Epitaxial substrate and semiconductor element

Abstract

The present invention provides a high electron mobility transistor element which has excellent performance by using a high resistance group III element nitride substrate. An epitaxial substrate according to an embodiment of the present invention comprises: a base substrate which is formed of a group III element nitride that is doped with a transition element, and has an upper surface and a lower surface opposite to each other; and an epitaxial layer which is formed on the upper surface of the base substrate. The epitaxial layer comprises a channel layer and a barrier layer from the base substrate side. The channel layer has a thickness of 0.8 µm or more. The sum of the transition element concentration in a portion at a depth of 0.5 µm from the upper surface of the channel layer and the average value of carbon concentrations in portions ranging from a depth of 0.15 µm to a depth of 0.50 µm from the upper surface of the channel layer is not less than 7 × 1015 cm- 3 but less than 1 × 1016 cm- 3.

Description

Epitaxial substrate and semiconductor element

[0001] The present invention relates to an epitaxial substrate and a semiconductor device.

[0002] Group III element nitrides have a wide direct transition band gap, a high breakdown field, and a high saturated electron velocity, and therefore are being actively developed as semiconductor materials for high-frequency / high-power electronic devices, for example. Examples of semiconductor elements using Group III element nitrides include high electron mobility transistor (HEMT) elements.

[0003] In recent years, development of HEMT devices has been progressing using a group III element nitride substrate as a base substrate and having a HEMT structure on this base substrate. The group III element nitride substrate on which the HEMT structure is laminated is desired to have high resistance. An example of a high resistance group III element nitride substrate is disclosed in Patent Document 1.

[0004] Patent No. 5451085

[0005] However, the HEMT device obtained using the high-resistivity Group III element nitride substrate may not have sufficient performance. For example, the HEMT device may have a large leakage current or an insufficient maximum current.

[0006] In view of the above, a main object of the present invention is to provide a HEMT device with excellent performance using a highly resistive Group III element nitride substrate.

[0007] 1. An epitaxial substrate according to an embodiment of the present invention comprises: a base substrate made of a Group III element nitride doped with a transition element, the base substrate having upper and lower surfaces facing each other; and an epitaxial layer formed on the upper surface of the base substrate, the epitaxial layer having, from the base substrate side, a channel layer and a barrier layer, the channel layer having a thickness of 0.8 μm or more; and a sum of a transition element concentration at a position 0.5 μm deep from the upper surface of the channel layer and an average carbon concentration from a position 0.15 μm deep to a position 0.50 μm deep from the upper surface of the channel layer being 7×10 １５ cm －３ 1x10 or more１６ cm －３ 2. In the epitaxial substrate according to the above item 1, the average value of the transition element concentration from a portion 0.2 μm deep from the upper surface of the channel layer to a portion 0.5 μm deep is less than T (cm －３ ), and the transition element concentration of the base substrate is S (cm －３ ), the thickness of the channel layer is D (μm), and the depth from the upper surface of the channel layer is d (μm), the channel layer may have a region approximated by the following formula (1): T=S×10^(k(D−d)) (1), where k is in μm. －１ 3. In the epitaxial substrate according to the above 2, the coefficient k may be 6.5 or less. 4. In the epitaxial substrate according to the above 2 or 3, the coefficient k may be 6.0 or less. 5. In the epitaxial substrate according to any one of the above 2 to 4, the coefficient k may be 4.5 or more. 6. In the epitaxial substrate according to any one of the above 2 to 5, the transition element concentration T is 5×10 １６ cm －３ From 5 x 10 １７ cm －３ 7. In the epitaxial substrate according to any one of 1 to 6 above, the thickness of the channel layer may be 2 μm or less. 8. In the epitaxial substrate according to any one of 1 to 7 above, the base substrate may contain gallium nitride. 9. In the epitaxial substrate according to any one of 1 to 8 above, the concentration of the transition element in the base substrate may be 1×10 １８ cm －３ 1x10 or more １９ cm －３ 10. A semiconductor device according to another embodiment of the present invention comprises the epitaxial substrate according to any one of 1 to 9 above, and a source electrode, a drain electrode, and a gate electrode provided on the epitaxial layer of the epitaxial substrate.

[0008] According to the embodiments of the present invention, it is possible to provide a HEMT device with excellent performance.

[0009] 2A is a schematic cross-sectional view showing the general configuration of an epitaxial substrate according to one embodiment of the present invention; FIG. 2B is a view showing a manufacturing process of a Group III element nitride substrate according to one embodiment; FIG. 2C is a view continuing from FIG. 2A; FIG. 2D is a view continuing from FIG. 2B; FIG. 2C is a schematic cross-sectional view showing the general configuration of a HEMT device according to one embodiment of the present invention; and FIG. 2D is a SIMS measurement result of the epitaxial substrate of Sample B-3 of Experimental Example 1.

[0010] Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to these embodiments. In order to clarify the description, the drawings may schematically show the width, thickness, shape, etc. of each part compared to the embodiments, but these are merely examples and do not limit the interpretation of the present invention. Furthermore, in the drawings, the same or equivalent elements are given the same reference numerals, and redundant explanations may be omitted.

[0011] A. Epitaxial Substrate FIG. 1 is a schematic cross-sectional view showing the general configuration of an epitaxial substrate according to one embodiment of the present invention. The epitaxial substrate 100 is plate-shaped and includes a base substrate 10 and an epitaxial layer 20. The epitaxial substrate 100 includes the base substrate 10 having a first main surface (upper surface) 11 and a second main surface (lower surface) 12 facing each other, and the epitaxial layer 20 formed on the first main surface 11 of the base substrate 10. The epitaxial substrate 100 can be obtained by epitaxially growing the epitaxial layer 20 on the base substrate 10. The epitaxial layer 20 has a layered structure in which a channel layer 21 and a barrier layer 22 are stacked from the base substrate 10 side.

[0012] The epitaxial substrate 100 may be manufactured in any suitable shape. In one embodiment, it is manufactured in a disk shape (in the form of a so-called wafer). The size of the epitaxial substrate 100 may be appropriately set depending on the purpose. For example, the diameter of the wafer is 50 mm to 200 mm. The epitaxial substrate 100 may have an orientation flat or a notch formed in a part thereof to indicate the crystal orientation (for example, the crystal orientation of the wafer).

[0013] A-1. Undersubstrate The thickness of the undersubstrate 10 is, for example, 250 μm or more and 800 μm or less. The undersubstrate 10 is a Group III element nitride substrate made of Group III element nitride crystals. Examples of Group III elements that make up the Group III element nitride include aluminum (Al), gallium (Ga), and indium (In). These may be used alone or in combination of two or more. Specific examples of Group III element nitrides include aluminum nitride (Al ｘ N), gallium nitride (Ga ｙ N), indium nitride (In ｚ N), aluminum gallium nitride (Al ｘ Ga ｙ N), gallium indium nitride (Ga ｙ In ｚ N), aluminum indium nitride (Al ｘ In ｚ N), aluminum gallium indium nitride (Al ｘ Ga ｙ In ｚ In each chemical formula in parentheses, typically, x+y+z=1.

[0014] The base substrate 10 may be semi-insulating. The resistivity of the base substrate 10 may be, for example, 1×10 ５ Ω・cm or more 1×10 １４ Ω cm or less, preferably 1×10 ６ Ω cm or more, more preferably 1×10 ７The resistivity is Ω·cm or more. The resistivity can be determined, for example, by measuring the change in the amount of charge over time. By measuring the change in the amount of charge over time, the resistivity can be determined without destroying the substrate. Specifically, the substrate is inserted into a capacitor consisting of a probe and a stage, a pulse voltage is applied, the change in the amount of charge on the substrate over time is measured, and the resistivity is calculated from the measured value. Since the probe does not contact the substrate, the resistivity can be determined without forming an ohmic contact electrode. The spatial resolution of the probe can be approximately 1 mm to 10 mm. The method for determining resistivity is described, for example, in the non-patent document "R. Stibal et al., "Contactless evaluation of semi-insulating GaAs wafer resistivity using the time-dependent charge measurement," Semiconductor Science and Technology, 6, p. 995 (1991)."

[0015] The base substrate 10 is doped with an element other than a group III element. Specifically, the group III element nitride contains an element other than a group III element as a dopant. By doping the base substrate 10 with an element other than a group III element, the above-mentioned resistivity can be satisfactorily satisfied. As the dopant, transition elements such as iron (Fe), manganese (Mn), zinc (Zn), vanadium (V), chromium (Cr), cobalt (Co), and nickel (Ni) are preferably used. These elements can be used alone or in combination of two or more. Among these, iron and manganese are preferably used, and manganese is more preferably used.

[0016] The abundance of transition elements in the base substrate 10 (unit: atoms / cm ３ ) can be set to any appropriate value. ３ ) will be simply referred to as the transition element concentration (unit: cm －３ The transition element concentration of the base substrate 10 is, for example, 6×10 １７ cm －３ or more, preferably 7×10１７ cm －３ More preferably, 8×10 １７ cm －３ or more, and more preferably 9 × 10 １７ cm －３ More preferably, it is 1×10 １８ cm －３ The transition element concentration in the base substrate 10 is, for example, 1×10 ２０ cm －３ or less, preferably 5 × 10 １９ cm －３ or less, and more preferably 3×10 １９ cm －３ or less, and more preferably 1×10 １９ cm －３ In one embodiment, the transition element concentration may be the sum of the iron concentration and the manganese concentration, or may be the iron concentration or the manganese concentration.

[0017] In the above-described Group III element nitride crystal, typically, the <0001> direction is the c-axis direction, the <1-100> direction is the m-axis direction, and the <11-20> direction is the a-axis direction. The crystal plane perpendicular to the c-axis is the c-plane, the crystal plane perpendicular to the m-axis is the m-plane, and the crystal plane perpendicular to the a-axis is the a-plane. In one embodiment, the thickness direction of the Group III element nitride substrate 10 is parallel or approximately parallel to the c-axis, the first main surface 11 is a Group III element polar plane on the (0001) plane side, and the second main surface 12 is a nitrogen polar plane on the (000-1) plane side. The first main surface 11 may be parallel to the (0001) plane or may be inclined with respect to the (0001) plane. The inclination angle of the first main surface 11 with respect to the (0001) plane is, for example, 10° or less, 5° or less, 2° or less, or 1° or less. The second main surface 12 may be parallel to the (000-1) plane or may be inclined with respect to the (000-1) plane. The inclination angle of the second main surface 12 with respect to the (000-1) plane is, for example, 10° or less, 5° or less, 2° or less, or 1° or less.

[0018] The Group III element nitride substrate constituting the base substrate can be obtained, for example, by growing a Group III element nitride crystal doped with an element other than a Group III element on a seed crystal film of a seed crystal substrate having a growth substrate and a seed crystal film.

[0019] 2A to 2C are diagrams showing a manufacturing process of a group III nitride substrate according to one embodiment, in which a seed crystal film 32 is formed on an upper surface 31 a of a growth substrate 31 having an upper surface 31 a and a lower surface 31 b facing each other, thereby forming a seed crystal substrate 33.

[0020] The growth substrate is, for example, a substrate having a shape and size that allows a Group III element nitride substrate having a desired shape and size to be produced. Typically, the growth substrate is disk-shaped with a diameter of 50 mm to 200 mm. The growth substrate has a thickness of, for example, 200 μm to 800 μm. Typically, the growth substrate is made of a single crystal. Examples of materials that can be used to form the growth substrate include sapphire, crystal-oriented alumina, silicon, gallium oxide, aluminum gallium nitride, gallium arsenide, and silicon carbide.

[0021] The thickness of the seed crystal film is, for example, 0.2 μm or more. From the viewpoint of preventing meltback or disappearance during growth of the Group III element nitride crystal, the thickness of the seed crystal film is preferably 1 μm or more, more preferably 2 μm or more. On the other hand, from the viewpoint of productivity, the thickness of the seed crystal film is preferably 10 μm or less, more preferably 5 μm or less. A Group III element nitride is typically used as the material constituting the seed crystal film. Details of the Group III element nitride are as described above.

[0022] The seed crystal film can be formed by any appropriate method. A typical method for forming the seed crystal film is a vapor phase growth method. Specific examples of the vapor phase growth method include metal organic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE), pulsed excited deposition (PXD), molecular beam epitaxy (MBE), evaporation, and sublimation. Among these, the MOCVD method is preferably used.

[0023] The formation of the seed crystal film by the MOCVD method includes, for example, a first formation step and a second formation step, in this order. Specifically, in the first formation step, a first layer (low-temperature growth buffer layer) (not shown) is formed on a growth substrate at a temperature T1 (e.g., 450°C to 550°C), and in the second formation step, a second layer (not shown) is formed at a temperature T2 (e.g., 1000°C to 1200°C) higher than temperature T1. The thickness of the first layer is, for example, 20 nm to 50 nm. The thickness of the second layer is, for example, 1 μm to 5 μm.

[0024] Next, a group III element nitride crystal is grown on the seed crystal film 32 of the seed crystal substrate 33 to form a group III element nitride crystal layer 34, thereby obtaining a laminated substrate 35 as shown in FIG. 2B . The degree of growth of the group III element nitride crystal (the thickness of the group III element nitride crystal layer 34) can be adjusted depending on the desired thickness of the group III element nitride substrate. Any appropriate direction can be selected as the growth direction of the group III element nitride crystal depending on the application, purpose, etc. Specific examples include the normal directions to the c-plane, a-plane, and m-plane, and the normal directions to planes inclined relative to the c-plane, a-plane, and m-plane.

[0025] The method for growing Group III element nitride crystals is not particularly limited as long as it can achieve a crystal orientation that roughly follows the crystal orientation of the seed crystal film. Specific examples of methods for growing Group III element nitride crystals include vapor phase growth methods such as metalorganic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE), pulsed xenon deposition (PXD), molecular beam epitaxy (MBE), and sublimation; and liquid phase growth methods such as flux deposition, ammonothermal deposition, hydrothermal deposition, and sol-gel deposition. These methods can be used alone or in combination of two or more.

[0026] Preferably, a flux method (e.g., a Na flux method) is employed as a method for growing Group III element nitride crystals. Details of such growth methods are described, for example, in Japanese Patent No. 5451085, and growth may be performed by adjusting various conditions of the described growth method as appropriate. Specifically, Group III element nitride crystals can be grown by adjusting various conditions using a crystal manufacturing apparatus that includes a pressure-resistant vessel capable of supplying high-pressure nitrogen gas, a turntable that can rotate within the pressure-resistant vessel, and an outer vessel placed on the turntable.

[0027] Growth of Group III nitride crystals by the flux method is typically carried out using a crucible as a growth container. Specifically, the seed crystal substrate is placed at a predetermined position in the crucible, and the crucible is then filled with raw materials. The crucible containing the seed crystal substrate is typically placed with a lid on in a nitrogen-containing atmosphere at a predetermined pressure and temperature for growth.

[0028] The raw material is, for example, a melt composition containing a flux, a Group III element, and a dopant. The flux preferably contains at least one of an alkali metal and an alkaline earth metal, and more preferably metallic sodium. Typically, the flux and a metal source material are mixed together. As the metal source material, an elemental metal, an alloy, a metal compound, etc. can be used, but from the viewpoint of handling, an elemental metal is preferably used.

[0029] The crucible (including the lid) can be made of any suitable material that can be used in the flux method. Examples of the crucible material include alumina, yttria, and YAG (yttrium aluminum garnet). The crucible material may be a single crystal or a polycrystal (ceramic). The ceramic may have a high relative density, such as by HIP treatment, to give it translucency.

[0030] As described above, the growth can be carried out in a nitrogen-containing atmosphere. The growth atmosphere can contain other gases in addition to nitrogen. The other gases are preferably inert gases such as argon, helium, and neon.

[0031] The pressure of the atmosphere during growth may be set to any appropriate pressure. For example, the pressure of the atmosphere during growth may be 1 MPa or more, 2 MPa or more, or 3 MPa or more, from the viewpoint of preventing evaporation of the flux. On the other hand, the pressure of the atmosphere during growth may be 50 MPa or less, or 10 MPa or less, from the viewpoint of preventing the growth apparatus from becoming large-scale.

[0032] The temperature of the atmosphere during growth can be set to any appropriate temperature. The temperature of the atmosphere during growth is preferably 700°C to 1000°C, and more preferably 800°C to 900°C. In one embodiment, the desired oxygen concentration can be achieved by adjusting the temperature of the atmosphere during growth.

[0033] The growth is preferably carried out while rotating the base substrate (crucible) from the viewpoint of, for example, promoting dissolution of high-pressure nitrogen gas into the melt composition. For example, the crucible with a lid is placed in the outer container of the crystal production apparatus and placed on a turntable, and the turntable is rotated (for example, on its axis) in this state to rotate the crucible.

[0034] After growth of the group III element nitride crystal, as shown in FIG. 2C , the growth substrate 31 is removed from the group III element nitride crystal (group III element nitride crystal layer 34) to obtain a free-standing substrate 36. Typically, as shown in the figure, the free-standing substrate 36 may include the group III element nitride crystal layer 34 and the seed crystal film 32. For example, the free-standing substrate 36 is obtained by separating the group III element nitride crystal layer 34 from the growth substrate 31. The group III element nitride crystal may be separated from the growth substrate by any appropriate method. Examples of methods for separating the group III element nitride crystal include a method of spontaneously separating the group III element nitride crystal from the growth substrate by utilizing the difference in thermal contraction between the group III element nitride crystal and the base substrate during a cooling step after growth of the group III element nitride crystal, a separation method using chemical etching, and a laser lift-off method using laser light irradiation. Alternatively, the free-standing substrate may be obtained by, for example, grinding and removing the growth substrate 31, or by slicing the growth substrate using a wire saw or the like.

[0035] The free-standing substrate 36 can be used as the above-mentioned Group III element nitride substrate as it is, but typically, the free-standing substrate 36 is subjected to any appropriate processing to obtain the above-mentioned Group III element nitride substrate.

[0036] One example of processing performed on the free-standing substrate is grinding of the peripheral portion (e.g., grinding using a diamond grinding wheel). Typically, the free-standing substrate is ground to have the desired shape and size (e.g., a disk shape having a desired diameter).

[0037] Other examples of processing performed on the freestanding substrate include grinding and polishing (e.g., lapping and chemical mechanical polishing (CMP)) of the main surfaces (upper and lower surfaces). Typically, the substrate is thinned and flattened to a desired thickness by grinding and polishing. In one embodiment, the seed crystal film 32 is removed by processing the main surface, leaving only the group III element nitride crystal layer 34 (only a single crystal growth layer).

[0038] Furthermore, examples of processing performed on the freestanding substrate include chamfering the peripheral edge, removing a process-affected layer formed on the surface by grinding or polishing, and removing residual stress that may result from the process-affected layer. In one embodiment, the primary surface (e.g., the group III element polar surface) of the group III element nitride substrate 10 is finished into an epiready surface by the above-mentioned various processes.

[0039] A-2. Epitaxial Layers Group III element nitride crystals may be used as materials for the channel layer 21 and the barrier layer 22. Examples of Group III elements that constitute the Group III element nitrides include Ga (gallium), Al (aluminum), and In (indium). These may be used alone or in combination of two or more. In one embodiment, the channel layer 21 is preferably made of gallium nitride. The barrier layer 22 is preferably made of at least one selected from aluminum gallium nitride, aluminum indium nitride, and aluminum indium gallium nitride.

[0040] The epitaxial layer 20 (channel layer 21 and barrier layer 22) can be formed by epitaxially growing a crystal on the undersubstrate 10. The crystal orientation of the epitaxial layer 20 and the crystal orientation of the undersubstrate 10 can be substantially aligned. Furthermore, the off-orientation (direction of the off-angle) of the epitaxial layer 20 and the off-orientation of the undersubstrate 10 can be substantially aligned. The off-angle of the epitaxial layer 20 and the off-angle of the undersubstrate 10 can be substantially the same.

[0041] A typical method for forming (growing) the epitaxial layer 20 (channel layer 21 and barrier layer 22) is vapor deposition. Specific examples of vapor deposition include metal organic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE), pulsed excited deposition (PXD), molecular beam epitaxy (MBE), evaporation, and sublimation. Among these, MOCVD is preferably used. In this case, the growth temperature of the epitaxial layer 20 is, for example, 800°C or higher and 1300°C or lower, and preferably 1000°C or higher and 1200°C or lower.

[0042] For example, when the channel layer 21 and the barrier layer 22 are formed by MOCVD, a metal-organic (MO) precursor gas may be used as the group III element source. For example, when a gallium nitride layer is formed as the channel layer 21 and an aluminum gallium nitride layer is formed as the barrier layer 22 by MOCVD, trimethylgallium (TMG) and trimethylaluminum (TMA) may be used as the Ga source and Al source, respectively. Ammonia gas may be used as the nitrogen source. At least one of hydrogen gas and nitrogen gas may be used as the carrier gas.

[0043] The thickness of the channel layer 21 is 0.8 μm or more, preferably 0.9 μm or more, more preferably 1.0 μm or more, and even more preferably 1.1 μm or more. On the other hand, the thickness of the channel layer 21 is, for example, 3 μm or less, preferably 2 μm or less, more preferably 1.5 μm or less, and even more preferably 1.3 μm or less. The thickness of the barrier layer 22 is, for example, 2 nm to 40 nm, and preferably 5 nm to 25 nm.

[0044] The channel layer 21 may contain transition elements that may be contained in the undersubstrate 10. Specifically, the transition elements may diffuse from the undersubstrate 10 during growth of the channel layer 21. This diffusion tends to be promoted, for example, as the growth temperature of the channel layer 21 increases. The abundance (concentration) of the transition elements in the channel layer 21 tends to decrease toward the upper surface 21a of the channel layer 21. In the channel layer 21, two-dimensional electron gas (2DEG) may be generated near the interface between the channel layer 21 and the barrier layer 22, and it is preferable that the abundance of the transition elements is low in the region where 2DEG can be generated. On the other hand, the concentration of the transition elements at a depth of 0.5 μm from the upper surface 21a of the channel layer 21 is, for example, 5×10 １４ cm －３ 1x10 or more １６ cm －３ may be less than 7×10 １４ cm －３ 9 x 10 or more １５ cm －３ or less, more preferably 1×10 １５ cm －３ 8 x 10 or more １５ cm －３ In one embodiment, such a good distribution of the transition element concentration can be achieved by setting the thickness of the channel layer 21 to, for example, 1.0 μm or more and 1.5 μm or less.

[0045] The channel layer 21 may contain carbon. The carbon contained in the channel layer 21 originates from, for example, the raw materials used for growing the channel layer 21. The amount of carbon present in the channel layer 21 (carbon concentration) tends to decrease as the growth temperature of the channel layer 21 increases. Therefore, by adjusting the growth conditions of the channel layer 21, the amount of carbon present in the channel layer 21 and its distribution can be controlled. The average carbon concentration from a portion 0.15 μm deep to a portion 0.50 μm deep from the upper surface 21 a of the channel layer 21 is, for example, 3×10 １５ cm －３ 1x10 or more １６ cm －３ may be less than 3×10 １５ cm －３ 8 x 10 or more １５ cm －３ The following is the result.

[0046] In one embodiment, the channel layer 21 has a region with a uniform carbon concentration in the depth direction (thickness direction) as the distribution of carbon concentration. Here, the region with a uniform carbon concentration refers to a region in which the variation in carbon concentration is within a range of ±15%. For example, the variation in carbon concentration from a portion 0.15 μm deep to a portion 0.50 μm deep from the upper surface 21 a of the channel layer 21 is within a range of ±15%. The depth direction (thickness direction) refers to a direction substantially perpendicular to the upper surface 11 of the base substrate 10.

[0047] The sum of the transition element concentration at a depth of 0.5 μm from the upper surface 21 a of the channel layer 21 and the average carbon concentration from a depth of 0.15 μm to a depth of 0.50 μm from the upper surface 21 a of the channel layer 21 (hereinafter, sometimes simply referred to as the “sum of concentrations”) is 7×10 １５ cm －３This is the result. This sum of concentrations can suppress leakage current in the resulting HEMT device. One possible cause of leakage current is a high-Si-concentration region that may exist near the interface between the base substrate 10 and the epitaxial layer 20. Specifically, silicon (Si) deposited on the upper surface 11 of the base substrate 10 before the epitaxial layer 20 is grown on the base substrate 10 remains accumulated at the interface between the base substrate 10 and the epitaxial layer 20 after epitaxial growth, forming a high-Si-concentration region, which is thought to cause current leakage at the interface between the base substrate 10 and the epitaxial layer 20. As described above, intentionally allowing transition elements and carbon to be present in the channel layer 21 can suppress leakage current. The high-Si-concentration region can be confirmed, for example, by secondary ion mass spectrometry (SIMS).

[0048] The sum of the transition element concentration at a depth of 0.5 μm from the upper surface 21 a of the channel layer 21 and the average carbon concentration from a depth of 0.15 μm to a depth of 0.50 μm from the upper surface 21 a of the channel layer 21 is 1×10 １６ cm －３ less than 9×10 １５ cm －３ Such a sum of concentrations allows a high maximum current to be obtained in the resulting HEMT device.

[0049] As the distribution of the transition element concentration in the channel layer 21, in one embodiment, the transition element concentration in the channel layer 21 is expressed as T (cm －３ ), and the average value of the transition element concentration from a portion 0.2 μm deep to a portion 0.5 μm deep from the upper surface 11 of the base substrate 10 is S (cm －３ ), the thickness of the channel layer 21 is D (μm), and the depth from the upper surface 21a of the channel layer 21 is d (μm), the region can be approximated by the formula T=S×10^(k(D−d)). Here, the unit of k is μm. －１ For example, if the transition element concentration T is 5×10 １６ cm －３ From 5 x 10 １７ cm －３In the range of

[0050] The coefficient k in the above formula indicates a linear slope and can vary depending on, for example, the transition element concentration in the base substrate 10 and the growth conditions (e.g., growth temperature and growth time) of the channel layer 21. The coefficient k is preferably 6.5 or less, and may be 6.0 or less. On the other hand, the coefficient k is preferably 4.5 or more.

[0051] B. HEMT Device Figure 3 is a schematic cross-sectional view showing the general configuration of a HEMT device according to one embodiment of the present invention. The HEMT device 200 includes an epitaxial substrate 100, and a source electrode 23, a drain electrode 24, and a gate electrode 25 provided on an epitaxial layer 20 having a channel layer 21 and a barrier layer 22 of the epitaxial substrate 100. Each of these electrodes may be a metal electrode having a thickness of approximately 10-15 nm. A HEMT device using the above epitaxial substrate may have excellent performance (e.g., output characteristics).

[0052] The present invention will be specifically explained below with reference to examples, but the present invention is not limited to these examples.

[0053] Experimental Example 1 (Preparation of Seed Crystal Substrate) A plurality of c-plane sapphire substrates having a diameter of 4 inches were prepared, and a gallium nitride film having a thickness of 2 μm was formed on each of the sapphire substrates by MOCVD to prepare seed crystal substrates.

[0054] (Growth of Gallium Nitride Crystals) Gallium nitride crystals were grown using a crystal manufacturing apparatus equipped with a pressure-resistant container capable of supplying high-pressure nitrogen gas, a turntable rotatable within the pressure-resistant container, an outer container placed on the turntable, and a crystal growth furnace for placing the outer container under a desired temperature environment. The obtained seed crystal substrate was placed in an alumina crucible in a nitrogen atmosphere glove box. Next, 40 g of metallic gallium, 80 g of metallic sodium, and 0.01 g to 0.10 g of manganese as a doping element were each melted in the glove box and filled into the crucible, and the seed crystal substrate was immersed in the flux melt and covered with an alumina plate. In this way, four crucibles with different manganese filling amounts were prepared. The prepared crucible was placed in a stainless steel inner container, which was then placed in a stainless steel outer container capable of accommodating the inner container, and the outer container was closed with a lid equipped with a nitrogen inlet pipe. In this state, the outer container was placed on a turntable located within the crystal manufacturing apparatus, and the pressure-resistant container of the crystal manufacturing apparatus was covered and sealed. Next, the heater unit was operated to heat the crystal growth furnace in the crystal manufacturing apparatus to a uniform temperature of 850°C, while nitrogen gas was introduced from a nitrogen gas cylinder into the pressure-resistant container until the pressure reached 4 MPa, and the outer container was rotated horizontally. This state was maintained for 35 hours, allowing gallium nitride crystal to grow. After that, the container was naturally cooled to room temperature and depressurized to atmospheric pressure, and the lid of the alumina crucible was opened, revealing that the grown gallium nitride crystal and the sapphire substrate had naturally peeled off. In this way, a gallium nitride crystal with a diameter of 4 inches and a thickness of 1 mm was obtained.

[0055] Thereafter, the surface of the gallium nitride crystal separated from the sapphire substrate and the opposite surface were polished and flattened using diamond abrasive grains to obtain Mn-doped gallium nitride substrates with a diameter of 4 inches and a thickness of 0.4 mm. Specifically, four types of Mn-doped gallium nitride substrates with different manganese concentrations (A: 1 × 10 １８ cm －３ , B: 2 × 10 １８ cm －３ , C: 5 × 10 １８ cm －３ , D: 1 × 10 １９ cm －３The manganese concentration in each gallium nitride substrate was measured by the following method.

[0056] <Measurement of manganese concentration in gallium nitride substrate> A measurement sample measuring 20 mm x 20 mm was cut out from the obtained substrate, and the manganese (Mn) concentration was measured by secondary ion mass spectrometry (SIMS). The measurement conditions were as follows. The Mn concentration is the average value of the Mn concentration in the measurement range below. (Measurement conditions) Type of secondary ion mass spectrometer: magnetic sector type Type of primary ion: O ２ ＋ Measurement range: from a depth of 0.2 μm to a depth of 0.5 μm from the substrate surface (the surface on the epitaxial growth side) Detection limit: 5 × 10 １４ cm －３

[0057] (Fabrication of Epitaxial Substrate) A gallium nitride layer (channel layer) and an aluminum gallium nitride layer (barrier layer) were epitaxially grown in this order on the main surface of the obtained gallium nitride substrate by MOCVD to fabricate an epitaxial substrate. Specifically, the obtained gallium nitride substrate was placed on a susceptor in an MOCVD furnace. A mixed flow of hydrogen gas and nitrogen gas was used in the MOCVD furnace, and the temperature was raised to a predetermined temperature within the range of 1100°C to 1200°C at a furnace pressure of 0.3 atm. After reaching the predetermined temperature, a 1 μm gallium nitride layer was formed using ammonia gas and trimethylgallium (TMG), a Ga source gas. Thereafter, the temperature was raised to 1200°C, and trimethylaluminum (TMA), an Al source gas, was further added to form a 20 nm aluminum gallium nitride layer (Al:Ga composition ratio 0.2:0.8), thereby fabricating an epitaxial substrate. Thereafter, the substrate temperature was lowered to room temperature and the pressure was returned to atmospheric pressure, after which the epitaxial substrate was removed from the MOCVD furnace.

[0058] Experimental Example 2 An epitaxial substrate was obtained in the same manner as in Experimental Example 1, except that the deposition time for the gallium nitride layer was increased by 1.2 times to deposit a 1.2 μm thick gallium nitride layer.

[0059] Experimental Example 3 An epitaxial substrate was obtained in the same manner as in Experimental Example 1, except that the deposition time for the gallium nitride layer was increased by 1.5 times to deposit a 1.5 μm thick gallium nitride layer.

[0060] <Evaluation> The following evaluations were carried out on the epitaxial substrates obtained in Experimental Examples 1, 2, and 3. The evaluation results for the epitaxial substrates obtained in Experimental Example 1 are shown in Table 1, the evaluation results for the epitaxial substrates obtained in Experimental Example 2 are shown in Table 2, and the evaluation results for the epitaxial substrates obtained in Experimental Example 3 are shown in Table 3. Representatively, sample B-3 of Experimental Example 1 (gallium nitride substrate with Mn concentration of 2×10 １８ cm －３ The SIMS measurement results of the epitaxial substrate under the gallium nitride layer deposition temperature condition 3) are shown in FIG. 4. In FIG. 4, the vertical axis represents the concentration T (cm －３ ) and the horizontal axis represents the depth d (μm) from the upper surface of the channel layer. Note that noise can be observed in the concentration measurement near the upper surface of the channel layer.

[0061] 1. Manganese Concentration and Carbon Concentration A measurement sample measuring 20 mm x 20 mm was cut out from the center of the obtained epitaxial substrate, and the manganese (Mn) concentration and carbon (C) concentration were determined by secondary ion mass spectrometry (SIMS). The measurement conditions were as follows: (Measurement Conditions) Type of secondary ion mass spectrometer: magnetic sector type Type of primary ion: O ２ ＋ (when measuring manganese), Cs － (When measuring carbon) Measurement range: from the surface to a depth of 5 μm Detection limit: 5 × 10 １４ cm －３ (Manganese measurement), 3 x 10 １５ cm －３ (When measuring carbon)

[0062] The Mn concentration (cm －３ ) indicates the Mn concentration at a depth of 0.5 μm from the top surface of the channel layer, and the C concentration (cm －３ ) indicates the average value of the C concentration from the portion 0.15 μm deep to the portion 0.50 μm deep from the top surface of the channel layer, and the sum (cm －３) is the sum of these.

[0063] The gradient k of the Mn concentration shown in each table is the Mn concentration in the channel layer 21 as a function of T (cm －３ ), the thickness of the channel layer 21 is D (μm), the depth from the upper surface 21a of the channel layer 21 is d (μm), and the Mn concentration of the gallium nitride substrate is S (cm －３ ) and 5 × 10 １６ ≦T≦5×10 １７ The value of T and d was extracted within the range, and the least squares approximation method was applied to the formula T = S × 10^(k(D-d)).

[0064] 2. Characteristics of Transistor Element (Fabrication of Transistor Element) Next, a transistor element was fabricated using the obtained epitaxial substrate. Prior to forming electrodes on the epitaxial substrate, a 10 nm thick silicon oxide film was formed on the obtained epitaxial substrate as a passivation film. Subsequently, the silicon oxide film was etched away by photolithography in the areas where the source electrode, drain electrode, and gate electrode were to be formed. Next, using photolithography and reactive ion etching (RIE), the epitaxial growth layer was etched away to a depth of approximately 400 nm at the boundaries of each transistor element to be obtained. Next, a photoresist was applied to the epitaxial growth layer, and openings were formed by photolithography in the areas where the source electrode and drain electrode were to be formed. Metal films of Ti, Al, Ni, and Au were sequentially formed by vacuum deposition to thicknesses of 15 nm, 220 nm, 40 nm, and 75 nm, respectively, to form a multilayer structure. The substrate was then immersed in an organic solvent or a stripping solution, and the photoresist film was removed by lift-off to obtain source and drain electrodes. Next, in order to improve the ohmic properties of the source and drain electrodes, which could become ohmic metal pattern electrodes, the substrate was subjected to a heat treatment at 700°C for 30 seconds in a nitrogen gas atmosphere. Subsequently, a Pt metal film was formed to a thickness of 15 nm using photolithography and vacuum deposition, similar to the formation of the source and drain electrodes, to form a gate electrode which could become a Schottky metal pattern electrode. In this way, a transistor element was fabricated, in which electrodes with a gate width of 1 mm, a source-to-gate spacing of 0.5 μm, a gate-to-drain spacing of 7.5 μm, and a gate length of 1.5 μm were formed.

[0065] The drain current density and gate leakage current of the transistor elements fabricated as described above were measured. Specifically, the drain current density was determined from the current flowing between the source and drain when a source-drain voltage of +8 V was applied and a gate voltage of +3 V was set. In addition, the leakage current density was determined from the current flowing between the source and drain when a source-drain voltage of +8 V was applied and a gate voltage of -6 V was set to the off state. The transistor elements obtained from each epitaxial substrate were evaluated according to the following criteria. (Evaluation criteria) ○: Drain current density is 1000 mA / mm ２ ×: Drain current density exceeds 1000 mA / mm ２ ○: Leak current density is 1 μA / mm or less ２ Less than ×: Leak current density is 1 μA / mm ２ End

[0066]

[0067]

[0068]

[0069] The epitaxial substrate according to the embodiment of the present invention can be applied to, for example, a semiconductor device.

[0070] 10 Undersubstrate (Group III element nitride substrate), 11 First main surface (upper surface), 12 Second main surface (lower surface), 31 Growth substrate, 31a Upper surface, 31b Lower surface, 32 Seed crystal film, 33 Seed crystal substrate, 34 Group III element nitride crystal layer, 35 Laminated substrate, 36 Freestanding substrate, 20 Epitaxial layer, 21 Channel layer, 22 Barrier layer, 23 Source electrode, 24 Drain electrode, 25 Gate electrode, 100 Epitaxial substrate, 200 HEMT element.

Claims

1. A semiconductor device comprising: a base substrate made of a Group III element nitride doped with a transition element, the base substrate having upper and lower surfaces facing each other; and an epitaxial layer formed on the upper surface of the base substrate; the epitaxial layer having, from the base substrate side, a channel layer and a barrier layer; the channel layer having a thickness of 0.8 μm or more; and a sum of the transition element concentration at a depth of 0.5 μm from the upper surface of the channel layer and the average carbon concentration from a depth of 0.15 μm to a depth of 0.50 μm from the upper surface of the channel layer being 7×10 １５ cm －３ 1x10 or more １６ cm －３ Less than an epitaxial substrate.

2. The transition element concentration in the channel layer is T (cm －３ ), and the average value of the transition element concentration from a portion 0.2 μm deep to a portion 0.5 μm deep from the top surface of the base substrate is defined as S (cm －３ 2. The epitaxial substrate according to claim 1, wherein the channel layer has a region approximated by the following formula (1): T=S×10^(k(D−d)) (1), where k is in μm. －１ is.

3. The epitaxial substrate of claim 2, wherein the coefficient k is 6.5 or less.

4. The epitaxial substrate of claim 2, wherein the coefficient k is 6.0 or less.

5. The epitaxial substrate of claim 2, wherein the coefficient k is 4.5 or greater.

6. The transition element concentration T is 5×10 １６ cm －３ From 5 x 10 １７ cm －３ 3. The epitaxial substrate of claim 2, wherein the substrate approximates the formula in the range:

7. The epitaxial substrate of claim 1, wherein the channel layer has a thickness of 2 μm or less.

8. The epitaxial substrate of claim 1, wherein the base substrate comprises gallium nitride.

9. The concentration of the transition element in the base substrate is 1×10 １８ cm －３ 1x10 or more １９ cm －３ 2. The epitaxial substrate of claim 1, wherein:

10. A semiconductor device comprising: an epitaxial substrate according to any one of claims 1 to 9; and a source electrode, a drain electrode, and a gate electrode provided on the epitaxial layer of the epitaxial substrate.

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