Patent Description:
The MBE (Molecular Beam Epitaxy) method and the PLD (Pulsed Laser Deposition) method are known as a growth method of β-Ga<NUM>O<NUM> single crystal film (see, e.g., PTL <NUM> and PTL <NUM>). Other growth methods thereof, the sol-gel process, the MOCVD (<NPL>, discloses a method for growing a Ga<NUM>O<NUM>-based crystal film by an HVPE method comprising a step of exposing a substrate to a gallium chloride-based gas and methanol, and growing a Ga<NUM>O<NUM>-based crystal film on a principal surface of the substrate at a growth temperature.

<CIT> discloses a method for producing a gallium trichloride gas.

<CIT> discloses a crystalline layered structure, comprising a Ga<NUM>O<NUM>-based substrate and a ß- Ga<NUM>O<NUM>-based single crystal film that is formed on a principal surface of the Ga<NUM>O<NUM>-based substrate and includes Cl.

<CIT> discloses a Ga<NUM>O<NUM> MISFET (<NUM>), which includes: an n-type β-Ga<NUM>O<NUM> single crystal film (<NUM>), which is formed on a high-resistance β-Ga<NUM>O<NUM> substrate (<NUM>) directly or with other layer therebetween; a source electrode (<NUM>) and a drain electrode (<NUM>), which are formed on the n-type β Ga<NUM>O<NUM> single crystal film (<NUM>); and a gate electrode (<NUM>), which is formed on the n-type β-Ga<NUM>O<NUM> single crystal film (<NUM>) between the source electrode (<NUM>) and the drain electrode (<NUM>).

The MBE method is conducted, however, such that crystal is grown in a high vacuum chamber. Thus, it is difficult to increase the diameter of a β-Ga<NUM>O<NUM> single crystal film. Although a high-quality film can be generally obtained by increasing the growth temperature, a sufficient film growth rate is not obtained due to an increase in re-evaporation of source gases and it is thus not suitable for mass production.

The PLD method is not suitable for growing a film with a large area since a source (a raw material supply source to a substrate) is a point source which causes a growth rate to be different between a portion immediately above the source and other portions and in-plane distribution of film thickness is likely to be non-uniform. In addition, it takes long time to form a thick film due to a low film growth rate, hence, not suitable for mass production.

In regard to the sol-gel method, the MOCVD method and the mist CVD method, it is relatively easy to increase a diameter but it is difficult to obtain single crystal films with high purity since impurities contained in the used materials are incorporated into the β-Ga<NUM>O<NUM> single crystal film during epitaxial growth.

Thus, it is an object of the invention to provide a method for growing a β-Ga<NUM>O<NUM>-based single crystal film that allows a high-quality and large-diameter β-Ga<NUM>O<NUM>-based single crystal film to grow efficiently, as well as a crystalline layered structure having the β-Ga<NUM>O<NUM>-based single crystal film grown by the method.

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

According to the invention, a method for growing a β-Ga<NUM>O<NUM>-based single crystal film can be provided that allows a high-quality and large-diameter β-Ga<NUM>O<NUM>-based single crystal film to grow efficiently, as well as a crystalline layered structure having the β-Ga<NUM>O<NUM>-based single crystal film grown by the method.

<FIG> is a vertical cross-sectional view showing a crystalline layered structure <NUM> in an embodiment. The crystalline layered structure <NUM> has a Ga<NUM>O<NUM>-based substrate <NUM> and a β-Ga<NUM>O<NUM>-based single crystal film <NUM> formed on a principal surface <NUM> of the Ga<NUM>O<NUM>-based substrate <NUM> by epitaxial crystal growth.

The Ga<NUM>O<NUM>-based substrate <NUM> is a substrate formed of a Ga<NUM>O<NUM>-based single crystal with a β-crystal structure. The Ga<NUM>O<NUM>-based single crystal here means a Ga<NUM>O<NUM> single crystal or 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. In addition, the Ga<NUM>O<NUM>-based substrate <NUM> may contain a conductive impurity such as Si.

The plane orientation of the principal surface <NUM> of the Ga<NUM>O<NUM>-based substrate <NUM> is, e.g., (<NUM>), (-<NUM>), (<NUM>) or (<NUM>).

To form the Ga<NUM>O<NUM>-based substrate <NUM>, for example, a bulk crystal of a Ga<NUM>O<NUM>-based single crystal grown by, e.g., 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 β-Ga<NUM>O<NUM>-based single crystal film <NUM> is formed of a Ga<NUM>O<NUM>-based single crystal with a β-crystal structure in the same manner as the Ga<NUM>O<NUM>-based substrate <NUM>. In addition, the β-Ga<NUM>O<NUM>-based single crystal film <NUM> may contain a conductive impurity such as Si.

A structure of a vapor phase deposition system used for growing the β-Ga<NUM>O<NUM>-based single crystal film <NUM> in the present embodiment will be described below as an example.

<FIG> is a vertical cross-sectional view showing a vapor phase deposition system <NUM> in the embodiment. The vapor phase deposition system <NUM> is a vapor phase deposition system using HVPE (Halide Vapor Phase Epitaxy) 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 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 Ga<NUM>O<NUM>-based substrate <NUM> is placed and the β-Ga<NUM>O<NUM>-based single crystal film <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) 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 ) as an oxygen source gas and a chloride gas (e.g., silicon tetrachloride, etc.) used to add a dopant such as Si to the β-Ga<NUM>O<NUM>-based single crystal film <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 β-Ga<NUM>O<NUM>-based single crystal film <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 β-Ga<NUM>O<NUM>-based single crystal film <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.

<FIG> is a graph showing a relation, based on calculation of thermal equilibrium, between a driving force for growth and a growth temperature of Ga<NUM>O<NUM> crystal respectively when a gallium chloride gas consists of only a GaCl gas and consists of only a GaCl<NUM> gas. The calculation conditions are as follows: a carrier gas is an inert gas such as N<NUM>, a furnace pressure is <NUM> kPa, the supplied partial pressures of GaCl gas and GaCl<NUM> gas are both <NUM>,<NUM> kPa, and an O<NUM>/GaCl partial pressure ratio is <NUM>.

In <FIG>, the horizontal axis indicates a growth temperature (°C) of Ga<NUM>O<NUM> crystal and the vertical axis indicates a driving force for crystal growth. The Ga<NUM>O<NUM> crystal grows more efficiently with a larger driving force for crystal growth.

<FIG> shows that the maximum temperature at which the driving force for growth is maintained is higher when using the GaCl gas as a Ga source gas than when using the GaCl<NUM> gas.

If hydrogen is contained in an atmosphere for growing the β-Ga<NUM>O<NUM>-based single crystal film <NUM>, surface flatness and a driving force for growth of the β-Ga<NUM>O<NUM>-based single crystal film <NUM> decrease. Therefore, Cl<NUM> gas which does not contain hydrogen is used.

<FIG> is a graph showing a relation, based on calculation of thermal equilibrium, between an atmosphere temperature during reaction and equilibrium partial pressures of GaCl gas, GaCl<NUM> gas, GaCl<NUM> gas and (GaCl<NUM>)<NUM> gas which are obtained by reaction of Ga with Cl<NUM>. The other calculation conditions are as follows: a carrier gas is, e.g., an inert gas such as N<NUM>, a furnace pressure is <NUM>,<NUM> kPa and the supplied partial pressure of Cl<NUM> gas is 3x0,<NUM> kPa.

In <FIG>, the horizontal axis indicates an atmosphere temperature (°C) and the vertical axis indicates an equilibrium partial pressure (atm). It is shown that more gas is produced at a higher equilibrium partial pressure.

<FIG> shows that when reacting the metal gallium chloride with the Cl-containing gas at an atmosphere temperature of about not less than <NUM>, the equilibrium partial pressure of GaCl gas particularly capable of increasing a driving force for growth of Ga<NUM>O<NUM> crystal is increased, i.e., a partial pressure ratio of the GaCl gas with respect to the gallium chloride gas becomes higher. Based on this, 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>.

Also, at the atmosphere temperature of, e.g., <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.

Meanwhile, in consideration of 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 Ga<NUM>O<NUM>-based substrate <NUM> is exposed to the mixed gas, thereby epitaxially growing the β-Ga<NUM>O<NUM>-based single crystal film <NUM> on the Ga<NUM>O<NUM>-based 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> kPa (<NUM> atm).

When forming the β-Ga<NUM>O<NUM>-based single crystal film <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 β-Ga<NUM>O<NUM>-based single crystal film <NUM>, surface flatness and a driving force for growth of the β-Ga<NUM>O<NUM>-based single crystal film <NUM> decrease. Therefore, the oxygen-containing gas is an O<NUM> gas which does not contain hydrogen.

<FIG> is a graph showing a relation, based on calculation of thermal equilibrium, between an equilibrium partial pressure of GaCl and an O<NUM>/GaCl supplied partial pressure ratio when the atmosphere temperature during Ga<NUM>O<NUM> crystal growth is <NUM>. Here, a ratio of the supplied partial pressure of the O<NUM> gas to the supplied partial pressure of the GaCl gas is referred to as "O<NUM>/GaCl supplied partial pressure ratio". It is calculated using the supplied partial pressure value of the GaCl gas fixed to <NUM>,<NUM> kPa, a furnace pressure of <NUM>,<NUM> kPa adjusted by using an inert gas such as N<NUM> as a carrier gas, and various values of the O<NUM> gas supplied partial pressure.

In <FIG>, the horizontal axis indicates the O<NUM>/GaCl supplied partial pressure ratio and the vertical axis indicates an equilibrium partial pressure (atm) of the GaCl gas. It is shown that the smaller the equilibrium partial pressure of the GaCl gas, the more the GaCl gas is consumed for growth of Ga<NUM>O<NUM> crystal, i.e., the Ga<NUM>O<NUM> crystal grows efficiently.

<FIG> shows that the equilibrium partial pressure of the GaCl gas sharply falls at the O<NUM>/GaCl supplied partial pressure ratio of not less than <NUM>.

Based on this, to efficiently grow the β-Ga<NUM>O<NUM>-based single crystal film <NUM>, the β-Ga<NUM>O<NUM>-based single crystal film <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>.

<FIG> is a graph showing X-ray diffraction spectra obtained by 2θ-ω scan on crystalline layered structures in each of which a Ga<NUM>O<NUM> single crystal film is epitaxially grown on a (<NUM>)-oriented principal surface of a β-Ga<NUM>O<NUM> substrate. The growth conditions are as follows: a furnace pressure is <NUM>,<NUM> kPa, a carrier gas is N<NUM> gas, the GaCl supplied partial pressure is <NUM> Pa (5x10-<NUM> atm), and the O<NUM>/GaCl supplied partial pressure ratio is <NUM>.

In <FIG>, the horizontal axis indicates an angle 2θ (degrees) formed between the incident direction and the reflected direction of X-ray and the vertical axis indicates diffraction intensity (arbitrary unit) of the X-ray.

<FIG> shows a spectrum from a β-Ga<NUM>O<NUM> substrate (without β-Ga<NUM>O<NUM> crystal film) and spectra from crystalline layered structures having β-Ga<NUM>O<NUM> crystal films respectively epitaxially grown at <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM>. The β-Ga<NUM>O<NUM> crystal films of these crystalline layered structures have a thickness of about <NUM> to <NUM> nm.

Diffraction peaks from a (-<NUM>) plane, a (-<NUM>) plane and a (-<NUM>) plane or a (<NUM>) plane resulting from the presence of non-oriented grains, which are observed in the spectra from the crystalline layered structures having the β-Ga<NUM>O<NUM> crystal films grown at growth temperatures of <NUM> and <NUM>, disappear in the spectra from the crystalline layered structures having the β-Ga<NUM>O<NUM> crystal films grown at growth temperatures of not less than <NUM>. This shows that a β-Ga<NUM>O<NUM> single crystal film is obtained when a Ga<NUM>O<NUM> single crystal film is grown at a growth temperature of not less than <NUM>.

Also in case that the principal surface of the β-Ga<NUM>O<NUM> substrate has a plane orientation of (-<NUM>), (<NUM>) or (<NUM>), a β-Ga<NUM>O<NUM> single crystal film is obtained when a β-Ga<NUM>O<NUM> crystal film is grown at a growth temperature of not less than <NUM>. In addition, also in case that another Ga<NUM>O<NUM>-based substrate is used in place of the Ga<NUM>O<NUM> substrate or another Ga<NUM>O<NUM>-based crystal film is formed instead of the Ga<NUM>O<NUM> crystal film, evaluation results similar to those described above are obtained. In other words, when the plane orientation of the principal surface of the Ga<NUM>O<NUM>-based substrate <NUM> is (<NUM>), (-<NUM>), (<NUM>) or (<NUM>), the β-Ga<NUM>O<NUM>-based single crystal film <NUM> is obtained by growing at a growth temperature of not less than <NUM>.

<FIG> is a graph showing an X-ray diffraction spectrum obtained by <NUM>θ-ω scan on a crystalline layered structure in which a β-Ga<NUM>O<NUM> single crystal film is epitaxially grown on a (-<NUM>)-oriented principal surface of a β-Ga<NUM>O<NUM> substrate. The growth conditions for this β-Ga<NUM>O<NUM> single crystal film are as follows: a furnace pressure is <NUM>,<NUM> kPa, a carrier gas is N<NUM> gas, the GaCl supplied partial pressure is <NUM> Pa (5x10-<NUM> atm), the O<NUM>/GaCl supplied partial pressure ratio is <NUM> and the growth temperature is <NUM>.

<FIG> shows a spectrum from a β-Ga<NUM>O<NUM> substrate (without β-Ga<NUM>O<NUM> crystal film) having a (-<NUM>)-oriented principal surface and a spectrum from a crystalline layered structure having a β-Ga<NUM>O<NUM> crystal film epitaxially grown on the β-Ga<NUM>O<NUM> substrate at <NUM>. The β-Ga<NUM>O<NUM> crystal film of this crystalline layered structure has a thickness of about <NUM> nm.

<FIG> is a graph showing an X-ray diffraction spectrum obtained by 2θ-ω scan on a crystalline layered structure in which a Ga<NUM>O<NUM> single crystal film is epitaxially grown on a (<NUM>)-oriented principal surface of a β-Ga<NUM>O<NUM> substrate. The growth conditions for this β-Ga<NUM>O<NUM> single crystal film are as follows: a furnace pressure is <NUM>,<NUM> kPa, a carrier gas is N<NUM> gas, the GaCl supplied partial pressure is <NUM> Pa (5x10-<NUM> atm), the O<NUM>/GaCl supplied partial pressure ratio is <NUM> and the growth temperature is <NUM>.

<FIG> shows a spectrum from a β-Ga<NUM>O<NUM> substrate (without β-Ga<NUM>O<NUM> crystal film) having a (<NUM>)-oriented principal surface and a spectrum from a crystalline layered structure having a β-Ga<NUM>O<NUM> crystal film epitaxially grown on the β-Ga<NUM>O<NUM> substrate at <NUM>. The β-Ga<NUM>O<NUM> crystal film of this crystalline layered structure has a thickness of about <NUM> µm.

<FIG> is a graph showing an X-ray diffraction spectrum obtained by <NUM>θ-ω scan on a crystalline layered structure in which a Ga<NUM>O<NUM> single crystal film is epitaxially grown on a (<NUM>)-oriented principal surface of a β-Ga<NUM>O<NUM> substrate. The growth conditions for this β-Ga<NUM>O<NUM> single crystal film are as follows: a furnace pressure is <NUM>,<NUM> kPa, a carrier gas is N<NUM> gas, the GaCl supplied partial pressure is <NUM> Pa (5x10-<NUM> atm), the O<NUM>/GaCl supplied partial pressure ratio is <NUM> and the growth temperature is <NUM>.

In <FIG>, <FIG> and <FIG>, the horizontal axis indicates an angle <NUM>θ (degrees) formed between the incident direction and the reflected direction of X-ray and the vertical axis indicates diffraction intensity (arbitrary unit) of the X-ray.

In <FIG>, <FIG> and <FIG>, diffraction peaks of the spectrum from the crystalline layered structure having the β-Ga<NUM>O<NUM> crystal film grown at a growth temperature of <NUM> are coincident with the diffraction peaks of the spectrum from the β-Ga<NUM>O<NUM> substrate. This result shows that a β-Ga<NUM>O<NUM> single crystal film is obtained when the β-Ga<NUM>O<NUM> crystal film is grown on the principal surface of the β-Ga<NUM>O<NUM> substrate having a plane orientation of (<NUM>), (-<NUM>), (<NUM>) or (<NUM>) at a growth temperature of <NUM>.

<FIG> are graphs showing concentrations of impurities in the crystalline layered structure measured by secondary ion mass spectrometry (SIMS).

In <FIG>, the horizontal axis indicates a depth (µm) of the crystalline layered structure from a principal surface <NUM> of the β-Ga<NUM>O<NUM> single crystal film and the vertical axis indicates concentration (atoms/cm<NUM>) of each impurity. Here, an interface between the β-Ga<NUM>O<NUM> substrate and the β-Ga<NUM>O<NUM> single crystal film is located at a depth of about <NUM> µm in the crystalline layered structure. In addition, horizontal arrows on the right side in <FIG> indicate the respective measurable lower limits of concentrations of the impurity elements.

The β-Ga<NUM>O<NUM> single crystal film of the crystalline layered structure used for the measurement is a film which is grown on the (<NUM>)-oriented principal surface of the β-Ga<NUM>O<NUM> substrate at a growth temperature of <NUM>.

<FIG> shows the concentrations of C, Sn, and Si in the crystalline layered structure and <FIG> shows the concentrations of H and Cl in the crystalline layered structure. According to <FIG>, the concentration of each impurity element in the β-Ga<NUM>O<NUM> single crystal film is close to the measurable lower limit and is almost unchanged from the concentration in the Ga<NUM>O<NUM> substrate. This shows that the β-Ga<NUM>O<NUM> single crystal film is a highly pure film.

Similar evaluation results are obtained also in case that the principal surface of the β-Ga<NUM>O<NUM> substrate has a plane orientation of (-<NUM>), (<NUM>) or (<NUM>). In addition, also in case that another Ga<NUM>O<NUM>-based substrate is used in place of the β-Ga<NUM>O<NUM> substrate or another Ga<NUM>O<NUM>-based single crystal film is formed instead of the β-Ga<NUM>O<NUM> single crystal film, evaluation results similar to those described above are obtained.

According to <FIG>, not more than about 5x10<NUM> (atoms/cm<NUM>) of Cl is contained in the β-Ga<NUM>O<NUM> single crystal film. This results from that the Ga<NUM>O<NUM> single crystal film 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 1x10<NUM> (atoms/cm<NUM>) or more of Cl.

<FIG> is a graph showing a carrier concentration profile in a depth direction of the crystalline layered structure in which a β-Ga<NUM>O<NUM> crystal film is epitaxially grown on a (<NUM>)-oriented principal surface of a β-Ga<NUM>O<NUM> substrate.

In <FIG>, the horizontal axis indicates a depth (µm) from the surface of the β-Ga<NUM>O<NUM> crystal film and the vertical axis indicates a carrier concentration, i.e., a difference (cm-<NUM>) between a donor concentration Nd as a net donor concentration and an acceptor concentration Na. Then, a dotted curved line in the drawing is a theoretical curve showing a relation between the donor concentration and depletion layer thickness when relative permittivity of β-Ga<NUM>O<NUM> is <NUM> and built-in potential of β-Ga<NUM>O<NUM> in contact with Pt is <NUM>.

The procedure used to obtain the data shown in <FIG> is as follows. Firstly, an undoped β-Ga<NUM>O<NUM> crystal film having a thickness of about <NUM> µm is epitaxially grown on an Sn-doped n-type β-Ga<NUM>O<NUM> substrate having a (<NUM>)-oriented principal surface by the HVPE method. "Undoped" here means that intentional doping is not carried out, and it does not deny the presence of unintentional impurities.

The β-Ga<NUM>O<NUM> substrate is a <NUM> mm-square substrate having a thickness of <NUM> µm and has a carrier concentration of about 6x10<NUM> cm-<NUM>. The growth conditions for this β-Ga<NUM>O<NUM> single crystal film are as follows: a furnace pressure is <NUM>,<NUM> kPa, a carrier gas is N<NUM> gas, the GaCl supplied partial pressure is <NUM> Pa (5x10-<NUM> atm), the O<NUM>/GaCl supplied partial pressure ratio is <NUM> and the growth temperature is <NUM>.

Next, the surface of the undoped β-Ga<NUM>O<NUM> crystal film is polished <NUM> µm by CMP to flatten the surface.

Next, a Schottky electrode is formed on the β-Ga<NUM>O<NUM> crystal film and an ohmic electrode on the β-Ga<NUM>O<NUM> substrate, and C-V measurement is conducted while changing bias voltage in a range of +<NUM> to -10V. Then, a carrier concentration profile in a depth direction is calculated based on the C-V measurement result.

The Schottky electrode here is an <NUM> µm-diameter circular electrode having a laminated structure in which a <NUM> nm-thick Pt film, a <NUM> nm-thick Ti film and a <NUM> nm-thick Au film are laminated in this order. Also, the ohmic electrode is a <NUM> mm-square electrode having a laminated structure in which a <NUM> nm-thick Ti film and a <NUM> nm-thick Au film are laminated in this order.

In <FIG>, no measurement point is present in a region shallower than <NUM> µm which is equal to the thickness of the β-Ga<NUM>O<NUM> crystal film, and all measurement points are <NUM> µm on the horizontal axis. This shows that the entire region of the β-Ga<NUM>O<NUM> crystal film is depleted in the bias voltage range of +<NUM> to -10V.

Therefore, the entire region of the β-Ga<NUM>O<NUM> crystal film is naturally depleted at the bias voltage of <NUM>. It is predicted that the residual carrier concentration in the β-Ga<NUM>O<NUM> crystal film is as very small as not more than 1x10<NUM> cm-<NUM> since the donor concentration is about 1x10<NUM> cm-<NUM> when the depletion layer thickness is <NUM> µm, based on the theoretical curve.

Since the residual carrier concentration in the β-Ga<NUM>O<NUM> crystal film is not more than 1x10<NUM> cm-<NUM>, for example, it is possible to control the carrier concentration in the β-Ga<NUM>O<NUM> crystal film in a range of 1x10<NUM> to 1x10<NUM> cm-<NUM> by doping a Group IV element.

<FIG> is a graph showing voltage endurance characteristics of the above-mentioned crystalline layered structure.

In <FIG>, the horizontal axis indicates applied voltage (V) and the vertical axis indicates current density (A/cm<NUM>). In addition, a dotted straight line in the drawing indicates the measurable lower limit value.

The procedure used to obtain the data shown in <FIG> is as follows. Firstly, the above-mentioned crystalline layered structure composed of a β-Ga<NUM>O<NUM> substrate and a β-Ga<NUM>O<NUM> crystal film is prepared.

Next, a Schottky electrode is formed on the β-Ga<NUM>O<NUM> crystal film and an ohmic electrode on the β-Ga<NUM>O<NUM> substrate, and current density at an applied voltage of 1000V is measured.

The Schottky electrode here is a <NUM> µm-diameter circular electrode having a laminated structure in which a <NUM> nm-thick Pt film, a <NUM> nm-thick Ti film and a <NUM> nm-thick Au film are laminated in this order. Also, the ohmic electrode is a <NUM> mm-square electrode having a laminated structure in which a <NUM> nm-thick Ti film and a <NUM> nm-thick Au film are laminated in this order.

<FIG> shows that, even when voltage of 1000V is applied to the crystalline layered structure, leakage current is as very small as about 1x10-<NUM> A/cm<NUM> and insulation breakdown does not occur. This result is considered to be due to that the β-Ga<NUM>O<NUM> crystal film is a high-quality crystal film with only few crystal defects and the donor concentration is low.

The procedure used to obtain the data shown in <FIG> is as follows. Firstly, an undoped β-Ga<NUM>O<NUM> crystal film having a thickness of about <NUM> µm is epitaxially grown on an Sn-doped n-type β-Ga<NUM>O<NUM> substrate having a (<NUM>)-oriented principal surface by the HVPE method.

Next, a Schottky electrode is formed on the undoped β-Ga<NUM>O<NUM> crystal film and an ohmic electrode on the β-Ga<NUM>O<NUM> substrate, and C-V measurement is conducted while changing bias voltage in a range of +<NUM> to -10V. Then, a carrier concentration profile in a depth direction is calculated based on the C-V measurement result.

In <FIG>, measurement points at a bias voltage of <NUM> are <NUM> µm on the horizontal axis (measurement points in a region deeper than <NUM> µm are measurement points when the bias voltage is close to 10V). It is predicted that the residual carrier concentration in the β-Ga<NUM>O<NUM> crystal film is as very small as not more than 3x10<NUM> cm-<NUM> since the donor concentration is about <NUM>. 3x10<NUM> cm-<NUM> when the depletion layer thickness is <NUM> µm, based on the theoretical curve.

According to the embodiment, by controlling the conditions of producing the gallium source gas and the growth conditions for the β-Ga<NUM>O<NUM>-based single crystal film in the HVPE method, it is possible to efficiently grow a high-quality and large-diameter β-Ga<NUM>O<NUM>-based single crystal film. In addition, since the β-Ga<NUM>O<NUM>-based single crystal film has excellent crystal quality, it is possible to grow a good-quality crystal film on the β-Ga<NUM>O<NUM>-based single crystal film. Thus, a high-quality semiconductor device can be manufactured by using the crystalline layered structure including the β-Ga<NUM>O<NUM>-based single crystal film in the present embodiment.

Claim 1:
A method for growing a β-Ga<NUM>O<NUM>-based single crystal film by Halide Vapor Phase Epitaxy method, comprising a step of exposing a Ga<NUM>O<NUM>-based substrate to a gallium chloride-based gas and an oxygen-including gas and growing a β-Ga<NUM>O<NUM>-based single crystal film on a principal surface of the Ga<NUM>O<NUM>-based substrate at a growth temperature of not lower than <NUM> in an inert gas atmosphere,
wherein the gallium chloride-based gas is a gas produced by reacting a metal gallium with Cl<NUM> gas which does not contain hydrogen,
wherein the oxygen-including gas is an O<NUM> gas which does not contain hydrogen.