUM>O<NUM>-based polycrystal, a vertically-movable upper rotating shaft <NUM> to transmit rotation to the raw material chuck <NUM>, and an oval mirror <NUM> which houses halogen lamps <NUM> and collects light emitted from the halogen lamps <NUM> to a predetermined position of the polycrystalline material <NUM>.

The quartz tube <NUM> houses the seed chuck <NUM>, the lower rotating shaft <NUM>, the raw material chuck <NUM>, the upper rotating shaft <NUM>, the seed crystal <NUM>, the polycrystalline material <NUM> and the Ga<NUM>O<NUM>-based single crystal <NUM>. A mixture gas of an oxygen gas and a nitrogen gas as an inert gas is supplied into the quartz tube <NUM> and is hermetically-sealed therein.

An upper edge of the seed crystal <NUM> is brought into contact with a lower edge of the polycrystalline material <NUM> by adjusting a vertical position of the upper rotating shaft <NUM>, and in this state, a contact portion therebetween is heated and melted by collecting light of the halogen lamp <NUM> thereto. Then, the heated portion is moved by pulling the polycrystalline material <NUM> upward while appropriately rotating both or one of the seed crystal <NUM> and the polycrystalline material <NUM>, thereby growing the Ga<NUM>O<NUM>-based single crystal <NUM> to which crystal information of the seed crystal <NUM> is transferred.

In <FIG> which shows the Ga<NUM>O<NUM>-based single crystal <NUM> in the middle of growth, the upper side of a melted portion <NUM> melted by heat is the polycrystalline material <NUM> and the lower side is the Ga<NUM>O<NUM>-based single crystal <NUM>.

Next, a specific process of growing a Ga<NUM>O<NUM> single crystal as the Ga<NUM>O<NUM>-based single crystal <NUM> using the infrared-heating single crystal manufacturing equipment <NUM> will be described.

Firstly, the seed crystal <NUM>, which is formed of a β-Ga<NUM>O<NUM> single crystal, and the polycrystalline material <NUM>, which is formed of a Fe-containing β-Ga<NUM>O<NUM> polycrystal produced by adding Fe to Ga<NUM>O<NUM> powder having a purity of <NUM> mass%, are prepared separately. Here, it is possible to use pure Fe or Fe oxide as a raw material of Fe to be added to the Ga<NUM>O<NUM> powder.

Next, in the quartz tube <NUM>, the seed crystal <NUM> is bought into contact with the polycrystalline material <NUM> and the contact portion is heated such that both the seed crystal <NUM> and the polycrystalline material <NUM> are melted at the contact portion. Once the molten polycrystalline material <NUM> is crystallized together with the seed crystal <NUM>, a Ga<NUM>O<NUM> single crystal as the Ga<NUM>O<NUM>-based single crystal <NUM> containing Fe is produced above the seed crystal <NUM>.

Here, the grown Ga<NUM>O<NUM> single crystal as the Ga<NUM>O<NUM>-based single crystal <NUM> has a size such that a Ga<NUM>O<NUM> single crystal substrate with a principal surface having a size and a shape enough to include a perfect circle of not less than <NUM> mm in diameter can be cut out.

Next, the Ga<NUM>O<NUM> single crystal is processed by cutting, etc., thereby obtaining a high-resistivity Ga<NUM>O<NUM> single crystal substrate.

When Fe was added in an amount of <NUM> mol% and <NUM> mol%, cracks were not generated on the Ga<NUM>O<NUM> single crystal in both cases and a Ga<NUM>O<NUM>-based single crystal substrate having a square principal surface of not less than <NUM> mm in each side was obtained.

The Fe concentration in the obtained Ga<NUM>O<NUM> single crystal was about 5x10<NUM> cm-<NUM> when adding <NUM> mol% of Fe and was about <NUM>×<NUM><NUM> cm-<NUM> when adding <NUM> mol% of Fe. Resistivity of the Ga<NUM>O<NUM> single crystal substrate with <NUM> mol% of Fe was about <NUM>×<NUM><NUM> Ωcm.

For the purpose of comparison with the present embodiment, an element other than Fe was doped as an acceptor impurity into the Ga<NUM>O<NUM> single crystal as Comparative Example. The experimental result is described below. The experimental conditions, except an acceptor impurity to be doped, were the same as those for the above-mentioned test using Fe for doping.

When the Ga<NUM>O<NUM> single crystal was grown with Mg added at <NUM> mol%, cracks were easily generated on the grown crystal and it was not possible to cut out a Ga<NUM>O<NUM> single crystal substrate having a square principal surface of not less than <NUM> in each side. It is considered that this is because the solid solubility limit of Mg in the Ga<NUM>O<NUM> single crystal is lower than that of Fe.

Based on this result, another Ga<NUM>O<NUM> single crystal was then grown with Mg added at <NUM> mol%. As a result, generation of cracks was reduced and a substrate having a suitable size for mass production was cut out but the substrate did not exhibit high resistivity. The Mg concentration in the substrate was about <NUM> to <NUM>×<NUM><NUM> cm-<NUM> and was less than the Si concentration which was about <NUM>×<NUM><NUM> cm-<NUM> in the high concentration region. From this result, it was found that it is necessary to use expensive Ga<NUM>O<NUM> powder having a purity of not less than <NUM>% and having a lower Si concentration when Mg is doped to increase resistivity of the Ga<NUM>O<NUM> single crystal without generation of cracks.

Meanwhile, when <NUM> mol% of Zn was added to the Ga<NUM>O<NUM> powder for the purpose of doping Zn into the Ga<NUM>O<NUM> single crystal, Zn was evaporated during calcination for making a rod-shaped polycrystalline material and the grown Ga<NUM>O<NUM> single crystal did not exhibit high resistivity. The Zn concentration in the grown Ga<NUM>O<NUM> single crystal analyzed by SIMS (secondary ion mass spectrometry) was below the lower detection limit (not more than <NUM>×<NUM><NUM> cm-<NUM>). It was found from this result that it is difficult to dope Zn which has a high vapor pressure.

According to the embodiment, use of Fe as an acceptor impurity allows a high-resistivity Ga<NUM>O<NUM>-based single crystal to be grown while inhibiting a reduction in crystal quality thereof, and a Ga<NUM>O<NUM>-based single crystal substrate having a suitable size for mass production is obtained from such a high-resistivity Ga<NUM>O<NUM>-based single crystal at low cost.

High-resistivity Ga<NUM>O<NUM>-based single crystal substrates can be used to manufacture, e.g., Ga<NUM>O<NUM>-based transistors and use of the Ga<NUM>O<NUM>-based single crystal substrate in the present embodiment thus allows Ga<NUM>O<NUM>-based transistors to be mass-produced. Since Ga<NUM>O<NUM>-based transistors are expected to have lower loss and higher breakdown voltage than GaN-based transistors or SiC-based transistors which have been being developed as next-generation power device materials, a global-scale significant energy-saving effect is expected if mass production of Ga<NUM>O<NUM>-based transistors is achieved.

For example, although Si is mentioned as an example of a donor impurity contained in the Ga<NUM>O<NUM>-based single crystal in the embodiment, the donor impurity is not limited to Si and may be a group IV element equivalent to Si. One electron is produced by substitution of a group IV element equivalent to Si for a Ga atom in the Ga<NUM>O<NUM>-based single crystal, meaning that the group IV element equivalent to Si functions as a donor impurity in the same manner as Si. Even in this case, the effects of the embodiment described above are obtained in the same manner as the case where Si is used as a donor impurity.

Claim 1:
A Ga<NUM>O<NUM>-based single crystal substrate having a high electrical resistance, consisting of a β-Ga<NUM>O<NUM> single crystal comprising Si as a donor impurity and Fe,
wherein a concentration of the Si is <NUM>×<NUM><NUM> cm-<NUM> at the most highly concentrated portion in the β-Ga<NUM>O<NUM> single crystal,
wherein a concentration of the Fe in the β-Ga<NUM>O<NUM> single crystal is not less than <NUM>×<NUM><NUM> cm-<NUM> and is higher than the concentration of the Si, and
wherein the substrate comprises a principal surface having a size and a shape that includes a perfect circle of <NUM> in diameter.