Semiconductor device

Provided is a semiconductor device in which electron mobility is improved by applying sufficiently large tensile stress in a predetermined direction without occurrence of cracks in a nitride semiconductor. The semiconductor device includes: substrate (101), electron transit layer (103) that is disposed on substrate (101) and is formed by GaN; and electron supply layer (104) that is disposed on electron transit layer (103) and is formed by AlGaN. A coefficient of thermal expansion of substrate (101) is different between a first direction in a main surface of substrate (101) and a second direction that is perpendicular to the first direction in the main surface, and tensile stress occurs in electron transit layer (103).

FIELD OF THE INVENTION

The present disclosure relates to a semiconductor device, and particularly, to a semiconductor device having high carrier mobility.

DESCRIPTION OF THE RELATED ART

A high electron mobility transistor (hereinafter, referred to as “HEMT”) is a device that uses a high-concentration two-dimensional gas, which occurs at a hetero junction interface, as a channel. The HEMT, which is formed by a nitride semiconductor such as gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN), or a mixed crystal thereof, can generate a two-dimensional gas in which a carrier concentration is as high as 1×1019cm−3or greater and electron mobility is high, at a hetero junction interface due to spontaneous polarization and piezo polarization of the nitride semiconductor even in an un-doped state. Accordingly, it is possible to obtain a transistor in which on-resistance is low and which operates with a large current.

As a method of increasing electron mobility in the HEMT, there is known a method of applying tensile stress to a channel. The reason for this is as follows. When applying the tensile stress to the channel, in a direction in which the tensile stress occurs, degeneration of a conduction band is solved, and thus an effective mass of electrons decreases.

When using a substrate formed by silicon (Si) or a substrate formed by gallium oxide (β-Ga2O3) as the nitride semiconductor, tensile stress occurs in the nitride semiconductor. This is because a coefficient of thermal expansion in silicon or gallium oxide is smaller than a coefficient of thermal expansion of the nitride semiconductor. Here, a configuration of using gallium oxide as a substrate for nitride semiconductor growth is described, for example, in PTL 1.

CITATION LIST

Patent Literature

SUMMARY OF THE INVENTION

However, when a difference in a coefficient of thermal expansion between a constituent material of a substrate and the nitride semiconductor is excessively large, there is a problem that cracks occur in the nitride semiconductor during cooling after crystal growth.

The present disclosure has been made in consideration of the above-described problem, and an object thereof is to improve electron mobility by applying sufficiently large tensile stress in a predetermined direction without occurrence of cracks in the nitride semiconductor.

To accomplish the object, the present disclosure employs a semiconductor device configuration in which a coefficient of thermal expansion or a substrate density is made different between intersecting directions in a plane of a substrate for semiconductor growth.

According to an aspect of the present disclosure, there is provided a semiconductor device including: a substrate; an electron transit layer that is disposed on the substrate and is formed by a first nitride semiconductor; and an electron supply layer that is disposed on the electron transit layer and is formed by a second nitride semiconductor. A coefficient of thermal expansion of the substrate is different between a first direction in a main surface of the substrate and a second direction that is perpendicular to the first direction in the main surface, and tensile stress occurs in the electron transit layer.

According to another aspect of the present disclosure, there is provided a semiconductor device including: a substrate having a first density; an electron transit layer that is disposed on the substrate and is formed by a first nitride semiconductor; and an electron supply layer that is disposed on the electron transit layer and is formed by a second nitride semiconductor. The substrate includes plural regions having a second density that is smaller than the first density.

According to the aspects of the present disclosure, sufficiently large tensile stress is applied in a predetermined direction without occurrence of cracks in a nitride semiconductor, and thus it is possible to improve electron mobility.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, description will be given of a gallium oxide (Ga2O3) substrate that is used in a first exemplary embodiment. Ga2O3having a monoclinic structure has the following characteristics. A β-phase is a stable phase, and respective lattice constants of an a-axis, a b-axis, and a c-axis are 1.223 nm, 0.304 nm, and 0.580 nm which are different from each other. Gallium oxide also has anisotropy in a coefficient of thermal expansion. Specifically, a coefficient of thermal expansion in a [100] direction in crystal axes is 1.9×106/K to 5.3×10−6/K, a coefficient of thermal expansion in a [010] direction is 5.9×10−6/K to 8.6×10−6/K, and a coefficient of thermal expansion in a [001] direction is 5.9×10−6/K to 8.6×10−6/K.

Typically, in a case where a semiconductor is subjected to crystal growth on a hetero substrate having a component different from that of the semiconductor, bending occurs due to a difference in a coefficient of thermal expansion between the substrate and the semiconductor in a temperature-lowering process (cooling process) after the crystal growth. In a case where a coefficient of thermal expansion of the semiconductor is greater than a coefficient of thermal expansion of the substrate, bending having a convex shape in a direction from the semiconductor toward the substrate occurs, and thus tensile stress is applied to the semiconductor. In contrast, in a case where the coefficient of thermal expansion of the semiconductor is smaller than the coefficient of thermal expansion of the substrate, bending having a convex shape in a direction from the substrate toward the semiconductor occurs, and thus compressive stress is applied to the semiconductor.

A β-Ga2O3substrate (hereinafter, simply referred to as “Ga2O3substrate”), in which a plane orientation of a main surface is (−201) plane, has a coefficient of thermal expansion different between a first direction in the main surface and a direction that is perpendicular to the first direction. For example, in a case where GaN, in which a main surface is a (0001) plane, that is, a c-plane, is subjected to crystal growth on the Ga2O3substrate, a coefficient of thermal expansion in a direction perpendicular to the c-axis of GaN is 5.6×10−6/K, and is greater than a coefficient of thermal expansion in a [100] direction in crystal axes of Ga2O3, and is smaller than a coefficient of thermal expansion in a [010] direction that is perpendicular to the [100] direction of Ga2O3. According to this, in GaN that is grown on Ga2O3, so-called uniaxial stress, in which tensile stress and compressive stress are mixed together in the c-plane, occurs. Furthermore, a negative symbol (−) in a Miller index representing a plane orientation or a crystal axis conveniently indicates inversion of any one index that continues from the symbol.

FIG. 1illustrates a cross-sectional configuration of high electron mobility transistor (HEMT) that is a semiconductor device according to this exemplary embodiment.

As illustrated inFIG. 1, the semiconductor device according to this exemplary embodiment includes Ga2O3substrate101, buffer layer102that is disposed on a main surface of Ga2O3substrate101and is formed by GaN, electron transit layer103that is disposed on the buffer layer102and is formed by un-doped GaN, electron supply layer104that is disposed on electron transit layer103and is formed by un-doped AlGaN, and cap layer105that is disposed on electron supply layer104and is formed by GaN. Hereinafter, a stacking structure of the semiconductor as described above may be referred to as an HEMT structure. In addition, a structure, which includes electron transit layer103and electron supply layer104disposed on electron transit layer103on an upper side of Ga2O3substrate101, may be referred to as AlGaN/GaN structure.

Here, for example, the thickness of Ga2O3substrate101is 650 μm, the thickness of buffer layer102is 30 nm, the thickness of electron transit layer103is 2 μm, the thickness of electron supply layer104is 50 nm, and the thickness of cap layer105is 20 nm. In addition, an Al content of AlGaN that forms electron supply layer104is, for example, 15%.

Gate electrode106is disposed on electron supply layer104. Source electrode107and drain electrode108are disposed on an upper side of electron supply layer104and on both sides of gate electrode106with a predetermined interval from gate electrode106. Gate electrode106has a stacking structure (not illustrated) of titanium (Ti), gold (Au), and palladium (Pd). Here, for example, the thickness of Ti is 10 nm, the thickness of Au is 100 nm, and the thickness of Pd is 100 nm. Source electrode107and drain electrode108have a stacking structure (not illustrated) of Ti, Al, and Ti. For example, the thickness of Ti is 20 nm, and the thickness of Al is 20 nm.

In addition, gate electrode106, source electrode107, and drain electrode108are disposed in such a manner that an electron transit direction (gate longitudinal direction) in electron transit layer103is parallel to a [−102] direction in crystal axes of Ga2O3.

An HEMT having a structure illustrated inFIG. 1is prepared on Ga2O3substrate101having a diameter of 5.08 cm (2 inches), and bending thereof is measured. Hereinafter, a direction from Ga2O3substrate101to electron transit layer103is referred to as “upward direction”, and a direction from electron transit layer103to Ga2O3substrate101is referred to as “downward direction”.

In a [010] direction in the Ga2O3substrate, bending having an upwardly convex shape occurs, and the size thereof is 25 μm. Hereinafter, the upwardly convex shape is described as +25 μm by using a positive symbol. On the other hand, in a [−102] direction that is perpendicular to the [010] direction in the Ga2O3substrate, bending having a downwardly convex shape occurs, and the size thereof is 15 μm. Hereinafter, the downwardly convex shape is described as −15 μm by using a negative symbol. That is, in electron transit layer103, compressive stress occurs in a direction parallel to the [010] direction of the Ga2O3substrate, and tensile stress occurs in a direction parallel to the [−102] direction. That is, a direction of an electron, which transits through electron transit layer103, matches a direction in which the tensile stress occurs. Furthermore, an expression of “matching” includes a manufacturing error, and represents “substantial matching”.

From more detailed investigation of the amount of bending in the Ga2O3substrate and the HEMT structure on the Ga2O3substrate, it can be seen that the amount of bending in an upward direction becomes maximum in the [010] direction, and the amount of bending in a downward direction becomes maximum in the [−102] direction. Hereinafter, a situation in which both of the tensile stress and the compressive stress occur in the same plane is defined as “uniaxial stress occurs”.

Furthermore, as is the case with PTL 1, in a Ga2O3substrate in which the main surface is a (100) plane or a plane that is inclined to the (100) plane by an angle of 2° to 4°, a coefficient of thermal expansion in the main surface is substantially uniform without depending on a direction. Accordingly, the uniaxial stress does not occur in the GaN crystal on the main surface.

FIG. 2illustrates a plane orientation relationship between Ga2O3substrate101and electron transit layer103formed by GaN as illustrated inFIG. 1. As illustrated inFIG. 2, GaN that is a nitride semiconductor pertains to “hexagonal”, and Ga2O3pertains to “monoclinic” as described above.

FIG. 3Aillustrates carrier mobility of an AlGaN/GaN structure which is measured by hall measurement. In addition,FIG. 3Billustrates a sheet carrier density of the AlGaN/GaN structure which is measured by the hall measurement.FIG. 3Aalso illustrates carrier mobility of the AlGaN/GaN structure in which an electrode is formed for electron transit in the [010] direction of the Ga2O3substrate as Comparative Example 1, and carrier mobility of the AlGaN/GaN structure on an Si substrate in which strain in an in-plane direction is isotropic as Comparative Example 2. In addition,FIG. 3Balso illustrates a sheet carrier density of the AlGaN/GaN structure in which an electrode is formed for electron transit in the [010] direction of the Ga2O3substrate as Comparative Example 1, and a sheet carrier density of the AlGaN/GaN structure on the Si substrate in which strain in an in-plane direction is isotropic as Comparative Example 2.

As illustrated inFIG. 3A, in a case where an electron transits in the direction in the AlGaN/GaN structure formed on the Ga2O3substrate (Comparative Example 1), the carrier mobility is greater than that of the AlGaN/GaN structure on the Si substrate (Comparative Example 2). In addition, the carrier mobility in a case where an electron transits in the [−102] direction of the Ga2O3substrate of this exemplary embodiment is greater than the carrier mobility of the case where an electron transits in the [010] direction of the Ga2O3substrate (Comparative Example 1) by approximately 20%. The reason for this difference is considered as follows. In electron transit layer103, particularly, tensile stress occurs in an electron transit direction, and thus degeneration of a conduction band in the tensile stress occurring direction is solved. As a result, an effective mass of electrons decreases.

The carrier mobility is inversely proportional to the effective mass of carriers. Accordingly, when the effective mass of electrons decreases, mobility thereof increases. At this time, in electron supply layer104, both tensile stress due to lattice mismatching with electron transit layer103, and uniaxial stress due to a difference in a coefficient of thermal expansion between Ga2O3substrate101and electron supply layer104occur. At this time, when the two kinds of stress are combined, a carrier density may vary. Furthermore, an evaluation target according to this exemplary embodiment is a sheet carrier density of an in-plane direction in the main surface of Ga2O3substrate101, and thus it is clear that orientation dependency does not occur in the sheet carrier density illustrated inFIG. 3B.

The magnitude of stress applied to electron transit layer103can be changed by changing a ratio between the thickness of Ga2O3substrate101and the thickness of the HEMT structure formed on Ga2O3substrate101. For example, as buffer layer102in the HEMT structure is made to be thicker by a hydride vapor phase epitaxy (HVPE) method and the like with a constant thickness of Ga2O3substrate101, a coefficient of thermal expansion in electron transit layer103becomes predominant, and thus the uniaxial stress applied to electron transit layer103decreases. Similarly, even when substrate101is made to be thinner through polishing on a rear surface of Ga2O3substrate101, the uniaxial stress decreases. In contrast, when the HEMT structure is made to be thinner, or Ga2O3substrate101is made to be thicker, a coefficient of thermal expansion of Ga2O3substrate101becomes predominant, and thus the uniaxial stress applied to electron transit layer103increases.

In this exemplary embodiment, a direction of an electron that transits through electron transit layer103in the HEMT structure is set to match the [−102] direction in the crystal axes of Ga2O3substrate101, but there is no limitation thereto. That is, when tensile stress is applied in an electron transit direction, the carrier mobility increases. Furthermore, it is needless to say that a direction in which an effect of the tensile stress is the greatest is a direction in which the amount of bending is greatest. However, the tensile stress becomes the cause for cracks, and thus it is preferable that stress that occurs in a direction different from the electron transit direction is relatively small tensile stress or compressive stress.

Furthermore, even in a substrate other than the β-Ga2O3substrate in which a plane orientation of the main surface is [−201] plane as described in this exemplary embodiment, when using a substrate having the same characteristics as in the β-Ga2O3substrate, the same effect can be attained.

As described above, according to this exemplary embodiment, the electron mobility in the semiconductor device is improved, and thus it is possible to realize high speed and low power consumption of the semiconductor device.

Hereinafter, description will be given of the principle in which a substrate including a stress application portion in a second exemplary embodiment applies stress to the electron transit layer.

A substrate formed by sapphire (single crystal Al2O3) or silicon (Si) is irradiated with high-output laser light such as a condensed femtosecond laser, a crystal in the vicinity of a focal point of the laser light is melted, and a volume of a region irradiated with the laser in a crystal is expanded. When plural volume expanded regions as described above are formed in a substrate, stress occurs in the substrate, and thus it is possible to bend the substrate. Furthermore, the shape of the bending and the magnitude of the bending depend on laser irradiation conditions (an output, an interval, a depth, and the like). Accordingly, when bending, which has a convex shape in a direction (hereinafter, referred to as “upward direction”) from a substrate for crystal growth to a semiconductor that is a crystal substance on the substrate, is provided, it is possible to apply compressive stress to the semiconductor. In contrast, when bending, which has a convex shape in a direction (hereinafter, referred to as “downward direction”) from the semiconductor on the substrate to the substrate, is provided, it is possible to apply tensile stress to the semiconductor.

FIG. 4illustrates a cross-sectional configuration of a high electron mobility transistor (HEMT) that is a semiconductor device according to this exemplary embodiment.

As illustrated inFIG. 4, the semiconductor device according to this exemplary embodiment includes sapphire substrate201in which a (0001) plane of a plane orientation is set to a main surface, buffer layer202that is disposed on the main surface of sapphire substrate201and is formed by GaN, electron transit layer203that is disposed on buffer layer202and is formed by un-doped GaN, electron supply layer204that is disposed on electron transit layer203and is formed by un-doped AlGaN, and cap layer205that is disposed on electron supply layer204and is formed by GaN.

In this exemplary embodiment, for example, the thickness of sapphire substrate201is 650 μm, the thickness of buffer layer202is 30 nm, the thickness of electron transit layer203is 2 μm, the thickness of electron supply layer204is 50 nm, and the thickness of cap layer205is 20 nm. In addition, an Al content of AlGaN that formes electron supply layer204is, for example, 15%.

Gate electrode206is disposed on an upper side of electron supply layer204. Source electrode207and drain electrode208are disposed on an upper side of electron supply layer204and on both sides of gate electrode206with a predetermined interval from gate electrode206. Gate electrode206has a stacking structure (not illustrated) of titanium (Ti), gold (Au), and palladium (Pd). Here, for example, the thickness of Ti is 10 nm, the thickness of Au is 100 nm, and the thickness of Pd is 100 nm. Source electrode107and drain electrode108have a stacking structure (not illustrated) of Ti, Al, and Ti. For example, the thickness of Ti is 20 nm, and the thickness of Al is 20 nm.

Stress application portions209, which are plural regions formed through laser irradiation, are provided in an upper portion and in a lower portion of sapphire substrate201. Among plural stress application portions209, stress application portion209ais formed in an upper portion (on a front surface side) of sapphire substrate201, and stress application portion209bis formed in a lower portion (rear surface side) of sapphire substrate201.

A volume of plural stress application portions209is expanded due to laser irradiation, and thus a density thereof is smaller than a density of sapphire substrate201. Sapphire substrate201having a diameter of 5.08 cm (2 inches) is designed in such a manner that bending in an a-axis direction and bending in an m-axis direction perpendicular to the a-axis direction respectively become −30 μm and +5 μm due to plural stress application portions209.

For example, the above-described structure can be obtained by a configuration illustrated inFIG. 4andFIG. 5.FIG. 5illustrates an in-plane cross-sectional configuration on a front surface side of sapphire substrate201, andFIG. 6illustrates an in-plane cross-sectional configuration on a rear surface side of sapphire substrate201.

As illustrated inFIG. 5, plural stress application portions209aon a front surface side are formed as laser irradiation rows which are disposed with an interval, and are more densely disposed in the m-axis direction in comparison to the a-axis direction. In the m-axis direction, stress application portions209amay be formed to come into contact with each other, that is, may be formed integrally with each other. According to this configuration, volume expansion of the m-axis direction in the vicinity of the surface of sapphire substrate201becomes predominant, and thus it is possible to bend substrate201with an upwardly convex shape in the m-axis direction.

In addition, as illustrated inFIG. 6, plural stress application portions209bon a rear surface side of sapphire substrate201are formed in laser irradiation rows which are disposed with an interval, and are more densely disposed in the a-axis direction in comparison to the m-axis direction. In the a-axis direction, stress application portions209bmay be formed to come into contact with each other, that is, may be formed integrally with each other. According to this configuration, volume expansion of the a-axis direction in the vicinity of the rear surface of sapphire substrate201becomes predominant, and thus it is possible to bend substrate201with a downwardly convex shape in the a-axis direction.

As described above, when an interval between plural regions irradiated with a laser is made to be changed in accordance with an orientation of a crystal axis, it is possible to allow uniaxial stress to occur. In addition, when a region of two or greater layers, which are different in a depth in sapphire substrate201, is irradiated with a laser, it is possible to change the magnitude of bending in two directions and the directions in an arbitrary manner.

In this exemplary embodiment, gate electrode206, source electrode207, and drain electrode208are disposed in such a manner that an electron transit direction (gate longitudinal direction) in electron transit layer203, to which stress is applied as described above, becomes parallel to the a-axis of sapphire substrate201. According to this configuration, electron transit layer203receives tensile stress due to bending having a downwardly convex shape. That is, a direction of an electron that transits through electron transit layer203matches a direction in which the tensile stress is applied. In addition, an expression of “matching” includes a manufacturing error, and represents “substantial matching”.

FIG. 7illustrates carrier mobility measured by hall measurement.FIG. 7also illustrates carrier mobility in a case where stress application portion209is not provided to sapphire substrate201as a comparative example. As illustrated inFIG. 7, from measurement of the carrier mobility in the a-axis direction, the carrier mobility of the HEMT according to this exemplary embodiment shows a value higher than carrier mobility of HEMT according to comparative example, in which the stress application portion is not provided, by approximately 20%.

The reason for this difference is that when tensile stress is applied to a crystal substance, degeneration of a conduction band in the stress occurring direction is solved, and thus an effective mass of electrons decreases. The carrier mobility is inversely proportional to the effective mass of carriers. Accordingly, when the effective mass of electrons decreases, mobility thereof increases.

In this exemplary embodiment, the tensile stress is applied in the a-axis direction of sapphire substrate201, but the tensile stress application direction is not limited to the a-axis direction, and the same effect can be attained as long as the tensile stress is applied in the electron transit direction.

In addition, stress that is different between the a-axis direction and the m-axis direction in sapphire substrate201is applied, but tensile stress may be applied with respect to both of the directions. However, the tensile stress becomes the cause for cracks, and thus it is preferable that stress that occurs in a direction different from the electron transit direction is relatively small tensile stress or compressive stress.

In addition, stress application portion209may be formed through the laser irradiation before crystal growth for formation of the HEMT structure or after the crystal growth, but it is preferable that stress application portion209is formed after the crystal growth. The reason for this preference is that when stress application portion209is formed before the crystal growth, stress may be mitigated in the subsequent crystal growth. Furthermore, it is preferable that stress application portion209is formed after an electrode forming process rather than before the electrode forming process. The reason for this preference is that in a state in which stress is applied to sapphire substrate201, that is, in a state in which bending occurs in substrate201, it is difficult to perform a minute process, and a risk such as a decrease in a yield ratio increases.

In addition, in this exemplary embodiment, sapphire (single crystal Al2O3) is used for a substrate for crystal growth, but a constituent material of the substrate is not limited to sapphire, and may be silicon (Si), gallium nitride (GaN), or gallium oxide (Ga2O3). The reason for this is as follows. A method of allowing stress to occur through laser irradiation uses a principle in which a crystal in a portion irradiated with laser light is modified and a volume of the portion is expanded. Accordingly, it is possible to control the stress through control of a laser-light irradiation density, a laser-light irradiation depth, and the like regardless of a coefficient of thermal expansion of the substrate, and the like.

As described above, according to this exemplary embodiment, electron mobility in a semiconductor device is improved, and thus it is possible to realize high speed and low power consumption of the semiconductor device.

Hereinbefore, the semiconductor device according to the present disclosure has been described on the basis of the exemplary embodiments. However, the present disclosure is not limited to the exemplary embodiments, and may be executed in various aspects in a range not departing from the gist of the present disclosure. For example, the HEMT structure is not limited to the structures in the exemplary embodiments, and a component and the like in the AlGaN/GaN structure are not limited to the configurations in the exemplary embodiments. In addition, a combination of the first exemplary embodiment and the second exemplary embodiment is also possible.

INDUSTRIAL APPLICABILITY

The semiconductor device according to the present disclosure is effective as a semiconductor device having high carrier mobility, and the like.

REFERENCE MARKS IN THE DRAWINGS

103electron transit layer

104electron supply layer

203electron transit layer

204electron supply layer