Patent Description:
A technique of forming a β-Ga<NUM>O<NUM> single crystal film on a β-Ga<NUM>O<NUM>-based substrate by epitaxial crystal growth using the MBE (Molecular Beam Epitaxy) method is conventionally known (see, e.g., <CIT>).

According to <CIT>, the β-Ga<NUM>O<NUM> single crystal film can be grown at a high growth rate by the MBE method when the plane orientation of the principal plane of the β-Ga<NUM>O<NUM>-based substrate is adjusted to a predetermined plane orientation. <CIT> discloses a crystal laminate structure, in which crystals can be epitaxially grown on a β-Ga<NUM>O<NUM>-based substrate with high efficiency to produce a high-quality β-Ga<NUM>O<NUM>-based crystal film on the substrate; and a method for producing the crystal laminate structure. Provided is a crystal laminate structure(<NUM>)comprising: a β-Ga<NUM>O<NUM>-based substrate, of which the major face is a face that is rotated by <NUM> to <NUM>° inclusive with respect to face (<NUM>);and a β-Ga<NUM>O<NUM>-based crystal film which is formed by the epitaxial crystal growth on the major face of the β-Ga<NUM>O<NUM>-based based substrate. <CIT> discloses a method for efficiently growing a high-quality, large diameter β-Ga<NUM>O<NUM>-based single crystal film; and a crystalline layered structure having a β-Ga<NUM>O<NUM>-based single crystal film grown using this growing method. As one embodiment, the present invention provides a method for growing a & β-Ga<NUM>O<NUM>-based single crystal film by using the HVPE method, and including a step for exposing a Ga<NUM>O<NUM>-based substrate to a gallium chloride gas and an oxygen-containing gas, and growing a β-Ga<NUM>O<NUM>-based single crystal film on the principal surface (<NUM>) of the Ga<NUM>O<NUM>-based substrate at a growing temperature of <NUM> or higher. <CIT> discloses a β-Ga<NUM>O<NUM>-based single crystal substrate including a β-Ga<NUM>O<NUM>-based single crystal. The β-Ga<NUM>O<NUM>-based single crystal includes a full width at half maximum of an x-ray rocking curve of less than <NUM> seconds. <CIT> discloses a substrate for epitaxial growth, which enables the improvement in quality of a Ga-containing oxide layer that is formed on a β-Ga<NUM>O<NUM>-single-crystal substrate. A substrate for epitaxial growth comprises β-Ga<NUM>O<NUM> single crystals, wherein face (<NUM>) of the single crystals or a face that is inclined at an angle equal to or smaller than <NUM>° with respect to the face (<NUM>) is the major face. A crystal laminate structure comprises: the substrate for epitaxial growth; and epitaxial crystals which are formed on the major face of the substrate for epitaxial growth and each of which comprises a Ga-containing oxide.

It is an object of the invention to provide a semiconductor substrate comprising a β-Ga<NUM>O<NUM>-based single crystal on which an epitaxial layer comprising a β-Ga<NUM>O<NUM>-based single crystal can be grown by the HVPE method at a high growth rate; an epitaxial wafer having the semiconductor substrate and an epitaxial layer; and a method for producing the epitaxial wafer.

It is another object of the invention to provide a semiconductor substrate comprising a β-Ga<NUM>O<NUM>-based single crystal on which an epitaxial layer comprising a β-Ga<NUM>O<NUM>-based single crystal having good surface morphology can be grown by the HVPE method; an epitaxial wafer having the semiconductor substrate and an epitaxial layer; and a method for producing the epitaxial wafer.

The present invention is defined in the independent claims <NUM> and <NUM>. The dependent claims define embodiments of the invention.

According to en embodiment, it is possible to provide a semiconductor substrate comprising a β-Ga<NUM>O<NUM>-based single crystal on which an epitaxial layer comprising a β-Ga<NUM>O<NUM>-based single crystal can be grown by the HVPE method at a high growth rate; an epitaxial wafer having the semiconductor substrate and an epitaxial layer; and a method for producing the epitaxial wafer.

According to an embodiment, it is possible to provide a semiconductor substrate comprising a β-Ga<NUM>O<NUM>-based single crystal on which an epitaxial layer comprising a β-Ga<NUM>O<NUM>-based single crystal having good surface morphology can be grown by the HVPE method; an epitaxial wafer having the semiconductor substrate and an epitaxial layer; and a method for producing the epitaxial wafer.

<FIG> is a vertical cross-sectional view showing an epitaxial wafer <NUM> in the first embodiment. The epitaxial wafer <NUM> has a semiconductor substrate <NUM> and an epitaxial layer <NUM> formed on a principal plane <NUM> of the semiconductor substrate <NUM> by epitaxial crystal growth using the HVPE (Halide Vapor Phase Epitaxy) method.

The semiconductor substrate <NUM> is a substrate formed of a β-Ga<NUM>O<NUM>-based single crystal. The β-Ga<NUM>O<NUM>-based single crystal here means a β-Ga<NUM>O<NUM> single crystal or a crystal of which base crystal is a β-Ga<NUM>O<NUM> single crystal doped with an element such as Al or In. The composition of the β-Ga<NUM>O<NUM> single crystal doped with, e.g., Al and In is expressed by β-(GaxAlyIn(<NUM>-x-y))<NUM>O<NUM> (<NUM><x≤<NUM>, <NUM>≤y≤<NUM>, <NUM><x+y≤<NUM>). The band gap of the β-Ga<NUM>O<NUM> single crystal is widened by adding Al and is narrowed by adding In. The semiconductor substrate <NUM> may also contain a conductive impurity such as Sn.

β-Ga<NUM>O<NUM>-based crystals have a β-gallia structure belonging to the monoclinic system and the typical lattice constants of β-Ga<NUM>O<NUM> crystal not containing impurities are a<NUM>=<NUM>Å, b<NUM>=<NUM>Å, c<NUM>=<NUM>Å, α=γ=<NUM>° and β=<NUM>°.

To form the semiconductor substrate <NUM>, for example, 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.

The principal plane <NUM> of the semiconductor substrate <NUM> is a plane parallel to a [<NUM>] axis of the β-Ga<NUM>O<NUM>-based single crystal which constitutes the semiconductor substrate <NUM>. This is determined based on the finding by the present inventors that a growth rate of a β-Ga<NUM>O<NUM>-based single crystal layer epitaxially grown by the HVPE method and surface morphology of the β-Ga<NUM>O<NUM>-based single crystal layer can be controlled by adjusting an angle formed between the principal plane and the (<NUM>) plane of the β-Ga<NUM>O<NUM>-based single crystal when a plane orientation of the principal plane of the β-Ga<NUM>O<NUM>-based single crystal substrate is parallel to the [<NUM>] axis.

When the semiconductor substrate <NUM> has the principal plane <NUM> parallel to the [<NUM>] axis and is configured so that an angle formed between the principal plane <NUM> and a (<NUM>) plane of a β-Ga<NUM>O<NUM>-based single crystal constituting the semiconductor substrate <NUM> (hereinafter, referred to as "angle θ") is not less than -<NUM>° and not more than <NUM>° where an angle generated by rotation in a rotation direction of a right-handed screw advancing in the a-axis direction of the semiconductor substrate <NUM> is defined as a positive angle, the growth rate of the epitaxial layer <NUM> formed by the HVPE method can be increased. Furthermore, the growth rate of the epitaxial layer <NUM> formed by the HVPE method can be further increased by adjusting the angle θ to not less than <NUM>° and not more than <NUM>°, or not less than -<NUM>° and not more than -<NUM>°.

In addition, surface morphology of the epitaxial layer <NUM> formed by the HVPE method can be improved by adjusting the angle θ to not less than <NUM>° and not more than <NUM>°, or not less than -<NUM>° and not more than -<NUM>° where an angle generated by rotation in a rotation direction of a right-handed screw advancing in the a-axis direction of the semiconductor substrate <NUM> is defined as a positive angle.

Furthermore, surface morphology of the epitaxial layer <NUM> can be further improved by setting the angle θ to a value close to <NUM>° or -<NUM>°. When the semiconductor substrate <NUM> has, e.g., an average dislocation density of not more than <NUM>x<NUM><NUM>/cm<NUM>, the angle θ is set to not less than <NUM>° and not more than <NUM>°, or not less than -<NUM>° and not more than - <NUM>°. Then, when the semiconductor substrate <NUM> has an average dislocation density of not more than <NUM>x<NUM><NUM>/cm<NUM>, the angle θ is set to not less than <NUM>° and not more than <NUM>°, or not less than -<NUM>° and not more than -<NUM>°.

The β-Ga<NUM>O<NUM>-based single crystal has symmetry. Therefore, a plane coinciding with the (<NUM>) plane rotated about the [<NUM>] axis in the positive direction is equivalent to a plane coinciding with the (<NUM>) plane rotated in the negative direction. That is, the plane orientation of the principal plane <NUM> is equivalent regardless of whether the angle θ is positive or negative.

The epitaxial layer <NUM> is formed of a β-Ga<NUM>O<NUM>-based single crystal in the same manner as the semiconductor substrate <NUM>. The epitaxial layer <NUM> may also contain a conductive impurity such as Si.

A structure of the vapor phase deposition system used for growing the epitaxial layer <NUM> in the first embodiment will be described below as an example.

<FIG> is a vertical cross-sectional view showing the vapor phase deposition system <NUM> in the embodiment. The vapor phase deposition system <NUM> is a vapor phase deposition system using HVPE technique, and has a reaction chamber <NUM> having a first gas introducing port <NUM>, a second gas introducing port <NUM>, a third gas introducing port <NUM> and an exhaust port <NUM>, and a first heating means <NUM> and a second heating means <NUM> which are placed around the reaction chamber <NUM> to heat predetermined regions in the reaction chamber <NUM>.

The film growth rate when using the HVPE technique is higher than that in the PLD method, etc. In addition, in-plane distribution of film thickness is highly uniform and it is possible to grow a large-diameter film. Therefore, it is suitable for mass production of crystal.

The reaction chamber <NUM> has a source reaction region R1 in which a reaction container <NUM> containing a Ga source is placed and a gallium source gas is produced, and a crystal growth region R2 in which the semiconductor substrate <NUM> is placed and the epitaxial layer <NUM> is grown thereon. The reaction chamber <NUM> is formed of, e.g., quartz glass.

Here, the reaction container <NUM> is formed of, e.g., quartz glass and the Ga source contained in the reaction container <NUM> is metal gallium.

The first heating means <NUM> and the second heating means <NUM> are capable of respectively heating the source reaction region R1 and the crystal growth region R2 of the reaction chamber <NUM>. The first heating means <NUM> and the second heating means <NUM> are, e.g., resistive heaters or radiation heaters.

The first gas introducing port <NUM> is a port for introducing a Cl-containing gas (Cl<NUM> gas or HCl gas) into the source reaction region R1 of the reaction chamber <NUM> using an inert carrier gas (N<NUM> gas, Ar gas or He gas).

The second gas introducing port <NUM> is a port for introducing an oxygen-containing gas (O<NUM> gas or H<NUM>O gas, etc.) as an oxygen source gas and a chloride gas (e.g., silicon tetrachloride, etc.) used to add a dopant such as Si to the epitaxial layer <NUM>, into the crystal growth region R2 of the reaction chamber <NUM> using an inert carrier gas (N<NUM> gas, Ar gas or He gas).

The third gas introducing port <NUM> is a port for introducing an inert carrier gas (N<NUM> gas, Ar gas or He gas) into the crystal growth region R2 of the reaction chamber <NUM>.

A process of growing the epitaxial layer <NUM> in the present embodiment will be described below as an example.

Firstly, the source reaction region R1 of the reaction chamber <NUM> is heated by the first heating means <NUM> and an atmosphere temperature in the source reaction region R1 is then maintained at a predetermined temperature.

Next, in the source reaction region R1, a Cl-containing gas introduced through the first gas introducing port <NUM> using a carrier gas is reacted with the metal gallium in the reaction container <NUM> at the above-mentioned atmosphere temperature, thereby producing a gallium chloride gas.

The atmosphere temperature in the source reaction region R1 here is preferably a temperature at which GaCl gas has the highest partial pressure among component gases of the gallium chloride gas produced by the reaction of the metal gallium in the reaction container <NUM> with the Cl-containing gas. The gallium chloride gas here contains GaCl gas, GaCl<NUM> gas, GaCl<NUM> gas and (GaCl<NUM>) <NUM> gas, etc..

The temperature at which a driving force for growth of Ga<NUM>O<NUM> crystal is maintained is the highest with the GaCl gas among the gases contained in the gallium chloride gas. Growth at a high temperature is effective to obtain a high-quality Ga<NUM>O<NUM> crystal with high purity. Therefore, for growing the epitaxial layer <NUM>, it is preferable to produce a gallium chloride gas in which a partial pressure of GaCl gas having a high driving force for growth at a high temperature is high.

If hydrogen is contained in an atmosphere for growing the epitaxial layer <NUM>, surface flatness and a driving force for crystal growth of the epitaxial layer <NUM> decrease. Therefore, it is preferable that a Cl<NUM> gas not containing hydrogen be used as the Cl-containing gas.

In addition, to increase a partial pressure ratio of the GaCl gas in the gallium chloride gas, it is preferable that the metal gallium in the reaction container <NUM> be reacted with the Cl-containing gas in a state that the atmosphere temperature in the source reaction region R1 is maintained at not less than <NUM> by using the first heating means <NUM>.

Meanwhile, at the atmosphere temperature of, e.g., not less than <NUM>, the partial pressure ratio of the GaCl gas is predominantly high (the equilibrium partial pressure of the GaCl gas is four orders of magnitude greater than the GaCl<NUM> gas and is eight orders of magnitude greater than the GaCl<NUM> gas) and the gases other than GaCl gas hardly contribute to the growth of Ga<NUM>O<NUM> crystal.

Considering the lifetime of the first heating means <NUM> and heat resistance of the reaction chamber <NUM> formed of quartz glass, etc., it is preferable that the metal gallium in the reaction container <NUM> be reacted with the Cl-containing gas in a state that the atmosphere temperature in the source reaction region R1 is maintained at not more than <NUM>.

Next, in the crystal growth region R2, the gallium chloride gas produced in the source reaction region R1 is mixed with the oxygen-containing gas introduced through the second gas introducing port <NUM> and the semiconductor substrate <NUM> is exposed to the resulting mixed gas, thereby epitaxially growing the epitaxial layer <NUM> on the semiconductor substrate <NUM>. At this time, in a furnace housing the reaction chamber <NUM>, pressure in the crystal growth region R2 is maintained at, e.g., <NUM> atm.

When forming the epitaxial layer <NUM> containing an additive element such as Si or Al, a source gas of the additive element (e.g., a chloride gas such as silicon tetrachloride (SiCl<NUM>)) is introduced, together with the gallium chloride gas and the oxygen-containing gas, into the crystal growth region R2 through the gas introducing port <NUM>.

If hydrogen is contained in an atmosphere for growing the epitaxial layer <NUM>, surface flatness and a driving force for crystal growth of the epitaxial layer <NUM> decrease. Therefore, it is preferable that an O<NUM> gas not containing hydrogen be used as the oxygen-containing gas.

In addition, to suppress an increase in the equilibrium partial pressure of the GaCl gas and to efficiently grow the epitaxial layer <NUM>, the epitaxial layer <NUM> is preferably grown in a state that a ratio of the supplied partial pressure of the O<NUM> gas to the supplied partial pressure of the GaCl gas in the crystal growth region R2 is not less than <NUM>.

In addition, the growth temperature is preferably not less than <NUM> to grow a high-quality epitaxial layer <NUM>.

The epitaxial layer <NUM> contains, e.g., not more than <NUM>x<NUM><NUM> (atoms/cm<NUM>) of Cl. This results from that the epitaxial layer <NUM> is formed by the HVPE method using Cl-containing gas. Generally, Cl-containing gas is not used to form a Ga<NUM>O<NUM> single crystal film when using a method other than the HVPE method, and the Ga<NUM>O<NUM> single crystal film does not contain Cl, or at least does not contain <NUM>x<NUM><NUM> (atoms/cm<NUM>) or more of Cl.

A relation between the angle θ, which is formed between the principal plane <NUM> and the (<NUM>) plane of the β-Ga<NUM>O<NUM> single crystal constituting the semiconductor substrate <NUM>, and surface morphology of the epitaxial layer <NUM> was evaluated. The result is as follows.

<FIG>, <FIG> and 5A to 5C are optical microscope images showing surfaces of epitaxial layers. For the angle θ of the semiconductor substrate <NUM> as an underlying for the epitaxial layer <NUM>, the value is indicated at the top left of each image.

The epitaxial layers <NUM> shown in <FIG>, <FIG> and 5A to 5C are β-Ga<NUM>O<NUM> single crystal layers grown on the semiconductor substrates <NUM> formed of β-Ga<NUM>O<NUM> single crystal.

<FIG>, <FIG> and 5A to 5C show that the more the angle θ becomes closer to <NUM>° from <NUM>°, the shorter the length of the V-shaped groove appeared on the surface of the epitaxial layer <NUM> and the better the surface morphology.

Now, the V-shaped groove which deteriorates surface morphology of the epitaxial layer <NUM> will be described using <FIG>.

<FIG> is an optical microscope image showing a surface of the epitaxial layer <NUM> and <FIG> is a schematic vertical cross-sectional view showing the epitaxial layer <NUM> taken along a line A-A in <FIG>"L" and "T" in <FIG>respectively indicate the length of the V-shaped groove and the thickness of the epitaxial layer <NUM> (the depth of the V-shaped groove). The angle θ of the semiconductor substrate <NUM> shown in <FIG> is about +<NUM>°.

It is considered that dislocations in the semiconductor substrate <NUM> propagate into the epitaxial layer <NUM> and the V-shaped grooves are thereby formed. The inventors of the present application found that a dislocation line <NUM> inside the semiconductor substrate <NUM>, which is a starting point of V-shaped groove on the epitaxial layer <NUM>, appears at a position where the [<NUM>] axis is located when rotated by about <NUM>° in a rotation direction of a left-handed screw advancing in the a-axis direction of the semiconductor substrate <NUM> (in a counterclockwise direction in <FIG>).

<FIG> are scanning transmission electron microscope (STEM) bright-field images respectively showing the upper surface and the vertical cross section of the epitaxial wafer <NUM>. The angle θ of the semiconductor substrate <NUM> shown in <FIG> is about -<NUM>°. The dislocation line <NUM> inside the semiconductor substrate <NUM> can be seen in these images.

An angle formed between a straight line <NUM> of the bottom of the V-shaped groove and the principal plane <NUM> of the semiconductor substrate <NUM> is equal to the angle θ as described later. The angle θ in the epitaxial wafer <NUM> shown in <FIG> is as small as -<NUM>° and, in the STEM image with high magnification in <FIG>, the straight line <NUM> of the bottom of the V-shaped groove thus appears to coincide with the principal plane <NUM>. Meanwhile, the side surface of the V-shaped groove is substantially perpendicular to the principal plane <NUM> as shown in <FIG> but can be inclined in a very thin region in the vicinity of the principal plane <NUM>. The semiconductor substrate <NUM> and the epitaxial layer <NUM> shown in <FIG> are thinned for the purpose of STEM observation, and the inclined side surface of the V-shaped groove in the vicinity of the principal plane <NUM> is shown in <FIG>.

When the angle θ is set to <NUM>°, the dislocation lines <NUM> inside the semiconductor substrate <NUM> become parallel to the principal plane <NUM> and dislocations do not appear on the principal plane <NUM>. Thus, propagation of dislocations from the semiconductor substrate <NUM> into the epitaxial layer <NUM> hardly occurs. Also when the angle θ is -<NUM>°, the dislocation lines <NUM> become parallel to the principal plane <NUM> and the length L of the V-shaped groove is thus substantially zero.

Even when the dislocation lines <NUM> are not completely parallel to the principal plane <NUM>, distances between dislocations appeared on the principal plane <NUM> (appeared at intersections of the principal plane <NUM> and the dislocation lines <NUM>) increase as closer to parallel.

<FIG> are conceptual diagrams illustrating a relation between an angle φ, which is formed between the dislocation line <NUM> and the principal plane <NUM>, and a distance D<NUM> between dislocations appeared on the principal plane <NUM>. These conceptual diagrams are given on the assumption that the dislocation lines <NUM> are aligned at regular intervals.

When a distance between the dislocation lines <NUM> is defined as D<NUM>, the distance D<NUM> between dislocations appeared on the principal plane <NUM> is equal to D<NUM>/sinφ. Therefore, when the angle φ is changed on the assumption that the distance D<NUM> is always the same, the dislocation lines <NUM> become perpendicular to the principal plane <NUM> at φ=<NUM>° (when the angle θ is <NUM>° or -<NUM>°), and the distance D<NUM> becomes equal to the distance D<NUM> and is the smallest. Then, with the angle φ of closer to <NUM>° (the angle θ of about <NUM>° or -<NUM>°), the dislocation lines <NUM> are closer to parallel to the principal plane <NUM> and the distance D<NUM> increases.

When the semiconductor substrate <NUM> has, e.g., an average dislocation density of not more than <NUM>x<NUM><NUM>/cm<NUM>, the average distance D<NUM> between the dislocation lines <NUM> is <NUM> µm. In this case, when the absolute value of φ is smaller than about <NUM>°, the distance D<NUM> between dislocations appeared on the principal plane <NUM> is greater than <NUM> µm. That is, an average number of dislocations appeared on the principal plane <NUM> is less than <NUM>/mm<NUM>.

Table <NUM> below shows a relation between the average dislocation density of the semiconductor substrate <NUM> and the angle φ when the average number of dislocations appeared on the principal plane <NUM> is less than <NUM>/mm<NUM>.

φ is <NUM>° when the angle θ is about <NUM>° or -<NUM>°. Therefore, according to Table <NUM>, when the semiconductor substrate <NUM> has an average dislocation density of not more than <NUM>x<NUM><NUM>/cm<NUM>, the average number of dislocations appeared on the principal plane <NUM> can be <NUM>/mm<NUM> by setting the angle θ to not less than <NUM>° and not more than <NUM>°, or not less than -<NUM>° and not more than -<NUM>°.

In addition, when the semiconductor substrate <NUM> has an average dislocation density of not more than <NUM>x<NUM><NUM>/cm<NUM>, the average number of dislocations appeared on the principal plane <NUM> can be <NUM>/mm<NUM> by setting the angle θ to not less than <NUM>° and not more than <NUM>°, or not less than -<NUM>° and not more than -<NUM>°.

Furthermore, when the semiconductor substrate <NUM> has an average dislocation density of not more than <NUM>x<NUM><NUM>/cm<NUM>, the average number of dislocations appeared on the principal plane <NUM> can be <NUM>/mm<NUM> by setting the angle θ to not less than <NUM>° and not more than <NUM>°, or not less than -<NUM>° and not more than -<NUM>°.

Furthermore, when the semiconductor substrate <NUM> has an average dislocation density of not more than <NUM>x<NUM><NUM>/cm<NUM>, the average number of dislocations appeared on the principal plane <NUM> can be <NUM>/mm<NUM> by setting the angle θ to a value other than -<NUM>° and <NUM>°.

Meanwhile, the straight line <NUM> of the bottom of the V-shaped groove is parallel to the [<NUM>] axis of the β-Ga<NUM>O<NUM> single crystal constituting the semiconductor substrate <NUM>. Since the cross section shown in <FIG> is parallel to the b-axis of the β-Ga<NUM>O<NUM>-based single crystal constituting the semiconductor substrate <NUM>, the (<NUM>) plane of the semiconductor substrate <NUM> seen in this cross section coincides with the [<NUM>] axis. Thus, the angle θ formed between the principal plane <NUM> of the semiconductor substrate <NUM> and the [<NUM>] axis is equal to an angle formed between the principal plane <NUM> and the (<NUM>) plane (an angle formed between the straight line <NUM> of the bottom of the V-shaped groove and the principal plane <NUM>) as shown in <FIG>. Therefore, according to the calculation, the length L of the V-shaped groove becomes shorter as the angle θ becomes closer to <NUM>°.

<FIG> is a graph plotting data showing a relation between the length L of the V-shaped groove and the angle θ where an angle generated by rotation in a rotation direction of a right-handed screw advancing in the a-axis direction of the semiconductor substrate <NUM> is defined as a positive angle. The length L of the V-shaped groove indicated on the vertical axis in <FIG> is the calculated value obtained from the equation L=T/tanθ when the thickness T of the epitaxial layer <NUM> is <NUM> µm.

Based on the curved line shown in <FIG>, the length L of the V-shaped groove sharply decreases as θ becomes further from <NUM>°, the curvature becomes substantially zero at around <NUM>°, and after that, the length L of the V-shaped groove decreases gradually. Thus, to reduce the length L of the V-shaped groove, the angle θ is preferably not less than <NUM>° and not more than <NUM>°, and not less than -<NUM>° and not more than -<NUM>° equivalent thereto.

Table <NUM> below shows a relation between the angle θ, where an angle generated by rotation in a rotation direction of a right-handed screw advancing in the a-axis direction of the semiconductor substrate <NUM> is defined as a positive angle, and the measured values and the calculated values of the length L of the V-shaped groove.

Among the measured values of the length L of the V-shaped groove shown in Table <NUM>, the value at the angle θ of <NUM>° is substantially zero. This is because the dislocation lines <NUM> become parallel to the principal plane <NUM> and dislocations do not appear on the principal plane <NUM> and do not propagate into the epitaxial layer <NUM> as described above. The calculated values do not become zero since the calculation is performed on the assumption that dislocations propagate into the epitaxial layer <NUM>.

Meanwhile, the measured values of the length L of the V-shaped groove at the angle θ of <NUM>° and <NUM>° are smaller than the calculated values and are substantially zero. This is because when the length L of the V-shaped groove is smaller than the thickness T of the epitaxial layer <NUM>, the V-shaped groove is filled with the growing crystal in the process of growth and becomes shallow, and the length L becomes too short to measure by an optical microscope. When the length L is not more than about <NUM> µm, it is difficult to find by an optical microscope. Also when the angle θ is -<NUM>° and -<NUM>°, the length L of the V-shaped groove is too short to measure by an optical microscope and the measured values are substantially zero.

The measured value of the length L of the V-shaped groove is substantially zero also when the angle θ is <NUM>°. It is considered that this is because the [<NUM>] axis of the semiconductor substrate <NUM> is perpendicular to the principal plane <NUM> and the straight line <NUM> of the bottom of the V-shaped groove is thus perpendicular to the principal plane <NUM>. Also when the angle θ is -<NUM>°, the [<NUM>] axis of the semiconductor substrate <NUM> is perpendicular to the principal plane <NUM> and the measured value of the length L of the V-shaped groove is substantially zero.

As such, by setting the angle θ to not less than <NUM>° and not more than <NUM>° or not less than - <NUM>° and not more than -<NUM>°, it is possible to reduce the length L of the V-shaped groove down to the unmeasurable level by an optical microscope.

In addition, according to Table <NUM>, the measured values of the length L of the V-shaped groove are smaller than the calculated values when the angle θ is small (θ=<NUM> and <NUM>). This is because the V-shaped groove is filled with the growing crystal in the process of growth of the epitaxial layer <NUM> and becomes shallow.

Based on the measured value of L, the depth of the V-shaped groove (the length from the surface of the epitaxial layer <NUM> to the lowest portion of the V-shaped groove) at θ=<NUM> and <NUM> was calculated and the obtained values were respectively <NUM> µm and <NUM> µm which are smaller than the thickness of the epitaxial layer <NUM> shown in Table <NUM>. This shows that the V-shaped groove was filled in the process of crystal growth when θ=<NUM> and <NUM>.

<FIG> are schematic diagrams illustrating change in shape of the V-shaped groove when the angle θ is increased in the positive direction (in a rotation direction of a right-handed screw advancing in the a-axis direction of the semiconductor substrate <NUM>).

<FIG> are schematic diagrams illustrating change in shape of the V-shaped groove when the angle θ is increased in the negative direction (in a rotation direction of a left-handed screw advancing in the a-axis direction of the semiconductor substrate <NUM>).

<FIG> and <FIG> show the states in which the V-shaped groove is shallow and the lowest portion thereof is distant from the principal plane <NUM> of the semiconductor substrate <NUM>. The states at the angle θ of more than <NUM>° and not more than <NUM>° and at the angle θ of less than <NUM>° and not less than -<NUM>°described above correspond to the states shown in <FIG> and <FIG>. Thus, it is possible to remove the V-shaped groove by performing a polishing process such as CMP (Chemical Mechanical Polishing) on the surface of the epitaxial layer <NUM>. On the other hand, when the lowest portion of the V-shaped groove reaches the principal plane <NUM> of the semiconductor substrate <NUM>, it is not possible to remove the V-shaped groove while leaving a portion of the epitaxial layer <NUM>.

At θ=<NUM>°, the straight line <NUM> of the bottom of the V-shaped groove is parallel to the principal plane <NUM> and the length L of the V-shaped groove thus is very large (theoretically, infinity). Also in this case, the V-shaped groove is filled with the growing crystal in the process of growth of the epitaxial layer <NUM> and becomes shallow.

In addition, when the length L of the V-shaped groove is smaller than the thickness T of the epitaxial layer <NUM>, the V-shaped groove is filled with the growing crystal in the process of growth and becomes shallow, as described above. In other words, when the angle θ is more than <NUM>° and not more than <NUM>° and when the angle θ is less than -<NUM>° and not less than -<NUM>°, the V-shaped groove becomes shallow. Thus, the V-shaped grooves shown in <FIG> and <FIG> are shallow by being filled with the growing crystal and the lowest portions thereof are distant from the principal plane <NUM> of the semiconductor substrate <NUM>.

Therefore, when the angle θ is not less than -<NUM>° and not more than <NUM>°, more than <NUM>° and not more than <NUM>°, or less than -<NUM>° and not less than -<NUM>°, it is possible to remove the V-shaped groove by polishing the surface of the epitaxial layer <NUM>.

<FIG> are optical microscope images showing the surface of the epitaxial layer <NUM> on the semiconductor substrate <NUM> with the angle θ of <NUM>°, wherein <FIG> is an image before CMP and <FIG> is an image after CMP. <FIG> show that the V-shaped groove was removed while leaving a portion of the epitaxial layer <NUM>.

Similar evaluation results are obtained also when the semiconductor substrate <NUM> and the epitaxial layer <NUM> are respectively formed of β-Ga<NUM>O<NUM>-based single crystals other than the β-Ga<NUM>O<NUM> single crystal.

Table <NUM> below shows a relation between offset angles of the principal plane of a β-Ga<NUM>O<NUM> substrate from the (<NUM>) plane in the [<NUM>] axis direction and the [<NUM>] axis direction and the growth rate of a β-Ga<NUM>O<NUM> layer formed on the β-Ga<NUM>O<NUM> substrate by the HVPE method.

Table <NUM> shows that the growth rate of the β-Ga<NUM>O<NUM> layer is, e.g., <NUM> µm/h when grown on the β-Ga<NUM>O<NUM> substrate of which principal plane is inclined by -<NUM>° in the [<NUM>] axis direction and <NUM>° in the [<NUM>] axis direction from the (<NUM>) plane.

<FIG> is a graph plotting data showing a relation between an offset angle from the (<NUM>) plane in the [<NUM>] axis direction and the growth rate of the β-Ga<NUM>O<NUM> layer shown in Table <NUM>. <FIG> is a graph plotting data showing a relation between an offset angle from the (<NUM>) plane in the [<NUM>] axis direction and the growth rate of the β-Ga<NUM>O<NUM> layer shown in Table <NUM>.

As shown in <FIG>, no regularity is found in the relation between the growth rate and the offset angle from the (<NUM>) plane in the [<NUM>] axis direction, and clear dependence of the growth rate on the offset angle in the [<NUM>] axis direction is not identified.

On the other hand, as shown in <FIG>, in the relation between the growth rate and the offset angle from the (<NUM>) plane in the [<NUM>] axis direction, the growth rate is the lowest at the offset angle of around <NUM>° and increases as the offset angle becomes further from <NUM>°.

Table <NUM> below shows a relation between the angle θ of the semiconductor substrate <NUM> formed of a β-Ga<NUM>O<NUM> single crystal, i.e., the offset angle from the (<NUM>) plane in the [<NUM>] axis direction, and the growth rate of the epitaxial layer <NUM> formed of a β-Ga<NUM>O<NUM> single crystal.

<FIG> is a graph plotting data showing a relation between the angle θ and the growth rate of the epitaxial layer <NUM> shown in Table <NUM>. <FIG> is an enlarged graph showing the range of <NUM>°≤θ≤<NUM>° in <FIG>.

As shown in <FIG>, the growth rate of the epitaxial layer <NUM> is relatively high (not less than <NUM> µm/h) with the angle θ of not less than <NUM>° and not more than <NUM>°, and is particularly high (not less than <NUM> µm/h) with the angle θ of not less than <NUM>° and not more than <NUM>°. Due the symmetry of the β-Ga<NUM>O<NUM>-based single crystal constituting the semiconductor substrate <NUM>, the relation between the growth rate of the epitaxial layer <NUM> and the absolute value of the negative angle θ is the same as the relation between the growth rate of the epitaxial layer <NUM> and the absolute value of the positive angle θ. For this reason, it can be said that the growth rate of the epitaxial layer <NUM> is relatively high with the angle θ of not less than -<NUM>° and not more than <NUM>°, and is particularly high with the angle θ of not less than <NUM>° and not more than <NUM>°, or not less than -<NUM>° and not more than -<NUM>°.

For the relation between the angle θ and the growth rate of the epitaxial layer <NUM>, similar results are obtained also when the semiconductor substrate <NUM> and the epitaxial layer <NUM> are respectively formed of β-Ga<NUM>O<NUM>-based single crystals other than the β-Ga<NUM>O<NUM> single crystal.

The second embodiment is an embodiment of a semiconductor element including the epitaxial wafer <NUM> in the first embodiment. A lateral transistor having a MESFET (Metal Semiconductor Field Effect Transistor) structure will be described below as an example of such a semiconductor element.

<FIG> is a vertical cross-sectional view showing a lateral transistor <NUM> in the second embodiment. The lateral transistor <NUM> includes the epitaxial layer <NUM> formed on the semiconductor substrate <NUM>, and a gate electrode <NUM>, a source electrode <NUM> and a drain electrode <NUM> which are provided on the epitaxial layer <NUM>. The gate electrode <NUM> is arranged between the source electrode <NUM> and the drain electrode <NUM>.

The source electrode <NUM> and the drain electrode <NUM> are in contact with an upper surface of the epitaxial layer <NUM> (a surface opposite to the surface in contact with the semiconductor substrate <NUM>) and form ohmic junctions. Meanwhile, the gate electrode <NUM> is in contact with the upper surface of the epitaxial layer <NUM> and forms a Schottky junction, and a depletion layer is thereby formed in the epitaxial layer <NUM> under the gate electrode <NUM>. The lateral transistor <NUM> functions as either a normally-off transistor or a normally-on transistor depending on the thickness of this depletion region.

The semiconductor substrate <NUM> is formed of a β-Ga<NUM>O<NUM>-based crystal containing a p-type dopant such as Mg, Be, Zn or Fe, and has high electrical resistance.

The epitaxial layer <NUM> contains an n-type dopant such as Si or Sn. The n-type dopant concentration is higher around the contact portion with the source electrode <NUM> and the drain electrode <NUM> than in other portions. The thickness of the epitaxial layer <NUM> is, e.g., <NUM> to <NUM> µm.

The gate electrode <NUM>, the source electrode <NUM> and the drain electrode <NUM> are formed of, e.g., a metal such as Au, Al, Ti, Sn, Ge, In, Ni, Co, Pt, W, Mo, Cr, Cu and Pb, an alloy containing two or more of such metals, a conductive compound such as ITO, or a conductive polymer. The conductive polymer to be used is, e.g., a polythiophene derivative (PEDOT: poly(<NUM>,<NUM>)-ethylenedioxythiophene) doped with polystyrene sulfonate (PSS) or a polypyrrole derivative doped with TCNA, etc. In addition, the gate electrode <NUM> may have a two-layer structure composed of two different metals, e.g., Al/Ti, Au/Ni or Au/Co.

In the lateral transistor <NUM>, the thickness of the depletion layer in the epitaxial layer <NUM> under the gate electrode <NUM> is changed by controlling bias voltage applied to the gate electrode <NUM>, thereby controlling a drain current.

The lateral transistor <NUM> described above is an example of the semiconductor element including the epitaxial wafer <NUM> in the first embodiment, and the epitaxial wafer <NUM> can be used to manufacture various other semiconductor elements.

It is possible to manufacture, e.g., MISFET (Metal Insulator Semiconductor Field Effect Transistor) and HEMT (High Electron Mobility Transistor) in which the epitaxial layer <NUM> is used as a channel layer, and Schottky diode in which the semiconductor substrate <NUM> and the epitaxial layer <NUM> are respectively connected to an ohmic electrode and a Schottky electrode. Type and concentration of dopants contained in the semiconductor substrate <NUM> and the epitaxial layer <NUM> are appropriately determined according to the type of semiconductor element to be manufactured.

A region of the epitaxial layer <NUM> without any V-shaped grooves and a crystal layer formed on such region are used to manufacture semiconductor elements including the lateral transistor <NUM>. That is, the better the surface morphology of the epitaxial layer <NUM>, the more the semiconductor elements obtained from one wafer.

In the embodiment described above, the growth rate of the epitaxial layer <NUM> formed by the HVPE method can be further increased by adjusting the angle θ of the semiconductor substrate <NUM> to not less than <NUM>° and not more than <NUM>°. As a result, it is possible to efficiently form the epitaxial layer <NUM>. In addition, by growing the epitaxial layer <NUM> at a high growth rate, it is possible to reduce diffusion of impurities from the semiconductor substrate <NUM> and thus possible to increase quality.

Claim 1:
A semiconductor substrate that is used as an underlying substrate for epitaxial crystal growth carried out by the HVPE method, the semiconductor substrate comprising:
a β-Ga<NUM>O<NUM>-based single crystal; and
a principal plane that is a plane parallel to a [<NUM>] axis of the β-Ga<NUM>O<NUM>-based single crystal,
wherein θ is an angle formed between the principal plane and a (<NUM>) plane of the β-Ga<NUM>O<NUM>-based single crystal, i. e. an angle formed from the principal plane to a (<NUM>) plane of the β-Ga<NUM>O<NUM>-based single crystal, where an angle generated by rotation in a rotation direction of a right-handed screw advancing in the a-axis direction is defined as a positive angle, and wherein
i) the angle θ is not less than <NUM>° and not more than <NUM>°, or not less than - <NUM>° and not more than -<NUM>°, or
ii) the angle θ is not less than <NUM>° and not more than <NUM>°, or not less than -<NUM>° and not more than -<NUM>.