Patent Description:
A conventional semiconductor element in which a Ga-containing oxide is deposited on an element substrate formed of a β-Ga<NUM>O<NUM> single crystal is known (see, e.g., PTL <NUM>).

This type of semiconductor element is formed by laminating a layer(s) of n-type or p-type conductivity on a main surface of a β-Ga<NUM>O<NUM> single crystal substrate using a physical vapor-phase growth method such as MBE (Molecular Beam Epitaxy) or a chemical vapor-phase growth method such as CVD.

In addition, a (<NUM>) plane having high cleavability and thus providing a flat surface easily is often used as the main surface of the β-Ga<NUM>O<NUM> single crystal substrate (see, e.g., PTL <NUM>).

Further prior art related to this field of technology can be found in <CIT> disclosing β-Ga<NUM>O<NUM> growth on β-Ga<NUM>O<NUM> (<NUM>), (<NUM>) and (<NUM>) substrate surfaces by pulsed laser deposition. Still further prior art is known from <CIT> and <CIT>.

In general, growth temperature needs to be high to some extent in order that high-quality crystals without mixture of heterogeneous phase are formed by epitaxial growth. However, when a crystal is epitaxially grown on a β-Ga<NUM>O<NUM> single crystal substrate having a (<NUM>) plane as a main surface, a growth rate tends to decrease with increasing growth temperature of the crystal. It is believed that this is because raw materials of the crystal re-evaporate from the substrate and this causes a problem that the raw materials are wasted.

It is an object of the invention to provide a crystal laminate structure that 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, and a method for producing the crystal laminate structure.

According to one embodiment of the invention, a method for producing a crystal laminate structure as defined in the claims is provided so as to achieve the above object.

According to an embodiment of the invention, a crystal laminate structure can be provided that 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, as well as a method for producing the crystal laminate structure.

According to the present embodiment, β-Ga<NUM>O<NUM>-based crystals can be epitaxially grown on a β-Ga<NUM>O<NUM>-based substrate with high efficiency to form a high-quality β-Ga<NUM>O<NUM>-based crystal film without mixture of heterogeneous phase. In conventional methods in which a β-Ga<NUM>O<NUM>-based crystal is epitaxially grown on a β-Ga<NUM>O<NUM>-based substrate having a (<NUM>) plane as a main surface, a sufficient growth rate is not obtained at a growth temperature required to grow high-quality crystals, e.g., at a growth temperature of not less than <NUM>°C, and it is not possible to efficiently grow crystals. However, the present inventors found that, when a β-Ga<NUM>O<NUM>-based substrate of which main face is a plane rotated by not less than <NUM>° and not more than <NUM>° with respect a (<NUM>) plane is used as a base for epitaxial crystal growth, high-quality β-Ga<NUM>O<NUM>-based crystals are grown at a sufficient rate. Examples of embodiments thereof will be described in detail below.

<FIG> is a cross sectional view showing a crystal laminate structure in the first embodiment. A crystal laminate structure <NUM> includes a β-Ga<NUM>O<NUM>-based substrate <NUM> and a β-Ga<NUM>O<NUM>-based crystal film <NUM> formed on a main surface <NUM> of the β-Ga<NUM>O<NUM>-based substrate <NUM>.

The main surface <NUM> of the β-Ga<NUM>O<NUM>-based substrate <NUM> is a plane which is rotated by not less than <NUM>° and not more than <NUM>° with respect the (<NUM>) plane. In other words, in the β-Ga<NUM>O<NUM>-based substrate <NUM>, an angle θ (<NUM><θ≤<NUM>°) formed between the main surface <NUM> and the (<NUM>) plane is not less than <NUM>°. Examples of the plane rotated by not less than <NUM>° and not more than <NUM>° with respect the (<NUM>) plane include a (<NUM>) plane, a (<NUM>) plane, a (-<NUM>) plane, a (<NUM>) plane and a (<NUM>) plane.

When the main surface <NUM> of the β-Ga<NUM>O<NUM>-bascd substrate <NUM> is a plane rotated by not less than <NUM>° and not more than <NUM>° with respect the (<NUM>) plane, it is possible to effectively suppress re-evaporation of raw materials of the β-Ga<NUM>O<NUM>-based crystal from the β-Ga<NUM>O<NUM>-based substrate <NUM> at the time of epitaxially growing the β-Ga<NUM>O<NUM>-based crystal on the β-Ga<NUM>O<NUM>-bascd substrate <NUM>. In detail, where a percentage of the re-evaporated raw material during growth of the β-Ga<NUM>O<NUM>-based crystal at a growth temperature of <NUM>°C is defined as <NUM>%, the percentage of the re-evaporated raw material can be suppressed to be not more than <NUM>% when the main surface <NUM> of the β-Ga<NUM>O<NUM>-based substrate <NUM> is a plane rotated by not less than <NUM>° and not more than <NUM>° with respect the (<NUM>) plane. It is thus possible to use not less than <NUM>% of the supplied raw material to form the β-Ga<NUM>O<NUM>-based crystal, which is preferable from the viewpoint of growth rate and manufacturing cost of the β-Ga<NUM>O<NUM>-bascd crystal.

The β-Ga<NUM>O<NUM>-based substrate <NUM> is formed of, e.g., a β-Ga<NUM>O<NUM> single crystal. The β-Ga<NUM>O<NUM> crystal has a monoclinic crystal structure and typically has lattice constants of a=<NUM>Å, b=<NUM>Å, c=<NUM>Å, α=γ=<NUM>° and β=<NUM>°.

In the β-Ga<NUM>O<NUM> crystal, the (<NUM>) plane comes to coincide with the (<NUM>) plane when rotated by <NUM>° around the c-axis and comes to coincide with the (<NUM>) plane when rotated by <NUM>°. Meanwhile, the (<NUM>) plane comes to coincide with the (<NUM>) plane when rotated by <NUM>° around the b-axis, comes to coincide with the (<NUM>) plane when rotated by <NUM>° and comes to coincide with the (-<NUM>) plane when rotated by <NUM>°.

It should be noted that, the β-Ga<NUM>O<NUM>-based substrate <NUM> is basically formed of a β-Ga<NUM>O<NUM> single crystal as described above but may be formed of an oxide including mainly Ga doped with one or more elements selected from the group consisting of Cu, Ag, Zn, Cd, Al, In, Si, Ge and Sn. It is possible to control a lattice constant, bandgap energy or electrical conduction properties by adding such elements. It is possible to use the β-Ga<NUM>O<NUM>-based substrate <NUM> formed of, e.g., a (GaxAlyIn(<NUM>-x-y))<NUM>O<NUM> (where <NUM><x≤<NUM>, <NUM>≤y≤<NUM>, <NUM><x+y≤<NUM>) crystal which is a β-Ga<NUM>O<NUM> crystal doped with Al and In. The band gap is widened by adding Al and is narrowed by adding In.

When the elements listed above are added to the β-Ga<NUM>O<NUM> crystal, lattice constants may slightly change. Even in such a case, the (<NUM>) plane, the (<NUM>) plane, the (-<NUM>) plane, the (<NUM>) plane and the (<NUM>) plane still fall under the category of a plane rotated by not less than <NUM>° and not more than <NUM>° with respect the (<NUM>) plane.

The β-Ga<NUM>O<NUM>-based crystal film <NUM> is formed of a β-(Al<NUM>-xGax)<NUM>O<NUM> crystal (<NUM><x≤<NUM>), e.g., a β-Ga<NUM>O<NUM> crystal (when x=<NUM>). In addition, the β-Ga<NUM>O<NUM>-based crystal film <NUM> may contain conductive impurities.

Firstly, an ingot for forming the β-Ga<NUM>O<NUM>-based substrate <NUM> is produced by, e.g., the FZ (Floating Zone) method or the EFG (Edge Defined Film Fed Growth) method, etc..

In case of using the FZ method, an ingot is formed using, e.g., an infrared-heating single crystal manufacturing system. In detail, firstly, an end of a seed crystal is held on a seed chuck and an upper end portion of a rod-like polycrystalline raw material is held on a raw-material chuck. After adjusting a vertical position of an upper rotating shaft, the top edge of the seed crystal is brought into contact with the lower edge of the polycrystalline raw material. The vertical position of the upper rotating shaft and that of a lower rotating shaft are adjusted so that light of halogen lamp is focused on the upper edge of the seed crystal and the lower edge of the polycrystalline raw material. After the adjustment, the upper edge of the seed crystal and the lower edge of the polycrystalline raw material are heated so that the heated portion is melted, thereby forming melt droplets. At this time, only the seed crystal is being rotated. The aforementioned portion is melted while rotating the polycrystalline raw material and the seed crystal in opposite directions so as to be mixed sufficiently and the polycrystalline raw material and the seed crystal are then pulled in directions opposite to each other to form a single crystal having appropriate length and thickness, thereby making an ingot.

In case of using the EFG method, a predetermined amount of β-Ga<NUM>O<NUM> powders, etc., to be a raw material is put in a crucible and is melted by heating, thereby producing β-Ga<NUM>O<NUM> melt. Through a slit formed on a slit die which is placed in the crucible, the β-Ga<NUM>O<NUM> melt is drawn up to an upper surface of the slit die by capillary action, the β-Ga<NUM>O<NUM> melt is cooled by contact with the seed crystal and an ingot having an arbitrary cross sectional shape is thereby formed.

It should be noted that, a desired conductive impurity may be added when producing β-Ga<NUM>O<NUM> ingots by such producing methods.

The β-Ga<NUM>O<NUM> ingot formed as described above is sliced by, e.g., a wire saw so that a cross section thereof is a plane rotated by not less than <NUM>° and not more than <NUM>° with respect the (<NUM>) plane, thereby obtaining the β-Ga<NUM>O<NUM>-based substrate <NUM> having a thickness of <NUM> mm. In the subsequent grinding and polishing process, the β-Ga<NUM>O<NUM>-based substrate <NUM> is ground and polished until the thickness becomes about <NUM> µm.

Next, the β-Ga<NUM>O<NUM>-based substrate <NUM> is subjected to organic cleaning using methanol, acetone and methanol in this order for three minutes each, running water cleaning using ultrapure water, hydrofluoric acid immersion cleaning for fifteen minutes, sulfuric acid/hydrogen peroxide mixture immersion cleaning for five minutes and running water cleaning using ultrapure water, and is further subjected to thermal cleaning under conditions of at <NUM>°C for ten minutes. After this, the main surface <NUM> of the β-Ga<NUM>O<NUM>-based substrate <NUM> is ready for epitaxial growth of the β-Ga<NUM>O<NUM>-based crystal film <NUM>.

The method of forming the β-Ga<NUM>O<NUM>-based crystal film <NUM> on the main surface <NUM> of the β-Ga<NUM>O<NUM>-based substrate <NUM> is MBE (Molecular Beam Epitaxy). In the present embodiment, a process using the MBE will be described in detail.

The MBE is a crystal growth method in which a single or compound solid is heated in an evaporation source called cell and vapor generated by heat is supplied as a molecular beam onto the surface of the substrate.

<FIG> is a cross sectional view showing an MBE system used for forming the crystal laminate structure <NUM>. The MBE system <NUM> is provided with a vacuum chamber <NUM>, a substrate holder <NUM> supported in the vacuum chamber <NUM> to hold the β-Ga<NUM>O<NUM>-based substrate <NUM>, heating devices <NUM> held on the substrate holder <NUM>, plural cells <NUM> (<NUM>a, <NUM>b, <NUM>c, <NUM>d) each containing one of raw materials of the β-Ga<NUM>O<NUM>-based crystal film <NUM>, heaters <NUM> (<NUM>a, <NUM>b, <NUM>c, <NUM>d) for respectively hearing the plural cells <NUM>, a gas supply pipe <NUM> for supplying oxygen gas into the vacuum chamber <NUM>, and a vacuum pump <NUM> for exhausting the air in the vacuum chamber <NUM>. It is configured that the substrate holder <NUM> can be rotated by a non-illustrated motor via a shaft <NUM>.

A Ga raw material and an Al raw material are respectively loaded in the first cell <NUM>a and the second cell <NUM>b. An n-type impurity raw material to be doped as a donor, such as Si or Sn, is loaded in the third cell <NUM>c. A p-type impurity raw material to be doped as an acceptor, such as Mg or Zn, is loaded in the fourth cell <NUM>d. Each of the first to fourth cells <NUM>a to <NUM>d is provided with a non-illustrated shutter and is configured such that the shutter can be closed when the raw material contained therein is not used.

Firstly, the β-Ga<NUM>O<NUM>-based substrate <NUM> is attached to the substrate holder <NUM> of the MBE system <NUM>. Next, the vacuum pump <NUM> is activated to reduce atmospheric pressure in the vacuum chamber <NUM> to about <NUM>-<NUM> Torr. Then, the β-Ga<NUM>O<NUM>-based substrate <NUM> is heated by the heating devices <NUM>. It should be noted that, radiation heat of heat source such as graphite heater of the heating device <NUM> is thermally transferred to the β-Ga<NUM>O<NUM>-based substrate <NUM> via the substrate holder <NUM> and the β-Ga<NUM>O<NUM>-based substrate <NUM> is thereby heated.

After the β-Ga<NUM>O<NUM>-based substrate <NUM> is heated to a predetermined temperature, oxygen-based gas is supplied into the vacuum chamber <NUM> through the gas supply pipe <NUM>.

After a period of time required for stabilization of gas pressure in the vacuum chamber <NUM> (e.g., after <NUM> minutes) since the oxygen-based gas was supplied into the vacuum chamber <NUM>, the cells <NUM>a and <NUM>b are heated by the heaters <NUM>a and <NUM>b while rotating the substrate holder <NUM> so that Ga vapor and Al vapor are started to be supplied. In case that a Ga<NUM>O<NUM> crystal film not containing Al is formed as the β-Ga<NUM>O<NUM>-based crystal film <NUM>, the first cell <NUM>a is heated to start supply of Ga vapor.

Meanwhile, when imparting an n-type conductivity to the β-Ga<NUM>O<NUM>-based crystal film <NUM>, vapor of the n-type impurity to be a donor such as Si or Sn is supplied from the third cell <NUM>c by heating the third heater <NUM>c. On the other hand, when imparting a p-type conductivity, vapor of the p-type impurity to be an acceptor such as Mg or Zn is supplied from the fourth cell <NUM>d by heating the fourth heater <NUM>d.

The vapor produced from each cell <NUM> is radiated as molecular beam onto the surface of the β-Ga<NUM>O<NUM>-based substrate <NUM>. Beam-equivalent pressure (BEP) of Ga and that of Al are, e.g., respectively <NUM>×<NUM>-<NUM> Pa and <NUM>×<NUM><NUM>-<NUM> Pa. Meanwhile, in case of not producing Al vapor, for example, the Ga beam-equivalent pressure is e.g., <NUM>×<NUM>-<NUM> Pa.

Then, a β-Ga<NUM>O<NUM>-based crystal is epitaxially grown on the main surface <NUM> of the β-Ga<NUM>O<NUM>-based substrate <NUM> and the β-Ga<NUM>O<NUM>-based crystal film <NUM> is thereby formed. Growth temperature and growth time of the β-Ga<NUM>O<NUM>-based crystal are, e.g., respectively <NUM>°C and <NUM> hour.

In addition, the β-Ga<NUM>O<NUM>-based crystal film <NUM> may be subjected to annealing treatment in an inert atmosphere, where necessary. The annealing treatment is performed in a heat treatment equipment such as lamp annealing apparatus. Alternatively, the annealing treatment may be performed in the MBE system <NUM>.

A high-electron-mobility transistor (HEMT), which is one of semiconductor devices including the β-Ga<NUM>O<NUM>-based substrate <NUM> and the β-Ga<NUM>O<NUM>-based crystal film <NUM> of the first embodiment, will be described as the second embodiment.

<FIG> is a cross sectional view showing a high-electron-mobility transistor in the second embodiment. This high-electron-mobility transistor <NUM> includes the β-Ga<NUM>O<NUM>-based substrate <NUM> and the β-Ga<NUM>O<NUM>-based crystal film <NUM> of the first embodiment. The high-electron-mobility transistor <NUM> further includes an n-type β-(AlGa)<NUM>O<NUM> layer <NUM> on the β-Ga<NUM>O<NUM>-based crystal film <NUM>, and a gate electrode <NUM>, a source electrode <NUM> and a drain electrode <NUM> on the n-type β-(AlGa)<NUM>O<NUM> layer <NUM>. The gate electrode <NUM> is arranged between the source electrode <NUM> and the drain electrode <NUM>.

The gate electrode <NUM> is in contact with a surface <NUM>a of the n-type β-(AlGa)<NUM>O<NUM> layer <NUM>, thereby forming a Schottky junction. Meanwhile, the source electrode <NUM> and the drain electrode <NUM> are in contact with the surface <NUM>a of the n-type β-(AlGa)<NUM>O<NUM> layer <NUM>, thereby forming an ohmic junction.

In the second embodiment, the β-Ga<NUM>O<NUM>-based substrate <NUM> contains group II elements such as Mg and has high electrical resistance.

In the present embodiment, the β-Ga<NUM>O<NUM>-based crystal film <NUM> is of an i-type and functions as an electron transit layer. This i-type β-Ga<NUM>O<NUM>-based crystal film <NUM> is formed by epitaxially growing a β-Ga<NUM>O<NUM>-based single crystal on the main surface <NUM> of the β-Ga<NUM>O<NUM>-based substrate <NUM>.

The n-type β-(AlGa)<NUM>O<NUM> layer <NUM> is an electron supply layer doped with a donor such as Si or Sn and is formed by epitaxial growth on the β-Ga<NUM>O<NUM>-based crystal film <NUM>.

Since the β-Ga<NUM>O<NUM>-based crystal film <NUM> and the n-type β-(AlGa)<NUM>O<NUM> layer <NUM> have different band gaps, discontinuity of bands occurs at the interface therebetween, electrons generated from the donor in the n-type β-(AlGa)<NUM>O<NUM> layer <NUM> are concentrated on the β-Ga<NUM>O<NUM>-based crystal film <NUM> side and are distributed in a region in the vicinity of the interface, and an electron layer called two-dimensional electron gas is thereby formed.

As such, a first depletion layer due to the Schottky junction with the gate electrode <NUM> and a second depletion layer due to the formation of two-dimensional electron gas are produced in the n-type β-(AlGa)<NUM>O<NUM> layer <NUM>. The n-type β-(AlGa)<NUM>O<NUM> layer <NUM> has a thickness at which the first depletion layer is in contact with the second depletion layer.

Voltage is applied to the gate electrode <NUM> to change the thicknesses of the first and second depletion layers and to adjust the concentration of the two-dimensional electron gas, thereby allowing drain current to be controlled.

The thickness of the β-Ga<NUM>O<NUM>-based crystal film <NUM> is not specifically limited but is desirably not less than <NUM> nm. In addition, the thickness of the n-type β-(AlGa)<NUM>O<NUM> layer <NUM> is set to <NUM> to <NUM> µm depending on a doping concentration.

A MESFET (Metal-Semiconductor Field Effect Transistor), which is one of semiconductor devices including the β-Ga<NUM>O<NUM>-based substrate <NUM> and the β-Ga<NUM>O<NUM>-based crystal film <NUM> of the first embodiment, will be described as the third embodiment.

<FIG> is a cross sectional view showing a MESFET in the third embodiment. This MESFET <NUM> includes the β-Ga<NUM>O<NUM>-based substrate <NUM> and the β-Ga<NUM>O<NUM>-based crystal film <NUM> of the first embodiment. The MESFET <NUM> further includes a gate electrode <NUM>, a source electrode <NUM> and a drain electrode <NUM> on the β-Ga<NUM>O<NUM>-based crystal film <NUM>. The gate electrode <NUM> is arranged between the source electrode <NUM> and the drain electrode <NUM>.

The gate electrode <NUM> is in contact with a surface <NUM>a of the β-Ga<NUM>O<NUM>-based crystal film <NUM>, thereby forming a Schottky junction. Meanwhile, the source electrode <NUM> and the drain electrode <NUM> are in contact with the surface <NUM>a of the β-Ga<NUM>O<NUM>-based crystal film <NUM>, thereby forming an ohmic junction.

In the third embodiment, the β-Ga<NUM>O<NUM>-based substrate <NUM> contains group II elements such as Mg and has high electrical resistance.

In the present embodiment, the β-Ga<NUM>O<NUM>-based crystal film <NUM> is of an n-type and the donor concentration thereof in the vicinity of contact areas with the source electrode <NUM> and with the drain electrode <NUM> is higher than that in the remaining portion.

The thickness of the depletion layer in the β-Ga<NUM>O<NUM>-based crystal film <NUM> under the gate electrode <NUM> is changed by controlling bias voltage applied to the gate electrode <NUM>, thereby allowing drain current to be controlled.

A Schottky-barrier diode, which is one of semiconductor devices including the β-Ga<NUM>O<NUM>-based substrate <NUM> and the β-Ga<NUM>O<NUM>-based crystal film <NUM> of the first embodiment, will be described as the fourth embodiment.

<FIG> is a cross sectional view showing a Schottky-barrier diode in the fourth embodiment. This Schottky-barrier diode <NUM> includes the β-Ga<NUM>O<NUM>-based substrate <NUM> and the β-Ga<NUM>O<NUM>-based crystal film <NUM> of the first embodiment. The Schottky-barrier diode <NUM> further includes a Schottky electrode <NUM> on the β-Ga<NUM>O<NUM>-based crystal film <NUM> and an ohmic electrode <NUM> on a surface <NUM> of the β-Ga<NUM>O<NUM>-based substrate <NUM> opposite to the β-Ga<NUM>O<NUM>-based crystal film <NUM>.

The Schottky electrode <NUM> is in contact with the surface <NUM>a of the β-Ga<NUM>O<NUM>-based crystal film <NUM>, thereby forming a Schottky junction. Meanwhile, the ohmic electrode <NUM> is in contact with the surface <NUM> of the β-Ga<NUM>O<NUM>-based substrate <NUM>, thereby forming an ohmic junction.

In the fourth embodiment, the β-Ga<NUM>O<NUM>-based substrate <NUM> and the β-Ga<NUM>O<NUM>-based crystal film <NUM> are of an n-type and the donor concentration of the β-Ga<NUM>O<NUM>-based crystal film <NUM> is lower than that of the β-Ga<NUM>O<NUM>-based substrate <NUM>.

When forward voltage (electric potential is positive on the Schottky electrode <NUM> side) is applied to the Schottky diode <NUM>, the number of electrons moving from the β-Ga<NUM>O<NUM>-based substrate <NUM> to the β-Ga<NUM>O<NUM>-based crystal film <NUM> is increased. As a result, a forward current flows from the Schottky electrode <NUM> to the ohmic electrode <NUM>.

On the other hand, when reverse voltage (electric potential is negative on the Schottky electrode layer <NUM> side) is applied to the Schottky diode <NUM>, substantially no electric current flows through the Schottky diode <NUM>.

According to the embodiments described above, since a β-Ga<NUM>O<NUM>-based substrate of which main face is a plane rotated by not less than <NUM>° and not more than <NUM>° with respect to the (<NUM>) plane is used as a base for epitaxial crystal growth, it is possible to grow a β-Ga<NUM>O<NUM>-based crystal at a sufficient rate and thereby to form a high-quality β-Ga<NUM>O<NUM>-based crystal film. In addition, use of such high-quality β-Ga<NUM>O<NUM>-based crystal films allows high-performance semiconductor devices excellent in operating characteristics to be formed.

In addition, since waste of the raw materials of the β-Ga<NUM>O<NUM>-based crystal can be reduced, it is possible to reduce the manufacturing cost of semiconductor devices which include a β-Ga<NUM>O<NUM>-based crystal film and a β-Ga<NUM>O<NUM>-based crystal film.

In the present example, the growth rate of the β-Ga<NUM>O<NUM>-based crystal was evaluated using plural β-Ga<NUM>O<NUM>-based substrates of which main surfaces respectively have different plane orientations.

Firstly, a β-Ga<NUM>O<NUM> ingot made by the FZ method was sliced using a wire saw, thereby forming β-Ga<NUM>O<NUM> single crystal substrates of <NUM> mm in thickness. Here, five types of β-Ga<NUM>O<NUM> single crystal substrates respectively having the (-<NUM>) plane, the (<NUM>) plane, the (<NUM>) plane, the (<NUM>) plane and the (<NUM>) plane as the main surface were formed as the β-Ga<NUM>O<NUM>-based substrate <NUM> while a β-Ga<NUM>O<NUM> single crystal substrate having the (<NUM>) plane as the main surface was formed as Comparative Example.

Next, in the grinding and polishing process, each β-Ga<NUM>O<NUM> single crystal substrate was ground and polished until the thickness became about <NUM> µm.

Next, each β-Ga<NUM>O<NUM> single crystal substrate was subjected to organic cleaning using methanol, acetone and methanol in this order for three minutes each, running water cleaning using ultrapure water, hydrofluoric acid immersion cleaning for fifteen minutes, sulfuric acid/hydrogen peroxide mixture immersion cleaning for five minutes and running water cleaning using ultrapure water, and was further subjected to thermal cleaning under conditions of at <NUM>°C for ten minutes.

Next, a β-Ga<NUM>O<NUM> crystal was grown on each β-Ga<NUM>O<NUM> single crystal substrate in an oxygen-based gas atmosphere by the MBE and a β-Ga<NUM>O<NUM> crystal film was thereby formed as the β-Ga<NUM>O<NUM>-based crystal film <NUM>. The Ga beam-equivalent pressure was <NUM>×<NUM>-<NUM> Pa.

The growth temperature and growth time of the β-Ga<NUM>O<NUM> crystal were respectively <NUM>°C and <NUM> hour. In addition, on the β-Ga<NUM>O<NUM> single crystal substrates having the (<NUM>) plane and the (<NUM>) plane as the main surface, crystal growth of β-Ga<NUM>O<NUM> was also performed under the condition of a growth temperature of <NUM>°C.

<FIG> is a graph showing growth rates of the respective β-Ga<NUM>O<NUM> crystals on the β-Ga<NUM>O<NUM> single crystal substrates.

As shown in <FIG>, in case of crystal growth at <NUM>°C which is a temperature allowing a β-Ga<NUM>O<NUM> crystal with sufficient quality to be grown, the growth rate was about <NUM> nm/hour on the β-Ga<NUM>O<NUM> single crystal substrate having the (<NUM>) plane as the main surface. On the other hand, the growth rate was about <NUM> to <NUM> nm/hour on the β-Ga<NUM>O<NUM> single crystal substrates having the (-<NUM>) plane, the (<NUM>) plane, the (<NUM>) plane, the (<NUM>) plane and the (<NUM>) plane as the main surface.

This result shows that, on the β-Ga<NUM>O<NUM>-based substrates having the (-<NUM>) plane, the (<NUM>) plane, the (<NUM>) plane, the (<NUM>) plane and the (<NUM>) plane as the main surface which were formed as the β-Ga<NUM>O<NUM>-based substrates <NUM> of the embodiments, the growth rate of the β-Ga<NUM>O<NUM> crystal is remarkably faster than that on the β-Ga<NUM>O<NUM> single crystal substrate having the (<NUM>) plane as the main surface. It should be noted that, it is generally rare that the growth rate of crystal is improved so much only by changing the plane orientation of the main surface of the substrate and it can be said that this result is beyond expectation of those skilled in the art.

It was also confirmed that, when the β-Ga<NUM>O<NUM> crystal is grown on the β-Ga<NUM>O<NUM> single crystal substrate having the (<NUM>) plane as the main surface at a growth temperature of <NUM>°C, the growth rate is about one-fifth of that of the β-Ga<NUM>O<NUM> crystal grown at a growth temperature of <NUM>°C. It is believed that this is because the raw materials of the β-Ga<NUM>O<NUM> crystal re-evaporate from the substrate.

On the other hand, it was confirmed that, when the β-Ga<NUM>O<NUM> crystal is grown on the β-Ga<NUM>O<NUM> single crystal substrates having the (<NUM>) plane and the (<NUM>) plane as the main surface at a growth temperature of <NUM>°C, the growth rate is substantially the same as that of the β-Ga<NUM>O<NUM> crystal grown at a growth temperature of <NUM>°C. It is believed that re-evaporation of the raw materials of the β-Ga<NUM>O<NUM> crystal from the substrate is suppressed when the β-Ga<NUM>O<NUM> single crystal substrate having the (<NUM>) plane as the main surface is used. It is considered that the same applies to the case where the β-Ga<NUM>O<NUM> single crystal substrates having the (-<NUM>) plane, the (<NUM>) plane and the (<NUM>) plane as the main surface are used.

<FIG> is a graph showing a relation between the growth rate of the β-Ga<NUM>O<NUM> crystal and a rotation angle with respect to the (<NUM>) plane as the main surface of the β-Ga<NUM>O<NUM> single crystal substrate. In <FIG>, the filled square indicates a value of the growth rate at a growth temperature of <NUM>°C when the main surface is rotated around the c-axis. The filled circle indicates a value of the growth rate at a growth temperature of <NUM>°C when the main surface is rotated around the c-axis. The filled diamond indicates a value of the growth rate at a growth temperature of <NUM>°C when the main surface is rotated around the b-axis.

It is understood from <FIG> that, when the rotation angle with respect to (<NUM>) plane as the main surface of the β-Ga<NUM>O<NUM> single crystal substrate is not less than <NUM>°, the growth rate when growing the β-Ga<NUM>O<NUM> crystal at a growth temperature of <NUM>°C is significantly improved as compared to the case where the main surface is the (<NUM>) plane. It is also understood that, when the β-Ga<NUM>O<NUM> crystal is grown at a growth temperature of <NUM>°C, the growth rate of the β-Ga<NUM>O<NUM> crystal hardly depends on the rotation angle with respect to (<NUM>) plane as the main surface of the β-Ga<NUM>O<NUM> single crystal substrate.

Claim 1:
A method for producing a crystal laminate structure (<NUM>) comprising:
a step of forming a β-Ga<NUM>O<NUM>-based crystal film (<NUM>) by epitaxial growth of a β-Ga<NUM>O<NUM>-based crystal on a main surface (<NUM>) of a β-Ga<NUM>O<NUM>-based substrate (<NUM>) at a growth temperature of not less than <NUM> by MBE method,
wherein the main surface comprises a plane rotated by not less than <NUM>° and not more than <NUM>° with respect to a (<NUM>) plane,
wherein the epitaxial growth of the β-Ga<NUM>O<NUM>-based crystal by the MBE method having a characteristics that a percentage of the re-evaporated raw material of the β-Ga<NUM>O<NUM>-based crystal from the β-Ga<NUM>O<NUM>-based substrate at the growth temperature is suppressed to be not more than <NUM>% where a percentage of the re-evaporated raw material from the β-Ga<NUM>O<NUM>-based substrate during the epitaxial growth of the β-Ga<NUM>O<NUM>-based crystal at a growth temperature of <NUM> is defined as <NUM>%,
wherein the main surface comprises one of a (<NUM>) plane, a (<NUM>) plane, a (<NUM>) plane, a (-<NUM>) plane, and a (<NUM>) plane, and
wherein a growth rate of the β-Ga<NUM>O<NUM>-based crystal on the main surface is not less than <NUM>/hr and not more than <NUM>/hr.