Patent Description:
Devices using group <NUM> nitride semiconductors such as GaN and InN are now put into practical use in a wide range of products. Conventionally, an MOCVD method and an MBE method have been used for crystal growth of such a group <NUM> nitride semiconductor. However, the MOCVD method requires a process temperature exceeding <NUM>. The MBE method allows a compound semiconductor film to be formed at a low temperature, but is not suitable to mass manufacturing because there is a limit on the area size of the film that may be formed and the manufacturing cost is high.

With the MBE method, if donors are incorporated at a high concentration, absorption by the high concentration donor level generated in the forbidden band in the vicinity of the conduction band of the crystal structure occurs. For this reason, the MBE method has a problem that the transparency of the manufactured compound semiconductor film is decreased (Non-patent Document <NUM>). For these reasons, the MOCVD method is now used to manufacture a compound semiconductor, mainly, to manufacture a nitride semiconductor for a practical use.

Currently, next-generation electronic devices having both a high withstand voltage and a low on-resistance are desired. In order to realize such an electronic device, it is desired to realize a two-, three-, or four-component compound semiconductor, more specifically, a compound semiconductor device using a group <NUM> nitride semiconductor. This requires further improvement in the quality of the crystal of such a compound semiconductor and improvement in the refinement of the doping technology. Especially for a vertical power device to be formed on a GaN substrate, it is urged to decrease the carbon concentration of an n-type drift layer and to improve the electron mobility. There are the following documents describing the prior art.

Patent Document <NUM> discloses a semiconductor device including a buffer layer formed of a metal nitride and a semiconductor layer, which are provided on a copper substrate.

Patent Document <NUM> discloses examples of a semiconductor substrate including a graphite plate having a thickness of <NUM> to <NUM>, containing a sintered polymer and having a heat resistance and flexibility, a buffer layer formed of HfN on the graphite plate, and a semiconductor layer formed of GaN on the buffer layer. Patent Document <NUM> discloses a method for manufacturing a group-III-V compound semiconductor by epitaxial growth on a ZnO substrate.

Non-patent Document <NUM> discloses research results on formation of a p-type GaN semiconductor layer. Non-patent Document <NUM> discloses research results on the contact resistance of a p-type GaN semiconductor layer. Non-patent Document <NUM> discloses research results on a low concentration doping technology into a nitride semiconductor. Non-patent Document <NUM> discloses research results on a transport model of electrons in a high electric field. Non-patent Document <NUM> discloses research results on a model of carrier mobility of GaN. Non-patent Document <NUM> discloses research results on evaluation of the contact resistance against a p-type GaN formed by a PSD method. Non-patent Document <NUM> discloses examples of experiments of producing an LED on glass. Non-patent Document <NUM> discloses research results on a nitride single crystal grown by the PSD method. Further prior art is known from <NPL>, and from <NPL>.

With the conventional technology, in the case where it is attempted to realize crystal growth of a group <NUM> nitride semiconductor by the MOCVD method, carbon and hydrogen contained in the material gas are incorporated into the film. This causes a problem that it is difficult to form a high quality film having a low concentration of impurities such as carbon, hydrogen and the like.

In addition, in the case where it is attempted to realize crystal growth of a group <NUM> nitride semiconductor by the MOCVD method, it is generally difficult that a film having a donor concentration of <NUM> × <NUM><NUM> cm-<NUM> or higher exhibits an electron mobility of about <NUM><NUM>/V·s or higher due to thermodynamic restrictions. The MOCVD method is based on a chemical reaction. Therefore, it is in fact impossible to realize crystal growth at a low temperature, and carbon and hydrogen contained in the material gas are easily incorporated into the manufactured film.

As a crystal growth method of a nitride semiconductor replacing the MOCVD method, a pulse sputter deposition (PSD) method is now proposed. It has been demonstrated that a p-type GaN thin film having a low concentration of residual hydrogen and exhibiting a high hole mobility is obtained by the PSD method (Non-patent Document <NUM>).

Currently, the MOCVD method is used to manufacture an electronic device and a light emitting device on a nitride semiconductor substrate for practical use. However, it is difficult to produce, by the MOCVD method, an n-type layer having a high donor concentration which are important to decrease the resistance of these devices. Therefore, there are very few reports on the characteristics of such an n-type layer.

As can be seen, it is desired to develop a group <NUM> nitride semiconductor of n-type conductivity that exhibits a high electron mobility even in a region of a high donor concentration. In such a situation, it is required to realize a semiconductor material exhibiting as high an electron mobility as possible in order to achieve the purposes of improving the performance, saving the energy, and improving the efficiency of electronic devices and light emitting devices.

The present invention, made in light of such a problem, has an object of easily manufacturing and providing a two-, three- or four-component compound semiconductor, more specifically, a group <NUM> nitride semiconductor film, of n-type conductivity that exhibits a high electron mobility even in a region of a high donor concentration.

The above-described problems are solved by a compound semiconductor as claimed in claim <NUM> and a corresponding method of fabricating such a compound semiconductor according to claim <NUM>.

A two-component nitride refers to a compound of one element among B, Al, Ga and In, and nitrogen. Namely, the two-component nitride is a two-component mixed crystal of BN (boron nitride), AIN (aluminum nitride), GaN (gallium nitride) or InN (indium nitride).

A three-component nitride refers to a compound obtained as a result of any one of the two-component group <NUM> elements mentioned above being partially replaced with another group <NUM> element. The three-component nitride is, for example, a three-component mixed crystal of InGaN (indium gallium nitride), AlGaN (aluminum gallium nitride), or AlInN (aluminum indium nitride). Regarding the three-component compound, it is known that the composition ratio thereof may be adjusted to adjust the bandgap within the range of the characteristics of the two-component compound.

Even a compound containing a trace amount of another group <NUM> element in addition to the group <NUM> acting as a main component of the compound semiconductor may also be encompassed.

The invention is directed to a GaN semiconductor having n-type conductivity with an electron concentration of <NUM> × <NUM><NUM> cm-<NUM> or higher and containing Si as the donor.

Preferably, the electron concentration is <NUM> × <NUM><NUM> cm-<NUM> or higher.

Preferably, the nitride semiconductor has a contact resistance of <NUM> × <NUM>-<NUM> Ω·cm <NUM> or lower against an n-type ohmic electrode metal.

The nitride semiconductor contains oxygen as an impurity at <NUM> × <NUM><NUM> cm<NUM> or higher.

Preferably, the nitride semiconductor has an absorption coefficient of <NUM>-<NUM> or lower to light having a wavelengthof <NUM>.

The nitride semiconductor has an RMS value of <NUM> or less obtained by a surface roughness measurement performed by an AFM.

The lower limit of the specific resistance is, for example, <NUM> × <NUM>-<NUM> Ω·cm, <NUM> × <NUM>-<NUM> Ω·cm, or <NUM> × <NUM>-<NUM> Ω·cm.

Preferably, the relationship between the electron concentration of the specific resistance of the nitride semiconductor fulfills a numerical range enclosed by four points at which (a) the electron concentration is <NUM> × <NUM><NUM> cm-<NUM> and the specific resistance is <NUM> × <NUM>-<NUM> Ω·cm, (b) the electron concentration is <NUM> × <NUM><NUM> cm-<NUM> and the specific resistance is <NUM> × <NUM>-<NUM> Ω·cm, (c) the electron concentration is <NUM> × <NUM><NUM> cm-<NUM> and the specific resistance is <NUM> × <NUM>-<NUM> Ω·cm, and (d) the electron concentration is <NUM> × <NUM><NUM> cm-<NUM> and the specific resistance is <NUM> × <NUM>-<NUM> Ω·cm.

The above-described invention is applicable to a contact structure, comprising the nitride semiconductor for a conductive portion. The above-described invention is also applicable to a contact structure, comprising the nitride semiconductor for an electrode. Such a contact structure is usable in a semiconductor device.

The nitride compound semiconductor according to the present invention exhibits a high electron mobility of <NUM><NUM>/V·s or higher even in a region of a high electron concentration of <NUM> × <NUM><NUM> cm-<NUM> or higher. The electron mobility is preferably <NUM><NUM>/V·s or higher.

However, the electron concentration and the oxygen content may be adjusted in consideration of the productivity thereof, so that a compound semiconductor exhibiting an electron mobility of <NUM><NUM>/V · s or higher is manufactured and applied to a structural portion of the device for which such a compound semiconductor is needed.

According to the present invention, the pulse sputtering method (PSD method) is used to form a sputtered single crystal film with no high-temperature process. More preferably, a compound semiconductor film is formed in a process performed generally at room temperature. There is no limit on the area size of the substrate, and films of various sizes from a small size to a large size may be manufactured.

For example, a compound semiconductor film of a rectangular outer shape having a length of a side of <NUM> (<NUM> inches) or longer or a compound semiconductor film of a circular outer shape having a diameter of <NUM> (<NUM> inches) or longer may be formed. Alternatively, a compound semiconductor film having an area size that is <NUM><NUM> or larger and having an allowable area within the restriction of the internal space of the sputtering apparatus may be formed.

In this case, a high quality compound semiconductor film is easily formed with no need of a buffer layer, which is required by the conventional technology.

Now, the properties of the compound semiconductor according to the present invention will be described. The specific resistance ρ of an n-type nitride semiconductor film is in inverse proportion to the electron mobility µn and the carrier concentration n. Therefore, the n-type nitride semiconductor film exhibiting a high electron mobility even at a high electron concentration indicates that a high quality film having a low electric resistance is formed. Namely, the present invention provides a high quality group <NUM> nitride semiconductor film easily usable for a semiconductor device. The compound semiconductor according to the present invention has a threading dislocation density of about <NUM> × <NUM><NUM>/cm<NUM> to about <NUM> × <NUM><NUM>/cm<NUM>.

Hereinafter, a compound semiconductor manufactured by pulse-sputtering using a group <NUM> nitride semiconductor will be described as an embodiment according to the present invention with reference to the drawings.

A group <NUM> nitride semiconductor according to an embodiment of the present invention is formed as a film by a pulse sputter deposition method (PSD method).

The "pulse sputtering method (PSD method)" used to manufacture a compound semiconductor of a nitride according to the present invention, and the materials and the manufacturing method used to manufacture the compound semiconductor, are basic items well known to a person of ordinary skill in the art.

For example, the standard technologies disclosed in the following publications are usable to work the present invention with no problem: "<NPL>), "<NPL>), "<NPL>), <CIT> "Pulse sputtering apparatus, and pulse sputtering method", <CIT> "Gallium nitride sintered body or gallium nitride molded article, and method for producing the same", and the like. Patent Documents <NUM> and <NUM>, and Non-patent Documents <NUM> and <NUM>, and the like may also be referred to.

According to the PSD method used in the present invention, crystal growth advances based on a physical reaction, and therefore, may be performed at a low temperature. In addition, carbon and hydrogen in a film formation environment are conspicuously removable. Since the crystal growth may be performed at a low temperature, generation of a thermal stress in the film is suppressed, and also a compound easily causing phase separation such as, for example, InGaN, is stably grown.

Single crystal growth of a compound semiconductor according to the present invention is not visually recognizable directly, but the principle of action of the crystal growth is generally considered as follows. <FIG> shows a crystal structure of GaN, which is one of two-component group <NUM> compounds. During the film formation of a compound semiconductor according to the present invention, it is considered that a polar surface at which Ga atoms of GaN are located to form a hexagonal shape (Ga atom surface) is aligned to a surface of a substrate acting as an underlying layer, so that a single crystal structure is formed.

In this step, with the manufacturing method used in the present invention, the film formation is allowed to be performed at a relatively low temperature, instead of at a high temperature exceeding <NUM> required by the MOCVD method or the like. The temperature range to be used is <NUM> or lower and may include room temperature of <NUM> (room temperature to <NUM>). Although the temperature varies in accordance with the film formation rate, a preferable temperature range may be, for example, <NUM> to <NUM>.

For this reason, it is estimated that a small number of oxygen atoms contained in the film formation atmosphere are present to cover a surface of the film to be formed during the film formation. It is considered that as a result of the above, the oxygen atoms act to prevent the bonding of the group <NUM> element and nitrogen, and therefore, the film formation process advances while main elements to form the desired compound are kept free.

In addition, it is considered that since the film formation conditions are the same for the entirety of the underlying layer in a planar direction, a crystal structure that is uniform and has a high level of crystallinity entirely is formed.

The GaN compound semiconductor formed as a sputtered film in this manner is considered to gradually grow in an axial direction of the hexagonal shape (thickness direction of the film), so that in a final step, a compound semiconductor film that is uniform in the plane and has at least a certain area size is manufactured.

It is preferred that the underlying layer to be used is formed of a material fulfilling the condition of having a lattice matched with, or matched in a pseudo manner with, the compound semiconductor to be grown. The film formation process by the PSD method is not performed at a high temperature exceeding <NUM>. Therefore, the material of the underlying layer does not need to be resistant against a high temperature. However, in order to improve the crystallinity, it is preferred that the crystal and the underlying layer fulfill the conditions of being lattice-matched or pseudo-lattice-matched with each other.

For the above-described reasons, according to the present invention, it is especially preferred that the material of the underlying layer is selected from the four types: SiC, sapphire, GaN, single crystalline silicon. Sapphire has a heat resistant temperature of <NUM>, and single crystalline silicon has a heat resistant temperature of <NUM>. These materials are usable to manufacture semiconductor devices such as AlGaN/GaN HEMTs, full-color LEDs, InGaN-TFTs, sensors and the like.

Alternatively, the material of the underlying layer may be, for example, metal foil or alkali-free glass for FPD having a heat resistant temperature of <NUM> to <NUM>, or the like, although the formed crystal quality of the compound semiconductor is inferior to the quality in the case where the above-listed materials are used. In this case, it is preferred that a buffer layer is formed on a surface of the material of the underlying layer for the crystal growth, for the purpose of making the underlying layer pseudo-lattice-matched with the compound semiconductor.

Regarding the size of the film to be formed according to the present invention, a device having a length of a shorter side or a diameter of a circle of <NUM> to <NUM> (<NUM> inches to <NUM> inches) may be manufactured. The present invention is also applicable to a medium-sized device having a diagonal line of a rectangle of <NUM> to <NUM> (<NUM> to <NUM> inches) and a large device having a diagonal line of a rectangle of <NUM> (<NUM> inches) or longer. The device or the substrate acting as the underlying layer may be circular, square, rectangular, or of an asymmetrical shape.

<FIG> respectively show a schematic view of a sputtering apparatus and a pulse sequence usable to manufacture a compound semiconductor according to the present invention. A sputtering apparatus <NUM> mainly includes a chamber <NUM>, a substrate electrode <NUM>, a target electrode <NUM>, a DC power supply <NUM>, a power supply controller <NUM>, a nitrogen supply source <NUM>, a heating device and the like.

The chamber <NUM> is sealable against the outside. The inner pressure of the chamber <NUM> is allowed to be decreased by a vacuum pump or the like (not shown). The substrate electrode <NUM> is located in the chamber <NUM>, and is capable of holding a heat dissipation sheet 12a.

The target electrode <NUM> is provided in the chamber <NUM> so as to face the substrate electrode <NUM>, and is capable of holding a target 13a. The target 13a is formed of a compound of a group <NUM> element and nitrogen. A high quality target material with little impurities that is currently available in general is used. For example, a high quality material such as the five-nine or six-nine level is needed.

The DC power supply <NUM> is electrically connected with the substrate electrode <NUM> and the target electrode <NUM>, and is a voltage source that applies a DC voltage between the substrate electrode <NUM> and the target electrode <NUM>.

The power supply controller <NUM> is connected with the DC power supply <NUM>, and performs control regarding the timing of the operation of the DC power supply <NUM>. The power supply controller <NUM> allows a pulse voltage to be applied between the substrate electrode <NUM> and the target electrode <NUM>.

The nitrogen supply source <NUM> is connected with the inside of the chamber <NUM> by, a supply tube or the like, and supplies nitrogen gas into the chamber <NUM>. Although not shown, an argon gas supply source that supplies argon gas into the chamber is also provided in addition to the nitrogen gas supply source <NUM>.

An oxygen supply source that supplies a predetermined amount of oxygen is also provided. The internal pressure is constantly allowed to be monitored while the film is formed. The content of oxygen in the chamber needs to be controlled to be kept at about <NUM> ppm substantially constantly during the film formation of the compound semiconductor.

In order to realize this, it is indispensable that the chamber used for the pulse sputtering, the supply system of the process gas and the discharge system of the process gas (main discharger, rough discharger) prohibit gas leak and invasion of external air, and it is important that the pressure is controlled to be highly stable during the film formation. It is considered to be fundamental to supply a trace amount of oxygen into the chamber intentionally. In order to realize this, the chamber needs to be confirmed to have been cleaned, and the materials to be used need to have a high purity.

The heating device <NUM> is secured to, for example, the substrate electrode <NUM>, so that the temperature around the heat dissipation sheet 12a on the substrate electrode <NUM> is adjustable. The representative examples of the film formation conditions to be used according to the present invention are as follows. <FIG> is an example of pulse sequence. The voltage PA of the driving pulse is adjustable. The film formation rate is generally <NUM> to <NUM>/sec. on average, and more preferably <NUM> to <NUM>/sec.

The film formation by the sputtering was performed in atmospheric gas containing argon as a main component, and the substrate temperature during the film formation was set to the range of <NUM> to <NUM>. In this case, doping gas such as SiH<NUM>, is usable as the doping material, and a target containing Si atoms is usable, in order to form a high concentration n-type group <NUM> nitride compound semiconductor.

Experiments were made in which oxygen were incorporated at a concentration of <NUM> ppm into the atmospheric gas to be used for the sputtering in order to introduce oxygen into the film of the target compound semiconductor to be manufactured, and in which oxygen was not incorporated. Physical characteristics of the compound semiconductor manufactured with oxygen and the compound semiconductor manufactured with no oxygen were checked in comparison with each other.

<FIG> is a schematic vertical cross-sectional view of a continuous film formation apparatus <NUM> of a roll-to-roll system. A plurality of film formation chambers <NUM> are provided inside the continuous film formation device <NUM>. The present invention is applicable to such a device as long as the substrate film <NUM> is a metal foil or a very thin film-like glass substrate that may be taken into a roll or taken out of a roll. While the flexible substrate film <NUM> is transported in a horizontal direction from a take-out roll <NUM> to a take-in roll <NUM>, the sputtering may be performed toward the substrate film <NUM> at a plurality of locations in the film formation chamber. As a result, a semiconductor device containing a desired compound semiconductor or the like is processed at a high speed. The table in the chamber is usable for, for example, a diameter of <NUM> to <NUM>.

According to the present invention, crystal growth of a compound semiconductor is realized on an underlying layer or a substrate having an area size defined by a shorter side of a rectangle or a diameter of at least <NUM> (<NUM> inches). The crystal is manufactured at a low temperature and at a high rate so as to have a certain area size and to be uniform. In addition, a novel compound semiconductor is mass-manufactured while the manufacturing cost thereof is suppressed.

<FIG> shows the relationship between the electron concentration (Ne) and the electron mobility (µe) of a Si-doped n-type GaN film produced by the PSD method by the present inventors. The electron concentration and the electron mobility were determined by a room temperature Hall effect measurement. The electron concentration (Ne) is considered to be substantially equal to the Si donor concentration. The film formation by the sputtering was performed in atmospheric gas containing argon gas as a main component, and the substrate temperature during the film formation was in the range of <NUM> to <NUM>.

Oxygen was incorporated at a concentration of <NUM> ppm into the atmospheric gas to be used for the sputtering for the purpose of introducing oxygen into this film, so that a crystal film exhibiting single crystallinity was formed. An n-type ohmic electrode metal stack structure (Ti (<NUM>)/Al (<NUM>)/Ti (<NUM>)/Au (<NUM>)) was formed on a surface of the resultant GaN thin film, and was annealed in nitrogen at <NUM>. The contact resistance of samples formed in this manner was evaluated by a TLM method and was found to be <NUM> × <NUM>-<NUM> Ω·cm<NUM>.

In this figure, the circles show the actually measured values, and the curve shows the fitting result based on the Caughey-Thomas-type empirical formula (formula <NUM> below; see Non-patent Document <NUM>), which is used to describe the mobility in a low electric field. In the formula below, ND is the donor concentration. Since the electron concentration (Ne) is considered to be substantially equal to the Si donor concentration as described above, the fitting is performed with an assumption that ND = Ne.

From the above-shown fitting result, <MAT> <MAT> were found. These values are comparable to the highest value of the mobility of the n-type GaN thin film formed by the MOCVD method conventionally reported (see, for example, Non-patent Document <NUM>). As can be seen, it has been confirmed that the carrier scattering is sufficiently suppressed in the film of the compound semiconductor manufactured according to the present invention.

With the MOCVD method of the conventional technology, it is considered to be difficult to form a GaN thin film exhibiting such a high electron mobility when the donor concentration is generally <NUM> × <NUM><NUM> cm-<NUM> or higher. According to the present invention, as shown in <FIG>, the Si-doped n-type GaN film produced by the PSD method exhibits the values matching the Caughey-Thomas-type empirical formula even at the donor concentration of at least <NUM> × <NUM><NUM> cm-<NUM>.

Namely, it has been found out that an n-type GaN film according to this example produced by the PSD method is a very high quality film exhibiting an electron mobility of <NUM><NUM>/V·s or higher even at an electron concentration of <NUM> × <NUM><NUM> cm-<NUM> or higher. Preferably, a film exhibiting an electron mobility of <NUM><NUM>/V·s or higher is usable. The specific resistance ρ of an n-type nitride semiconductor film is in inverse proportion to the electron mobility µn and the carrier concentration n. Therefore, the n-type nitride semiconductor film exhibiting a high electron mobility even at a high electron concentration indicates that a high quality film having a low resistance is formed.

The samples shown in <FIG> are all Si-doped.

When the donor concentration of the nitride semiconductor film is increased in order to realize a high electron concentration, the transparency of the film to visible light is decreased. This causes a concern that an inconvenience may occur in the case where the nitride semiconductor film according to the present invention is used for a transparent electrode or the like.

Under such circumstances, according to the present invention, the decrease in the transparency caused by the increase in the electron concentration of the film of the compound semiconductor is compensated for as follows. The nitrogen site is replaced, so that oxygen, which is a dopant acting as a donor, is incorporated as an impurity to expand the bandgap of the film.

The bandgap of an oxygen-doped film depends on the amount of doping. For example, in the case of GaN, the bandgap at room temperature may be varied in the range of <NUM> eV to <NUM> eV (value of the bandgap of gallium oxide). In GaN, when oxygen is incorporated as an impurity at <NUM> × <NUM><NUM> cm-<NUM> or higher into the film, the bandgap at room temperature is generally about <NUM> to about <NUM> eV.

Such an effect of oxygen, for example, allows the nitride semiconductor film according to this example to have an absorption coefficient of <NUM>-<NUM> or less to light having a wavelengthof <NUM> or to have an absorption coefficient of <NUM>-<NUM> or less to light having a wavelength of <NUM>. In this manner, the nitride semiconductor film according to this example is usable for a transparent electrode with no inconvenience.

<FIG> provides graphs each showing an oxygen concentration of the GaN film according to this embodiment manufactured by the PSD method. <FIG> shows SIMS data representing a profile, in a depth direction, of the oxygen concentration of a GaN film having a Si concentration of <NUM> × <NUM><NUM> cm-<NUM>, among the samples shown in <FIG>. It is understood that the oxygen is contained at a concentration of about <NUM> to <NUM> × <NUM><NUM> cm-<NUM>. This film exhibits an electron mobility of <NUM><NUM>/V·s.

The RMS value of an AFM image representing the surface roughness of this film was <NUM> as seen from <FIG>. The present inventors performed an AFM measurement on the film samples formed by the present inventors at various electron concentrations and containing oxygen at an electron concentration of <NUM> × <NUM><NUM> cm-<NUM> or higher. All the samples had an RMS value of <NUM> or less.

In the meantime, crystal growth was performed under substantially the same conditions but with no incorporation of <NUM> ppm oxygen into the atmospheric gas. The results were as follows. As shown in the profile in <FIG>, the oxygen concentration was about <NUM> x <NUM><NUM> cm-<NUM>, and the mobility at this point was <NUM><NUM>/V·s. As seen from <FIG>, the RMS value representing the surface roughness of this thin film was <NUM>.

Now, the two conditions, namely, the condition of incorporating oxygen and the condition of not incorporating oxygen, will be discussed. In the case with oxygen, it is considered that oxygen atoms in the atmosphere covering a surface of the film that is being formed cause the stress to alleviate and the migration of the atoms at the surface to promote. It is considered that this suppression on the surface roughness suppresses introduction of point defects and thus improves the mobility. At a high temperature used by the MOCVD method or the like of the conventional technology, oxygen evaporates from the surface. Therefore, it is considered to be difficult to provide the effect of improving the quality realized by the low-temperature growth performed by the PSD method.

By contrast, in the case with no oxygen, it is considered that the above-described action is not easily provided and thus the crystal of the film formed by the PSD method is likely to include defects.

<FIG> provides graphs showing the absorption coefficient (<FIG>) and the refractive index (<FIG>) of a GaN film having a Si concentration (electron concentration) of <NUM> × <NUM><NUM> cm-<NUM> measured by an ellipsometer. This film exhibits an electron mobility of <NUM><NUM>/V·s. This film has an absorption coefficient of <NUM>-<NUM> at a wavelength of <NUM>, which is used for a blue LED as a standard wavelength, and has an absorption coefficient of <NUM>-<NUM> at a wavelength of <NUM>, which is used for a blue-violet laser as a standard wavelength.

As can be seen, the oxygen doping allows the film to have an absorption coefficient of <NUM>-<NUM> or less to light having a wavelength of <NUM> or to have an absorption coefficient of <NUM>-<NUM> or less to light having a wavelength of <NUM>. As a result, the obtained compound semiconductor is usable as a transparent material.

Hereinafter, various forms of electronic device to which a compound semiconductor according to the present invention is applicable.

<FIG> is a schematic cross-sectional view of a compound semiconductor device <NUM> including a group <NUM> nitride semiconductor formed on a substrate. Reference sign <NUM> represents the substrate (sapphire), and reference sign <NUM> represents GaN.

<FIG> is a schematic cross-sectional view of a contact structure formed using a compound semiconductor according to the present invention. Reference sign <NUM> represents a GaN substrate, reference sign <NUM> represents GaN (film of a compound semiconductor formed by the PSD method), reference sign <NUM> represents an insulating film, reference sign <NUM> represents a wiring electrode connectable with an external device, and reference sign <NUM> represents a contact hole.

<FIG> is a schematic cross-sectional view of a contact structure <NUM> formed using a group <NUM> nitride compound semiconductor according to the present invention. In <FIG>, reference sign <NUM> represents an n-type GaN contact layer, reference sign <NUM> represents a Ti layer, reference sign <NUM> represents an Al layer, reference sign <NUM> represents an Ni layer, and reference sign <NUM> represents an Au layer. In this example, a composite metal electrode is used. After the film formation, heat treatment is performed at about <NUM>.

<FIG> is a schematic cross-sectional view of a thin film transistor to which the present invention is applicable. A high concentration n-type GaN layer is applicable as a contact layer of an electrode of the thin film transistor.

In the figure, reference sign <NUM> represents a substrate formed of alkali-free glass or the like, reference sign <NUM> represents an interlayer insulating layer, reference sign <NUM> represents a source-side contact layer (high concentration n+ GaN layer), reference sign <NUM> represents a source region, reference sign <NUM> represents an active layer, reference sign 54D represents a drain region, reference sign 53D represents a drain-side contact layer (high concentration n+ GaN layer), reference sign <NUM> represents a gate oxide film, reference sign <NUM> represents a source electrode, reference sign <NUM> represents a gate electrode, and reference sign <NUM> represents a drain electrode. The source region <NUM> and the drain region 54D are each formed such that the concentration of the impurity is gradually changed between the corresponding contact layer and the active layer.

<FIG> is a schematic cross-sectional view of a HEMT device to which the present invention is applicable. A high concentration n-type GaN layer according to the present invention is applicable as contact layers located below, and in contact with, source and drain electrodes of the HEMT device of AlGaN/GaN. In the figure, reference sign <NUM> represents a substrate formed of GaN, sapphire, SiC, Si or the like, reference sign <NUM> represents a buffer layer formed of GaN, AlN or the like, reference sign <NUM> represents a GaN undoped layer, reference sign <NUM> represents an AlGaN barrier layer, and reference sign <NUM> represents a contact layer formed using a high concentration n-type GaN layer. A source electrode <NUM>, a gate electrode <NUM> and a drain electrode <NUM> are provided in a top part of the device.

In the thin film transistor (<FIG>) and the HEMT device (<FIG>) described above, the high concentration n-type GaN layer is applicable as the contact layer. The contact resistance of such a contact layer against an electrode in a circuit element in which an operating current flows (the circuit element is each of source and drain in these devices) is significantly decreased. This significantly contributes to the improvement in performance of the electronic device.

<FIG> is a schematic cross-sectional view of an LED device as an example of GaN-based semiconductor device to which the present invention is applicable.

As shown in this figure, a plurality of compound semiconductor layers are sequentially stacked from the side of a substrate <NUM> formed of GaN, sapphire, SiC or Si. A buffer layer <NUM>, an n-type GaN layer <NUM>, a GaInN/GaN MQW light emitting layer <NUM>, a p-type GaN layer <NUM>, a tunnel junction <NUM> including a p-type GaN layer 76a and a high concentration n-type GaN layer 76b, an n-type GaN layer <NUM>, a contact layer <NUM> formed of a high concentration n-type GaN layer, and electrodes 79A and 79B are provided.

<FIG> is a schematic cross-sectional view of an InGaN/GaN VCSEL (surface emitting laser) structure to which the present invention is applicable. In such a vertical cavity surface emitting laser (VCSEL), a resonator is formed to be perpendicular to a surface of a semiconductor substrate. Therefore, laser light is output perpendicularly to the substrate surface.

In the figure, reference sign <NUM> represents a GaN substrate, reference sign 82D represents an inner multi-layer reflection mirror, reference sign <NUM> represents an n-type GaN layer, reference sign <NUM> represents an MQW active layer formed of GaInN/GaN, reference sign <NUM> represents a p-type AlGaN layer, reference sign 86a represents a p-type InGaN layer, and reference sign 86b represents a high concentration n-type GaN layer. 86a and 86b form a tunnel junction <NUM>. Reference sign <NUM> represents an n-type GaN layer, reference sign <NUM> represents a high concentration n-type GaN layer (contact layer), reference sign 89A and 89B represent electrodes, and reference sign 82U represents an upper multi-layer reflection mirror.

As described above, the compound semiconductor according to the present invention is usable for, for example, regions of a light emitting device or an electronic device in which a large amount of electric current flows, a contact portion of a semiconductor device, or an electrode structure such as a transparent electrode or the like. The compound semiconductor according to the present invention is preferably usable for a wire or the like of an electronic device drivable at a very low voltage. The compound semiconductor according to the present invention is adaptable to the specifications of large electric current and large electric power, which are not easily dealt with by the conventional technology.

The compound semiconductor according to the present invention exhibits a high electron mobility and thus has a low resistance, and therefore is considered to contribute to improvement in the operation speed of devices.

So far, a compound semiconductor according to the present invention, namely, a two-, three- or four-component compound semiconductor that contains nitrogen and one element selected from the group consisting of B, Al, Ga and In, which are group <NUM> elements, contains oxygen as an impurity at <NUM> × <NUM><NUM> cm-<NUM> or higher, has an electron concentration of <NUM> × <NUM><NUM> cm-<NUM> or higher, has n-type conductivity by Si donors and exhibits an electron mobility of <NUM><NUM>/V·s or higher has been described.

Hereinafter, a nitride semiconductor according to the invention made by the present inventors will be described.

The nitride semiconductor has a conspicuous feature of exhibiting a lower specific resistance (namely, exhibiting a higher mobility) than a conventional semiconductor although being in the form of a crystal doped with a donor at a high concentration.

Specifically, the nitride semiconductor is GaN has n-type conductivity, exhibits an electron concentration of <NUM> × <NUM><NUM> cm-<NUM> or higher, and exhibits a specific resistance of <NUM> × <NUM>-<NUM> Ω·cm or lower. Si is contained as donor impurity.

Conventionally, a nitride semiconductor doped with Ge grown by the MBE method at a high concentration and exhibiting a relatively low specific resistance is known. As compared with such a nitride semiconductor, the nitride semiconductor according to the present invention realizes a lower specific resistance in a region having a higher electron concentration.

Such a nitride semiconductor exhibiting a low specific resistance (exhibiting a high mobility) although being in the form of a crystal doped with donors at a high concentration is expected to be used for various uses, for example, to decrease the parasitic resistance of an electronic device such as an HEMT or the like, to provide a material replacing a transparent conductive film of ITO or the like, and to realize cascade connection of LED modules.

<FIG> shows the relationship between the electron concentration (cm-<NUM>) and the resistivity (mΩ·cm) of GaN according to the present invention. In the figure, star marks represent the GaN according to the present invention. Among the start marks, white star marks represent Si-doped GaN, and gray star marks represent Ge-doped GaN. The figure also shows, for comparison, data of GaN obtained by the MOCVD method (diamond-shaped marks) and the MBE method (circular marks) reported so far, and also shows the relationship between the electron concentration and the resistivity obtained by a theoretical calculation. In the figure, θ represents the compensation ratio of the concentration of ionized impurities (ratio of the acceptor concentration NA and the donor concentration ND; NA/ND).

The GaN crystal conventionally reported exhibits a tendency that the specific resistance is decreased as the electron concentration is increased regardless of whether the crystal is obtained by the MBE method or the MOCVD method. However, the specific resistance is increased when the electron concentration is above a certain level.

For example, in the case of GaN obtained by the MOCVD method, Si-doped GaN shows an increase in the specific resistance from when the electron concentration exceeds about <NUM> × <NUM><NUM> cm-<NUM>, and Ge-doped GaN shows an increase in the specific resistance from when the electron concentration exceeds about <NUM> × <NUM><NUM> cm-<NUM>. In the case of GaN obtained by the MBE method, Si-doped GaN shows an increase in the specific resistance from when the electron concentration exceeds about <NUM> × <NUM><NUM> cm-<NUM>, and Ge-doped GaN shows an increase in the specific resistance from when the electron concentration exceeds about <NUM> × <NUM><NUM> cm-<NUM>.

By contrast, in the case of GaN according to the present invention, neither Si-doped GaN (white marks) nor Ge-doped GaN (gray marks) shows any such increase in the specific resistance even when the electron concentration is <NUM> × <NUM><NUM> cm-<NUM>.

In addition, in the case of the conventional GaN, even Ge-doped GaN, obtained by the MBE method and exhibiting the lowest specific resistance in a region of a high electron concentration, exhibits a specific resistance of merely <NUM> mΩ·cm (<NUM> × <NUM>-<NUM> Ω·cm) at the minimum at an electron concentration of about <NUM> × <NUM><NUM> cm-<NUM>. By contrast, the GaN according to the present invention exhibits a specific resistance of <NUM> mΩ·cm (<NUM> × <NUM>-<NUM> Ω·cm) at generally the same electron concentration.

As is clear from the results shown in this figure, unlike the conventional GaN, the GaN according to the present invention has a feature of exhibiting a conspicuously low specific resistance of <NUM> × <NUM>-<NUM> Ω·cm or lower especially when the electron concentration is <NUM> × <NUM><NUM> cm-<NUM> or higher, and this feature is not lost even when the electron concentration is <NUM> × <NUM><NUM> cm-<NUM> or higher. As shown in the table below, this tendency has been experimentally confirmed in the range of specific resistance down to <NUM> × <NUM>-<NUM> Ω·cm. The theoretical value of the lowest limit of the resistance value caused by scattering of ionized impurities is <NUM> × <NUM>-<NUM> Ω·cm, but is varied to, for example, <NUM> × <NUM>-<NUM> Ω·cm, <NUM> × <NUM>-<NUM> Ω·cm, <NUM> × <NUM>-<NUM> Ω·cm or the like depending on the film formation conditions or the like.

<FIG> shows the relationship between the concentration of the donor impurities and the electron concentrations, of the GaN according to the present invention, obtained by a SIMS measurement. It is understood from these results that the activation ratio of the donors is about <NUM> in the GaN according to the present invention obtained by the PSD method. Namely, it is understood that for the GaN according to the present invention, the electron concentration is controllable by merely controlling the doping concentration of the donor impurity.

The various characteristics (electron concentration, electron mobility, specific resistance, and surface roughness) of the GaN according to the present invention are shown in Table <NUM> (Si-doped GaN). Table <NUM> shows the corresponding characteristics for Ge-doped GaN.

The GaN shown in Table <NUM> and Table <NUM> is all obtained in generally the same conditions as the crystal growth conditions by the PSD method described above. The materials and the like each having the following purity were used. The electron concentration was changed by changing the power applied to the cathode from <NUM> to <NUM> W.

The present inventors note that the vacuum level of the film formation environment and the quality of the vacuum state are important for growing a high quality crystal, and appropriately adjusted the conditions of pulse sputtering (pulse voltage, pulse width, duty ratio, etc.) in order to obtain a crystal of a desired film quality. It is one of advantages of the PSD method that such fine adjustments may be made quickly.

The measurement conditions and the like for the above-mentioned various properties are as follows.

The electron concentration and the electron mobility were measured by use of a Hall measurement device (ResiTest8400, Toyo Corporation) while the applied current was varied in the range of <NUM> mA to <NUM> mA and the applied magnetic field was varied in the range of <NUM> to <NUM> T (tesla) in accordance with the resistivity of the sample. The temperature for the measurement was room temperature.

The surface roughness was measured by use of an AFM device (JSPM4200 produced by JEOL Ltd.

<FIG> shows AFM images of surfaces of the Ge-doped GaN samples as examples of surface state of the above-described GaN. These samples all have an RMS value less than <NUM>. In general, a surface having an RMS value, obtained by a surface roughness measurement by an AFM device, of <NUM> or less may be evaluated to be sufficiently flat. In consideration of this, it is understood that the nitride semiconductor according to the present invention has a highly flat surface.

Nitride semiconductor crystals having the Ga site of GaN be partially replaced with Al or In (AlGaN or InGaN) were also produced, and various properties thereof were examined. The results are shown in Table <NUM> and Table <NUM>. These samples each have an Al concentration of <NUM>% and an In concentration of <NUM>%. The purity and the like of each of the materials used for the crystal growth are as follows.

The contact resistance of each of the nitride semiconductors shown in Table <NUM> to Table <NUM> was measured. It has been confirmed that all the samples have a contact resistance of <NUM> × <NUM>-<NUM> Ω·cm <NUM> or less against an n-type ohmic electrode metal. Such a value is sufficiently low. A contact structure including any of the above-described nitride semiconductors for a conductive portion is expected to be used in various uses, for example, to decrease the parasitic resistance of an electronic device such as a HEMT or the like, to provide a material replacing a transparent conductive film of ITO or the like, and to realize cascade connection of an LED module.

The contact resistance was measured by use of a TLM (Transmission Line Model) measurement apparatus (semiconductor parameter analyzer Agilent 4155C) on a TLM pattern including Ti/Al/Ti/Au electrode structures (<NUM> × <NUM>) located at an inter-electrode distance of <NUM> to <NUM>.

As described above, the nitrogen site of the nitride semiconductor may be replaced, so that oxygen, which is a dopant acting as a donor, is incorporated as an impurity to expand the bandgap of the film. In this manner, the decrease in the transparency caused by the increase in the electron concentration of the film of the nitride semiconductor is compensated for.

For this purpose, for example, oxygen as an impurity is incorporated at <NUM> × <NUM><NUM> cm-<NUM> or higher into the above-described nitride semiconductor. Such incorporation of oxygen as an impurity allows the nitride semiconductor to have an absorption coefficient of <NUM>-<NUM> or less to light having a wavelength of <NUM> or to have an absorption coefficient of <NUM>-<NUM> or less to light having a wavelength of <NUM>.

The above-described nitride semiconductor according to the present invention is formed by the PSD method. The present inventors consider that the above-described characteristics are obtained for the following reason: with the other crystal growth methods, the crystal growth advances in a thermal equilibrium state, whereas with the PSD method, the crystal growth advances in a thermal non-equilibrium state.

A nitride semiconductor such as GaN or the like doped with a donor at a high concentration is thermodynamically unstable, and therefore, is partially decomposed even while the crystal growth is advancing. Namely, the growth and the decomposition of the crystal occur at the same time. Therefore, the donor impurities once incorporated into the crystal is are pushed out at the time of decomposition. When it is attempted to dope the nitride semiconductor with donor impurities at a high concentration, this phenomenon that the donor impurities are pushed out reaches to an unignorable level, and as a result, the crystallinity itself is decreased. Namely, in the case where the nitride semiconductor is doped with the donor impurities at a high concentration, the decrease in the crystallinity is unavoidable under the crystal growth conditions close to the thermal equilibrium state.

By contrast, with the PSD method, the crystal growth advances in a thermal non-equilibrium state. Therefore, the donor impurities are not easily pushed out, and thus the crystallinity is not easily decreased.

In general, the Ge donor tends to be more easily incorporated into the nitride semiconductor crystal at a high concentration than the Si donor. One conceivable reason for this is the following. Since the radius of the Ge ion is close to the radius of Ga ion, the Ge ion easily replaces the Ga ion site. As a result, the accumulation of stress in the nitride semiconductor film is alleviated, and thus the surface of the film tends to be flat.

As described above, the nitride semiconductor according to the present invention realizes a lower specific resistance in a region of a higher electron concentration than the conventional nitride semiconductor.

There are the following documents that disclose inventions relating to a nitride semiconductor device having a low on-resistance.

<CIT> (Patent Document <NUM>) discloses an invention relating to a nitride semiconductor device having a low on-resistance. Paragraph <NUM> describes that "as described above, the source-side nitride semiconductor regrowth layer 205a and the drain-side nitride semiconductor regrowth layer 206a each may contain n-type impurities at a high concentration. However, as shown in <FIG>, when the impurity is silicon (Si), even if an impurity amount to be supplied during the growth of a nitride semiconductor layer is increased, the carrier concentration of the impurity in the nitride semiconductor layer to be formed is not increased. That is, the impurity carrier concentration has a certain upper limit. On the other hand, when germanium (Ge) is used as the impurity, a higher carrier concentration than that of silicon can be realized".

Paragraph <NUM> describes that "in order to investigate the characteristics of the composite electrode of the nitride semiconductor device <NUM> thus formed, the sheet resistance of the nitride semiconductor regrowth layer itself and the contact resistance thereof with the 2DEG were measured by a transmission line measurement (TLM) method. <FIG> shows the relationship between the sheet resistance of the nitride semiconductor regrowth layer itself and the supply amount of Ge. It was found that when the flow rate ratio of TEGe to TMG is increased to <NUM> or more with an increase in supply amount of TEGe, a nitride semiconductor regrowth layer having a lowered sheet resistance of approximately <NUM> × <NUM>-<NUM> Ω·cm can be obtained. It was found that when a nitride semiconductor regrowth layer formed under the conditions described above is used, the nitride semiconductor device <NUM> has a contact resistance of <NUM> to <NUM> × <NUM>-<NUM>Ω·cm, and a preferable contact with the 2DEG can be obtained".

Patent Document <NUM> and the corresponding United States Patent Application Publication <CIT> (Patent Document <NUM>) filed claiming the benefit of priority to Patent Document <NUM> were compared against each other regarding the above description. As a result, it has been found out that the name and the unit of the vertical axis of <FIG> are variously changed. It is presumed that Patent Document <NUM> includes some typographical error.

A technological document written by the inventors of Patent Document <NUM> (<NPL>") will be referred to. This document discloses a Ge-doped nitride semiconductor regrowth layer exhibiting a low on-resistance. <FIG> is exactly the same as <FIG> of Patent Document <NUM>.

The vertical axis is labeled as "Specific contact resistance (Ω·cm<NUM>)". Regarding <FIG>, there is a description "the measured specific contact resistance as a function of TEGe supply is shown in <FIG>, where extremely low specific contact resistance of <NUM> × <NUM>-<NUM> Ω·cm<NUM> was achieved". For this reason, it is considered that the vertical axis of <FIG> of Patent Document <NUM> should be "contact resistance" and the unit should be "Ω·cm<NUM>".

If, as shown in <FIG> of Patent Document <NUM>, the specific resistance is about <NUM> × <NUM>-<NUM> Ω·cm and the Ge concentration (electron concentration) is <NUM> × <NUM><NUM> cm-<NUM>, the electron mobility is about <NUM>,<NUM><NUM>/Vs. This value is far from the normal value known as the electron mobility of GaN crystal (about <NUM>,<NUM><NUM>/V·s). Based on this also, it is obvious that the above-described portion includes typographical errors.

As described above, Patent Document <NUM> is considered to disclose a "nitride semiconductor regrowth layer having a lowered contact resistance of approximately <NUM> × <NUM>-<NUM> Ω·cm<NUM>".

The above-described nitride semiconductor according to the present invention has a feature of exhibiting a low specific resistance (exhibiting a high mobility) although being in the form of a crystal doped with donors at a high concentration, and utilizing such a feature, is expected to be used for various uses, for example, to decrease the parasitic resistance of an electronic device such as an HEMT or the like, to provide a material replacing a transparent conductive film of ITO or the like, and to realize cascade connection of an LED module. For example, the nitride semiconductor according to the present invention may be applied as follows.

<FIG> is a schematic cross-sectional view of a vertical power MOSFET. This vertical power MOSFET <NUM> includes an n+-GaN layer <NUM> of a nitride semiconductor, according to the present invention, formed on a stack structure including an n+-GaN layer <NUM>, an n--GaN layer <NUM> and a p-GaN layer <NUM>. The n+-GaN layer <NUM> according to the present invention may be patterned as follows. After being deposited on the entire surface, the n+-GaN layer is patterned by lithography. Alternatively, a selective growth technology may be used, according to which, a crystal surface of gallium nitride is exposed to only a part of a surface of the sample, and the n+-GaN layer is epitaxially grown selectively on the exposed part. Reference sign <NUM> represents an insulating film, reference sign <NUM> represents a drain, reference sign <NUM> represents a source, and reference sign <NUM> represents a gate.

<FIG> is a schematic cross-sectional view of a GaN-based LED. The LED <NUM> includes an n-type nitride semiconductor layer <NUM>, an active layer <NUM> including a quantum well layer, a p-type nitride semiconductor layer <NUM>, and an n+-GaN layer <NUM> according to the present invention sequentially stacked on a substrate <NUM> formed of a nitride semiconductor.

A cathode electrode <NUM> is formed on a region of the n-type nitride semiconductor layer <NUM> that is exposed as a result of the n+-GaN layer <NUM>, the p-type nitride semiconductor layer <NUM> and the active layer <NUM> being partially removed. An anode electrode <NUM> is formed above the p-type nitride semiconductor layer <NUM> with the n+-GaN layer <NUM> being located therebetween. The n+-GaN layer <NUM> according to the present invention is conductive with the p-type nitride semiconductor layer <NUM> via a tunnel junction.

<FIG> is a schematic cross-sectional view of a Schottky diode. In this Schottky diode <NUM>, an n+-GaN substrate <NUM> has an n+-GaN layer <NUM> according to the present invention formed on a rear surface thereof. An n--GaN layer <NUM> is formed on a front surface of the n+-GaN substrate <NUM>. An ohmic electrode <NUM> is formed on the n+-GaN layer <NUM> side, and a Schottky electrode <NUM> is formed on the n--GaN layer <NUM> side. In the figure, reference sign <NUM> represents an insulating film.

The nitride semiconductor according to the present invention exhibiting a low specific resistance (exhibiting a high mobility) although being in the form of a crystal doped with a donor at a high concentration is usable for an n+-GaN layer of, for example, an IGBT (Insulated Gate Bipolar Transistor) in addition to the above-described devices.

As described above, the compound semiconductor according to the invention made by the present inventors may be summarized as follows.

The invention is directed to a gallium nitride semiconductor having n-type conductivity and an electron concentration of <NUM> × <NUM><NUM> cm-<NUM> or higher from Si donors.

The nitride semiconductor contains oxygen as an impurity at <NUM> × <NUM><NUM> cm-<NUM> or higher.

Preferably, the nitride semiconductor has an absorption coefficient of <NUM>-<NUM> or lower to light having a wavelength of <NUM>.

Preferably, the nitride semiconductor has an RMS value of <NUM> or less obtained by a surface roughness measurement performed by an AFM.

The relationship between the electron concentration and the specific resistance of the nitride semiconductor fulfills a numerical range enclosed by four points at which (a) the electron concentration is <NUM> × <NUM><NUM> cm-<NUM> and the specific resistance is <NUM> × <NUM>-<NUM> Ω·cm, (b) the electron concentration is <NUM> × <NUM><NUM> cm-<NUM> and the specific resistance is <NUM> × <NUM>-<NUM> Ω·cm, (c) the electron concentration is <NUM> × <NUM><NUM> cm-<NUM> and the specific resistance is <NUM> × <NUM>-<NUM> Ω·cm, and (d) the electron concentration is <NUM> × <NUM><NUM> cm-<NUM> and the specific resistance is <NUM> × <NUM>-<NUM> Ω·cm.

The nitride semiconductor according to the present invention exhibits an electron mobility of <NUM><NUM>/V·s or higher in a silicon-doped region of a high electron concentration of <NUM> × <NUM><NUM> cm-<NUM> or higher.

The present invention is applicable to an important circuit element that determines the performance of an electronic circuit, such as a contact portion of a wiring structure that is included in an electronic device having a low electric resistance and requiring a large amount of electric current, for example, a horizontal power semiconductor device such as an HEMT or the like, a vertical power semiconductor device, a high withstand voltage diode, a thin film transistor, a display device or the like, an active layer or the like.

Claim 1:
A GaN compound semiconductor,
wherein
the compound semiconductor is manufactured by a pulse sputtering method in a process atmosphere containing oxygen and a substrate temperature of <NUM> or lower,
the compound semiconductor contains Si as a donor,
the compound semiconductor contains oxygen as an impurity at <NUM> × <NUM><NUM> cm-<NUM> or higher,
the compound semiconductor has an electron concentration of <NUM> × <NUM><NUM> cm-<NUM> or higher and has n-type conductivity,
the compound semiconductor exhibits an electron mobility of <NUM><NUM>/V·s or higher, and
the compound semiconductor has an RMS value of <NUM> or less obtained by a surface roughness measurement performed by an AFM.