Patent Grant
10818757
The present invention relates to a nitride semiconductor substrate, a semiconductor device, and a method for manufacturing a nitride semiconductor substrate.
Group III nitride semiconductors such as gallium nitride have a higher saturated free electron velocity and a higher dielectric breakdown voltage than those of silicon. Therefore, the nitride semiconductors are expected to be applied to power devices that control electric power and the like, and to high-frequency devices such as those for base stations of cellular phones. For example, semiconductor devices such as Schottky barrier diodes (SBD) and pn junction diodes can be used as specific devices. In these semiconductor devices, a drift layer having a low donor concentration is thickly formed in order to improve a breakdown voltage when reverse bias is applied (for example, see Patent Document 1).
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2015-185576
In the nitride semiconductors, carbons can be incorporated due to a group III organometallic material during crystal growth. At least a part of the carbons incorporated into the nitride semiconductor functions as acceptors. Therefore, in donor doped n-type nitride semiconductor, at least a part of the carbons captures electrons from donors and compensates for the donors.
In the semiconductor device as a power device or a high frequency device, as described above, the donor concentration in the drift layer is low in order to improve the breakdown voltage. Therefore, there has been a case that a desired free electron concentration cannot be obtained in the drift layer, due to a great influence of compensating for a small amount of donors by a part of the carbons, even if the donor concentration in the drift layer is a predetermined value for example, in a low concentration region of 5×1016/cm3 or less. As a result, there is a possibility that a performance of the semiconductor device is deteriorated.
An object of the present invention is to provide a technique of improving the performance of the semiconductor device.
According to an aspect of the present invention, there is provided a nitride semiconductor substrate, including:
According to another aspect of the present invention, there is provided a semiconductor device, including:
According to further another aspect of the present invention, there is provided a method for manufacturing a nitride semiconductor substrate, including: forming a drift layer as a gallium nitride layer containing donors and carbons, on an n-type semiconductor substrate,
According to the present invention, a performance of a semiconductor device can be improved.
An embodiment of the present invention will be described hereafter, with reference to the drawings.
(1) Nitride Semiconductor Substrate
As shown in
Hereinafter, a “laminating direction” means a direction in which nitride semiconductor layers such as the underlying n-type semiconductor layer 120 are laminated in an upward direction (direction away from a main surface of the substrate 100) from the substrate 100 side in the drawing. In other words, regarding the drift layer 140, the “laminating direction” means “the direction from the substrate 100 side toward the surface side of the drift layer 140”. The surface (second main surface) of the drift layer 140 is the surface opposite to the substrate 100 side surface (first main surface) in the drift layer 140.
(Substrate)
A substrate 100 is configured as an n-type monocrystalline gallium nitride (GaN) substrate (free-standing GaN substrate) containing predetermined donors. As the donors in the substrate 100, for example, silicon (Si) or germanium (Ge) can be used. The donor concentration in the substrate 100 is, for example, 5.0×1017/cm3 or more and 5.0×1018/cm3 or less. The donor concentration, the carbon concentration described later, and the like can be measured by, for example, secondary ion mass spectrometry (SIMS).
A plane orientation of the main surface of the substrate 100 is, for example, a c-plane ((0001) plane). The GaN crystal constituting the substrate 100 may have a predetermined off-angle with respect to the main surface of the substrate 100. The off-angle is an angle formed by a normal direction of the main surface of the substrate 100 and the c-axis of the GaN crystal constituting the substrate 100. Specifically, the off-angle of the substrate 100 is, for example, 0.15° or more and 0.8° or less. If the off-angle of the substrate 100 is less than 0.15°, there is a possibility of increasing the concentration of the carbons (C) added when growing the nitride semiconductor layer such as the drift layer 140 on the substrate 100. In contrast, since the off-angle of the substrate 100 is 0.15° or more, the concentration of the carbons added when growing the nitride semiconductor layer such as the drift layer 140 on the substrate 100, can be decreased to a predetermined amount or less. Meanwhile, if the off-angle of the substrate 100 is more than 0.8°, there is a possibility that a morphology of the main surface of the substrate 100 is deteriorated. In contrast, since the off-angle of the substrate 100 is 0.8° or less, the morphology of the main surface of the substrate 100 can be flattened.
Further, a dislocation density on the main surface of the substrate 100 is, for example, 1×107/cm2 or less. If the dislocation density on the main surface of the substrate 100 is more than 1×107/cm2, there is a possibility of increasing the dislocations that lower a local breakdown voltage, in the nitride semiconductor layer such as the drift layer 140 formed on the substrate 100. Further, if the dislocation density on the main surface of the substrate 100 exceeds 1×107/cm2, the concentration (for example, carbon concentration) of impurities unintentionally added during growth of the nitride semiconductor layer on the substrate 100 tends to be high. In contrast, since the dislocation density on the main surface of the substrate 100 is 1×107/cm2 or less as in this embodiment, it is possible to suppress an increase of the dislocations that lower the local breakdown voltage in the nitride semiconductor layer such as the drift layer 140 formed on the substrate 100. Further, since a threading dislocation density on the main surface of the substrate 100 is 1×107 atoms/cm2 or less, the concentration of the impurities unintentionally added during growth of the nitride semiconductor layer can be decreased.
(Underlying N-Type Semiconductor Layer)
The underlying n-type semiconductor layer 120 is provided between the substrate 100 and the drift layer 140, as a buffer layer for taking over the crystallinity of the substrate 100 and stably epitaxially growing the drift layer 140. Further, the underlying n-type semiconductor layer 120 is configured as an n+-type GaN layer containing donors in the same concentration as that of the substrate 100. As the donors in the underlying n-type semiconductor layer 120, for example, Si or Ge can be used similarly to the donors in the substrate 100. Further, the donor concentration in the underlying n-type semiconductor layer 120 is, for example, 5.0×1017/cm3 or more and 5.0×1018/cm3 or less, similarly to the donor concentration in the substrate 100.
The underlying n-type semiconductor layer 120 contains carbons which are added (autodoped) due to the group III organometallic material used at the time of crystal growth. A total concentration of the carbons in the underlying n-type semiconductor layer 120 is, for example, 1.0×1015/cm3 or more and 5.0×1016/cm3 or less. The “total concentration of the carbons” means the concentration of all kinds of carbons, including not only the concentration of the carbons that function as acceptors but also the concentration of the carbons that do not function as acceptors, as described later.
In an n-type nitride semiconductor layer such as the underlying n-type semiconductor layer 120, at least a part of the carbons function as acceptors (compensating dopant) and compensates for the donors. Therefore, an effective free electron concentration in the underlying n-type semiconductor layer 120 is obtained as a difference obtained by subtracting the concentration of the carbons that function as acceptors from the donor concentration. However, in the underlying n-type semiconductor layer 120, the donor concentration is high and the concentration of the carbons that function as acceptors is relatively low enough to be negligible. Therefore, the free electron concentration in the underlying n-type semiconductor layer 120 can be regarded as approximately equal to the donor concentration, and for example, it is 5.0×1017/cm3 or more and 5.0×1018/cm3 or less.
The donor concentration and the total concentration of the carbons in the underlying n-type semiconductor layer 120 are substantially constant toward a laminating direction, respectively. If the underlying n-type semiconductor layer 120 includes a region in which each added concentration is constant toward the laminating direction, the underlying n-type semiconductor layer 120 may include a concentration gradient region near the substrate 100 side or the drift layer 140 side.
A thickness of the underlying n-type semiconductor layer 120 is smaller than a thickness of the drift layer 140 described later, and is, for example, 0.1 μm or more and 3 μm or less.
(Drift Layer)
The drift layer 140 is provided on the underlying n-type semiconductor layer 120 and is configured as an n−-type GaN layer containing donors in a low concentration. As the donors in the drift layer 140, for example Si or Ge can be used, similarly to the donors in the underlying n-type semiconductor layer 120.
The donor concentration in the drift layer 140 is lower than the donor concentration of the substrate 100 and the donor concentration of the underlying n-type semiconductor layer 120, and for example, it is 1.0×1015/cm3 or more and 5.0×1016/cm3 or less. If the donor concentration is less than 1.0×1015/cm3, there is a possibility that resistance of the drift layer 140 becomes higher. In contrast, since the donor concentration is 1.0×1015/cm3 or more, excessive increase in the resistance of the drift layer 140 can be suppressed. Meanwhile, if the donor concentration exceeds 5.0×1016/cm3, there is a possibility that the breakdown voltage is lowered when reverse bias is applied. In contrast, since the donor concentration is 5.0×1016/cm3 or less, a predetermined breakdown voltage can be secured.
The drift layer 140 also contains carbons added due to the group III organometallic material used at the time of crystal growth. At least a part of the carbons in the drift layer 140 functions as an acceptor and compensates for the donors. Here, as described above, in the underlying n-type semiconductor layer 120, the donor concentration is high at a level of 1018 order. Therefore, in the underlying n-type semiconductor layer 120, the carbon concentration compared to the donor concentration is low enough to be negligible. In contrast, in the drift layer 140, the donor concentration is as low as 5.0×1016/cm3 or less. Therefore, in the drift layer 140, the carbon concentration compared to the donor concentration is not negligible, and the concentration of free electrons in the drift layer 140 is easily influenced by the compensation for a small amount of the donors by a part of the carbons that function as acceptors. Accordingly, in the drift layer 140, it is impossible to obtain a desired free electron concentration distribution unless the relative relationship between the donor concentration and the concentration of the carbons that function as acceptors is controlled.
Therefore, in this embodiment, the donor concentration in the drift layer 140 is adjusted to be equal to or more than the concentration of the carbons that function as acceptors in the drift layer 140 over the entire area of the drift layer 140, and a difference obtained by subtracting the concentration of the carbons that function as acceptors in the drift layer 140 from the donor concentration in the drift layer 140, is adjusted to be gradually decreased from the substrate 100 side toward the surface side of the drift layer 140 (namely, toward the laminating direction). As a result, a desired free electron concentration distribution can be obtained in the drift layer 140. The relative relationship between the donor concentration and the carbon concentration in the drift layer 140 will be described later in detail.
In order to improve the breakdown voltage when reverse bias is applied, the drift layer 140 is provided so as to be thicker than the underlying n-type semiconductor layer 120 for example. Specifically, the thickness of the drift layer 140 is for example 3 μm or more and 40 μm or less. If the thickness of the drift layer 140 is less than 3 μm, there is a possibility that the breakdown voltage is lowered when reverse bias is applied (depending on the donor concentration in the drift layer 140). In contrast, since the thickness of the drift layer 140 is 3 μm or more, a predetermined breakdown voltage can be secured. Meanwhile, if the thickness of the drift layer 140 exceeds 40 μm, there is a possibility that on-resistance becomes higher when forward bias is applied. In contrast, since the thickness of the drift layer 140 is 40 μm or less, it is possible to suppress an excessive increase of the on-resistance when forward bias is applied.
(First P-Type Semiconductor Layer)
A first p-type semiconductor layer 220 is provided on the drift layer 140 and is configured as a p-type GaN layer containing acceptors. As the acceptors in the first p-type semiconductor layer 220, for example magnesium (Mg) can be used. Further, the acceptor concentration in the first p-type semiconductor layer 220 is, for example, 1.0×1017/cm3 or more and 2.0×1019/cm3 or less.
(Second P-Type Semiconductor Layer)
A second p-type semiconductor layer 240 is provided on the first p-type semiconductor layer 220 and is configured as a p+-type GaN layer containing the acceptors in a high concentration. As the acceptor in the second p-type semiconductor layer 240, for example Mg can be used similarly to the first p-type semiconductor layer 220. Further, the acceptor concentration in the second p-type semiconductor layer 240 is higher than the acceptor concentration in the first p-type semiconductor layer 220, and for example, it is 5.0×1019/cm3 or more and 2.0×1020/cm3 or less. Since the acceptor concentration in the second p-type semiconductor layer 240 is within the above range, a contact resistance between the second p-type semiconductor layer 240 and an anode described later can be lowered.
(Relative Relationship Between the Donor Concentration and the Carbon Concentration in the Drift Layer)
Next, with reference to
In
Here, in this embodiment, the donor concentration ND in the drift layer 140 is equal to or more than the concentration NA of the carbons that function as acceptors in the drift layer 140 (ND≥NA). If the donor concentration ND in the drift layer 140 is less than the concentration NA of the carbons that function as acceptors in the drift layer 140 in at least a part of the drift layer 140, there is a possibility that a region where free electrons are not obtained may be generated in a part of the drift layer 140. In contrast, since the donor concentration ND in the drift layer 140 is equal to or more than the concentration NA of carbons that function as acceptors in the drift layer 140 over the entire area of the drift layer 140, a predetermined amount of the free electrons can be generated over the entire area of the drift layer 140, even if at least a part of the carbons in the drift layer 140 compensates for the donors, in a state that the concentration of the donors is as low as 5.0×1016/cm3 or less. As a result, the drift layer 140 can function as an n-type layer.
Further, as shown in
As a result of intensive study by inventors, etc., it is found that not all of the carbons added to the nitride semiconductor compensates for the donors, but at least ⅓ or more of all of the carbons added to the nitride semiconductor functions as acceptors, and compensates for the donors. Namely, in the drift layer 140 of this embodiment, the concentration NA of the carbons that function as acceptors is, at least ⅓ times or more of the total concentration NC of the carbons (NC/3≤NA≤NC).
Therefore, in this embodiment, the donor concentration ND in the drift layer 140 is at least ⅓ or more times of the total concentration NC of the carbons in the drift layer 140 over the entire area of the drift layer 140 (ND≥NC/3), in consideration of a proportion of the carbons that function as acceptors as described above. If the donor concentration ND in the drift layer 140 is less than ⅓ of the total concentration NC of the carbons, there is a possibility that a predetermined amount of the free electrons are not generated in the drift layer 140, because most of the donors in the drift layer 140 are compensated by the carbons that function as acceptors. Therefore, there are possibilities that the drift layer 140 is not n-type, and the resistance of the drift layer 140 becomes higher. In contrast, since the donor concentration ND in the drift layer 140 is equal to or more than ⅓ times of the total concentration NC of the carbons, the amount of the donors in the drift layer 140 can be made larger than the amount compensated by carbons as acceptors, and a predetermined amount of the free electrons can be generated in the drift layer 140. As a result, it is possible to make the drift layer 140 function as an n-type layer and prevent the resistance of the drift layer 140 from becoming excessively high. When the donor concentration ND in the drift layer 140 is equal to or more than ⅓ times of the total concentration NC of the carbons in the drift layer 140, the donor concentration ND in the drift layer 140 may be lower than the total concentration NC of the carbons in the drift layer 140.
The donor concentration ND in the drift layer 140 is preferably made equal to or more than the total concentration NC of the carbons in the drift layer 140 over the entire area of the drift layer 140. Thereby, the amount of the donors in the drift layer 140 can be surely made larger than the amount compensated by the carbons as acceptors. As a result, the drift layer 140 can stably function as the n type layer.
Here, a band diagram near the drift layer 140 will be described, with reference to
In
In the vicinity of the junction interface between the drift layer 140 and the underlying n-type semiconductor layer 120, the free electron concentration of the drift layer 140 is gradually increased toward the underlying n-type semiconductor layer 120, and approaches the free electron concentration of the underlying n-type semiconductor layer 120. Thereby, the conduction band of the drift layer 140 and the conduction band of the underlying n-type semiconductor layer 120 are gently joined, and an energy barrier becomes low between the conduction band of the drift layer 140 and the conduction band of the underlying n-type semiconductor layer 120. As a result, when forward bias is applied, electrons can move smoothly from the underlying n-type semiconductor layer 120 toward the drift layer 140, and ON-resistance can be lowered.
Meanwhile, in the vicinity of the junction interface between the drift layer 140 and the first p-type semiconductor layer 220, the free electron concentration of the drift layer 140 is gradually decreased toward the first p-type semiconductor layer 220, and is more decreased than a hole concentration of the first p-type semiconductor layer 220. A depletion layer in the vicinity of the junction interface does not expand so much from the junction interface toward the first p-type semiconductor layer 220, but expands from the junction interface toward the drift layer 140. Thereby, a slope (electric field intensity) of the conduction band in the vicinity of the junction interface is gentle. When reverse bias is applied, the depletion layer expands further toward the underlying n-type semiconductor layer 120 from a state before applying reverse bias. At this time, the slope of the conduction band becomes largest in the vicinity of the junction interface between the drift layer 140 and the first p-type semiconductor layer 220. However, as described above, due to a low free electron concentration on the surface side of the drift layer 140 and expansion of the depletion layer in the drift layer 140, the slope of the conduction band in the vicinity of the junction interface is suppressed from becoming excessively steep, even in a case of applying reverse bias. Thereby, occurrence of an avalanche breakdown in the vicinity of the junction interface between the drift layer 140 and the first p-type semiconductor layer 220 can be suppressed, and the breakdown voltage can be improved.
More specifically, as shown in
When the concentration difference ND−NA in the drift layer 140 is linearly increased (in a straight line) in the laminating direction as in this embodiment, an absolute value of the gradient of ND−NA with respect to a depth from the surface side of the drift layer 140 is, for example, 5.0×1014 cm−3·μm−1 or more and 3.0×1016 cm−3·μm−1 or less. If the absolute value of the gradient of ND−NA is less than 5.0×1014 cm−3·μm−1 and ND−NA is low, there are possibilities that the energy barrier of the conduction band becomes high at the junction interface between the drift layer 140 and the underlying n-type semiconductor layer 120, and the ON-resistance becomes high when forward bias is applied. Meanwhile, if ND−NA is high, there are possibilities that the slope of the conduction band becomes large in the vicinity of the junction interface between the drift layer 140 and the first p-type semiconductor layer 220, and the breakdown voltage is lowered when reverse bias is applied. Accordingly, if the absolute value of the gradient of ND−NA is less than 5.0×1014 cm−3·μm−1, it becomes difficult to achieve both of lowering the ON-resistance when forward bias is applied and improving the breakdown voltage when reverse bias is applied. In contrast, since the absolute value of the gradient of ND−NA is 5.0×1014 cm−3·μm−1 or more, the slope of the conduction band can be made small in the vicinity of the junction interface between the drift layer 140 and the first p-type semiconductor layer 220, while making the energy barrier of the conduction band low in the vicinity of the junction interface between the drift layer 140 and the underlying n-type semiconductor layer 120. As a result, it is possible to achieve both of lowering the ON-resistance when forward bias is applied and improving the breakdown voltage when reverse bias is applied. Meanwhile, if the absolute value of the gradient of ND−NA is more than 3.0×1016 cm−3·μm−1, the maximum value or the minimum value of the donor concentration ND in the drift layer 140 is hardly within the above-described predetermined range. In contrast, since the absolute value of the gradient of ND·NA is 3.0×1016 cm−3·μm−1 or less, the maximum value or the minimum value of the donor concentration ND in the drift layer 140 can be within the above-described predetermined range.
(Regarding each Concentration of the Donor and the Carbon etc. in the Drift Layer)
A specific distribution of the donor concentration ND and the total carbon concentration NC in the drift layer 140 and the like will be described next, using
(Donor Concentration)
As shown in
(Carbon Concentration)
Meanwhile, by satisfying the following three conditions 1) the donor concentration ND in the drift layer 140 is 5.0×1016/cm3 or less; 2) the donor concentration ND in the drift layer 140 is equal to or more than the concentration NA of the carbons that function as acceptors in the drift layer 140 over the entire area of the drift layer 140; 3) the concentration difference ND−NA in the drift layer 140 is gradually decreased in the laminating direction, the total concentration NC of the carbons in the drift layer 140 can be arbitrarily distributed in the laminating direction.
Specifically, for example, as in the case of (A) in
Alternatively, for example, as in the case of (B) in
Alternatively, for example, as in the case of (C) of
Regarding a specific range of the total concentration NC of the carbons in the drift layer 140, the total concentration NC of the carbons in the drift layer 140 is, for example, 5.0×1016/cm3 or less. If the total concentration NC of the carbons in the drift layer 140 is more than 5.0×1016/cm3, there is a possibility that the crystallinity of the drift layer 140 is deteriorated and the loss of the semiconductor device 20 described later is increased. In contrast, since the concentration NC of the carbons in the drift layer 140 is 5.0×1016/cm3 or less, the crystallinity of the drift layer 140 can be improved and the loss of the semiconductor device 20 can be reduced. Note that it is much better that the total concentration NC of the carbons in the drift layer 140 is low as much as possible, and therefore a lower limit value of the total concentration NC of the carbons is not particularly limited.
(Hydrogen Concentration)
In addition to the donors and carbons, the drift layer 140 also contains hydrogen (H). Hydrogen (H) is incorporated into the drift layer 140, due to the group III organometallic material, the donor material, or the like used at the time of crystal growth of the drift layer 140. The hydrogen concentration in the drift layer 140 is, for example, 5.0×1016/cm3 or less, preferably 1.0×1016/cm3 or less. If the hydrogen concentration in the drift layer 140 is more than 5.0×1016/cm3, there is a possibility that the crystallinity of the drift layer 140 is deteriorated and the loss of the semiconductor device 20 described later is increased. In contrast, since the hydrogen concentration in the drift layer 140 is 5.0×1016/cm3 or less, the crystallinity of the drift layer 140 can be improved and the loss of the semiconductor device 20 can be reduced. Note that it is much better that the hydrogen concentration in the drift layer 140 is low as much as possible, and therefore the lower limit value of the hydrogen concentration is not particularly limited.
(2) Semiconductor Device
With reference to
As shown in
The drift layer 140, the first p-type semiconductor layer 220, and the second p-type semiconductor layer 240 form a mesa structure 180. The mesa structure 180 is, for example, a quadrangular pyramidal trapezoid or a truncated cone trapezoid, and a cross sectional area of the mesa structure 180 in a plan view becomes gradually small in the laminating direction. Thereby, the mesa structure 180 has a forward tapered side face. By forming such a mesa structure 180, an electric field concentration on an end portion of a first anode 320 described later is relaxed, and the breakdown voltage of the semiconductor device 20 can be improved.
Further, a part of the region where the concentration difference ND−NA in the drift layer 140 is gradually decreased toward the laminating direction, forms a part of the mesa structure 180. Here, in the mesa structure 180 as in this embodiment, electric field concentration is likely to occur in the vicinity of the pn junction interface near the side surface of the mesa structure 180. However, in this embodiment, the concentration difference ND−NA in the drift layer 140 is gradually decreased toward the laminating direction, also in the region in the vicinity of the pn junction interface near the side surface of the mesa structure 180. Thereby, the depletion layer expands from the pn junction interface toward the drift layer 140, also in the region in the vicinity of the pn junction interface near the side surface of the mesa structure 180, and the electric field in this region is relaxed. As a result, it becomes possible to suppress the occurrence of the avalanche breakdown in the region in the vicinity of the pn junction interface near the side surface of the mesa structure 180 when reverse bias is applied, and improve the breakdown voltage of the semiconductor device 20.
The first anode (p-type contact electrode) 320 of the anode (p-side electrode) 310 is provided on an upper surface of the mesa structure 180, that is, on the second p-type semiconductor layer 240. The first anode 320 includes a material in ohmic contact with the second p-type semiconductor layer 240, and includes, for example, an alloy of palladium (Pd), Pd and nickel (Ni) (Pd/Ni), or an alloy of Ni and gold (Ni/Au).
The insulating film 400 is provided so as to cover the surface of the drift layer 140 outside of the mesa structure 180, the side surface of the mesa structure 180, and a part of the surface of the second p-type semiconductor layer 240 (around the upper surface of the mesa structure 180). Thereby, the insulating film 400 functions to insulate the drift layer 140 and the like from the second anode 340 described later and to protect the drift layer 140 and the like. Note that the insulating film 400 has an opening for bringing the first anode 320 into contact with the second anode 340 described later.
The insulating film 400 of this embodiment has a two-layer structure, for example, so as to have a first insulating film 420 and a second insulating film 440. The first insulating film 420 is configured, for example, as a SOG (Spin On Glass) film formed by a coating method such as a spin coating method. The second insulating film 440 is configured, for example, as a silicon oxide (SiO2) film formed by sputtering or the like.
The second anode (p-side electrode pad) 340 of the anode 310 is in contact with the first anode 320 in the opening of the insulating film 400, and is provided so as to extend to the outside of the first anode 320 on the insulating film 400, and to cover the mesa structure 180. Specifically, the second anode 340 is provided so as to overlap on a part of the surface of the drift layer 140 outside of the mesa structure 180, the side surface of the mesa structure 180, and an upper surface of the mesa structure 180, when the semiconductor device 20 is planarly viewed from above. Thereby, it is possible to suppress the concentration of the electric field in the vicinity of the end portion of the first anode 320 or the pn junction interface near the side surface of the mesa structure 180. Note that the second anode 340 includes, for example, an alloy (Ti/Al) of titanium (Ti) and aluminum (Al).
The cathode 360 is provided on a back side of the substrate 100. The cathode 360 includes a material in ohmic contact with the n-type GaN substrate 100, and includes Ti/Al, for example.
(3) Method for Manufacturing a Nitride Semiconductor Substrate (Method for Manufacturing a Semiconductor Device)
With reference to
(Step 1: Preparation of a Substrate)
As shown in
(Step 2: Formation of the Underlying N-Type Semiconductor Layer)
Next, by the following procedure, the nitride semiconductor layer such as the underlying n-type semiconductor layer 120 is formed on the substrate 100, for example using a Metal Organic Vapor Phase Epitaxy (MOVPE) apparatus.
First, the substrate 100 is loaded into a processing chamber of the MOVPE apparatus. Then, hydrogen gas (or mixed gas of hydrogen gas and nitrogen gas) is supplied into the processing chamber of the MOVPE apparatus, and the substrate 100 is heated to a predetermined growth temperature (for example, 1000° C. or more and 1100° C. or less). When the temperature of the substrate 100 reaches a predetermined growth temperature, for example, trimethylgallium (TMG) as a group III organometallic material and ammonia (NH3) gas as a group V material are supplied to the substrate 100. At the same time, for example, monosilane (SiH4) gas is supplied to the substrate 100 as a donor material. Thereby, the underlying n-type semiconductor layer 120 as the n+-type GaN layer is epitaxially grown on the n-type monocrystalline gallium nitride substrate 100 as a GaN substrate. The crystal growth at this time is a homoepitaxial growth in which the same GaN crystal is grown in the laminating direction, and therefore the underlying n-type semiconductor layer 120 having good crystallinity can be formed on the substrate 100.
(Step 3: Formation of the Drift Layer)
Next, the drift layer 140 as an n−-type GaN layer is epitaxially grown on the underlying n-type semiconductor layer 120. At this time, each growth condition is adjusted so that the donor concentration ND in the drift layer 140 is 5.0×1016/cm3 or less, and the donor concentration ND in the drift layer 140 is equal to or more than the concentration NA of the carbons that function as acceptors in the drift layer 140 over the entire area of the drift layer 140, and further the concentration difference ND−NA in the drift layer 140 is gradually decreased in the laminating direction.
Specifically, as shown in
Further, the flow rate of the donor material and other growth conditions are relatively adjusted so that the donor concentration ND in the drift layer 140 is at least ⅓ times of the total concentration NC of the carbons over the entire area of the drift layer 140, in consideration of the concentration of the carbons incorporated due to the group III organometallic material. Specifically, the total concentration NC of carbons can be adjusted by adjusting the flow rate (growth rate) of TMG, the V/III ratio (ratio of the flow rate of the group V material to the flow rate of the group III organometallic material), the growth temperature, etc., during the growth of the drift layer 140
As described above, by satisfying the following three conditions 1) the donor concentration ND in the drift layer 140 is 5.0×1016/cm3 or less; 2) the donor concentration ND in the drift layer 140 is equal to or more than the concentration NA of the carbons that function as acceptors in the drift layer 140 over the entire area of the drift layer 140; 3) the concentration difference ND−NA in the drift layer 140 is gradually decreased in the laminating direction, the total concentration NC of the carbons in the drift layer 140 can be arbitrarily distributed in the laminating direction.
For example, as in the case of (A) in
Alternatively, for example, as in the case of (B) in
Alternatively, for example as in the case of (C) in
(Step 4: Formation of the First P-Type Semiconductor Layer)
Next, the first p-type semiconductor layer 220 as the p-type GaN layer is epitaxially grown on the drift layer 140. At this time, in place of the donor material, for example, biscyclopentadienyl magnesium (Cp2Mg) is supplied to the substrate 100 as an acceptor material.
(Step 5: Formation of the Second P-Type Semiconductor Layer)
Next, the second p-type semiconductor layer 240 as a p+-type GaN layer is epitaxially grown on the first p-type semiconductor layer 220 by the same processing procedure as in step 4.
(Step 6: Unloading)
When the growth of the second p-type semiconductor layer 240 is completed, supply of the group III organometallic material and heating of the substrate 100 are stopped. Then, when the temperature of the substrate 100 reaches 500° C. or lower, supply of the group V material is stopped. Thereafter, an atmosphere in the processing chamber of the MOVPE apparatus is replaced with N2 gas and an atmospheric pressure is restored, and the temperature of an inside of the processing chamber is lowered to a temperature at which the substrate can be unloaded, and thereafter the substrate 100 after growth is unloaded from the processing chamber.
The nitride semiconductor substrate 10 of this embodiment is manufactured through the above steps 1 to 6. Thereafter the nitride semiconductor substrate 10 is supplied to a manufacturer or the like of the semiconductor device 20, as an epitaxial wafer for manufacturing the semiconductor device 20.
(Step 7: Manufacture of a Semiconductor Device)
Next, as shown in
The semiconductor device 20 of this embodiment is manufactured through the above step 7.
The semiconductor device 20 of this embodiment is manufactured through the above steps 7.
(4) Effect Obtained by this Embodiment
According to this embodiment, one or a plurality of effects shown below can be obtained.
(a) The donor concentration of the drift layer 140 is 5.0×1016/cm3 or less, and meanwhile which is equal to or more than the concentration NA of the carbons that function as acceptors in the drift layer 140 over the entire area of the drift layer 140 (ND≥NA). Thereby, a predetermined amount of free electrons can be generated over the entire area of the drift layer 140, even if at least a part of the carbons in the drift layer 140 compensates for the donor, in a state that the donor concentration is as low as 5.0×1016/cm3 or less. As a result, the drift layer 140 can function as an n-type layer.
(b) The difference ND−NA obtained by subtracting the concentration NA of the carbons that function as acceptors in the drift layer 140, from the donor concentration ND in the drift layer 140 is, gradually decreased from the substrate 100 side toward the surface side of the drift layer 140. Since the concentration difference ND−NA in the drift layer 140 has such a predetermined distribution, a desired free electron concentration distribution can be obtained, even if at least a part of the carbons in the drift layer 140 compensates for the donors. In this case, for example, the concentration of the free electrons can be gradually decreased from the substrate 100 side toward the surface side of the drift layer 140.
(c) Since the concentration difference ND−NA in the drift layer 140 is gradually decreased from the substrate 100 side toward the surface side of the drift layer 140, the free electron concentration of the drift layer 140 is gradually decreased toward the first p-type semiconductor layer 220, in the vicinity of the junction interface between the drift layer 140 and the first p-type semiconductor layer 220. Thereby, the depletion layer in the vicinity of the junction interface between the drift layer 140 and the first p-type semiconductor layer 220 expands from the junction interface toward the drift layer 140, and the slope of the conduction band (electric field intensity) in the vicinity of the junction interface is gentle. Thereby, the slope of the conduction band in the vicinity of the junction interface between the drift layer 140 and the first p-type semiconductor layer 220 can be suppressed from becoming excessively steep when reverse bias is applied. As a result, the occurrence of the avalanche breakdown can be suppressed in the vicinity of the junction interface between the drift layer 140 and the first p-type semiconductor layer 220, and the breakdown voltage can be improved.
(d) Since the concentration difference ND−NA in the drift layer 140 is gradually decreased from the substrate 100 side toward the surface side of the drift layer 140, the free electron concentration of the drift layer 140 is gradually increased toward the underlying n-type semiconductor layer 120, in the vicinity of the junction interface between the drift layer 140 and the base n-type semiconductor layer 120. Thereby, the conduction band of the drift layer 140 and the conduction band of the underlying n-type semiconductor layer 120 are gently joined, and the energy barrier becomes low between the conduction band of the drift layer 140 and the conduction band of the underlying n-type semiconductor layer 120. As a result, electrons can move smoothly from the underlying n-type semiconductor layer 120 toward the drift layer 140 when forward bias is applied, and the ON-resistance can be lowered. Accordingly, in this embodiment, it is possible to achieve both of lowering the resistance when forward bias is applied and improving the breakdown voltage when reverse bias is applied.
(e) Since the concentration NA of the carbons that function as acceptors is at least ⅓ times or more of the total concentration NC of carbons in the drift layer 140, the donor concentration ND in the drift layer 140 of this embodiment is ⅓ times or more of the total concentration NC of carbons in the drift layer 140 over the entire area of the drift layer 140. Thereby, the amount of the donor in the drift layer 140 can be made larger than the amount compensated by the carbons as acceptors, and a predetermined amount of the free electrons can be generated in the drift layer 140. As a result, it is possible to make the drift layer 140 function as the n-type layer and prevent the resistance of the drift layer 140 from becoming excessively high.
(f) The concentration of hydrogen in the drift layer 140 is 5.0×1016/cm3 or less. Thereby, the crystallinity of the drift layer 140 can be improved and the loss of the semiconductor device 20 can be reduced.
As described above, embodiments of the present invention have been specifically described. However, the present invention is not limited to the above-described embodiments, and can be variously modified in a range not departing from the gist of the invention.
(a) In the above-described embodiment, explanation is given for a case that the concentration difference ND−NA in the drift layer 140 is linearly decreased from the substrate 100 side toward the surface side of the drift layer 140. However, when the concentration difference ND−NA in the drift layer 140 is gradually decreased from the substrate 100 side toward the surface side of the drift layer 140, the following modified example may be applied.
As shown in the modified example 1 of
As shown in the modified example 2 of
(b) In the above-described embodiment, explanation is given for case that the nitride semiconductor substrate 10 is configured as a wafer for manufacturing a pn junction diode, and the semiconductor device 20 is configured as a pn junction diode. However, the following modified example 3 may be applied to the nitride semiconductor substrate and the semiconductor device.
As shown in
As shown in
According to the modified example 3, even if the semiconductor device 22 is SBD, it is possible to obtain the same effect as in the above embodiment. Further, SBD like the semiconductor device 22 of this modified example is known to have a lower breakdown voltage than the pn junction diode. However, according to this modified example, by giving the above-described gradient to the concentration difference ND−NA in the drift layer 142, it is possible to improve the breakdown voltage of the semiconductor device 22 as the SBD.
(c) In the above-described embodiment, explanation is given for a case that the substrate 100 is the n-type GaN substrate. However, the substrate may be configured as a semiconductor substrate other than GaN as long as it is configured as the n-type semiconductor substrate. Specifically, the substrate may be configured as, for example, an n-type SiC substrate. However, in order to improve the crystallinity of the nitride semiconductor layer on the substrate, the substrate is preferably the n-type GaN substrate.
(d) In the above-described embodiment, explanation is given for a case that the underlying n-type semiconductor layer 120 is interposed between the substrate 100 and the drift layer 140. However, the underlying n-type semiconductor layer is not required to be provided. Namely, the drift layer may be directly provided on the substrate.
(e) In the above-described embodiment, explanation is given for a case that the first p-type semiconductor layer 220 and the second p-type semiconductor layer 240 are provided on the drift layer 140. However, the p-type semiconductor layer on the drift layer may be only one layer.
(f) In the above-described embodiment, explanation is given for a case that the nitride semiconductor layer such as the drift layer 140 is formed, by using the MOVPE apparatus. However, the nitride semiconductor layer such as the drift layer 140 may be formed using a hydride vapor phase epitaxy (HYPE) apparatus. However, in this case, when forming the drift layer 140, hydrocarbon gas is supplied to the substrate 100 as a carbon material, and the flow rate of the carbon material is adjusted. Thereby, the total concentration NC of the carbons in the drift layer 140 can have a predetermined distribution in the laminating direction.
<Preferable Aspects of the Present Invention>
Preferable aspects of the present invention will be supplementarily described here after.
(Supplementary Description 1)
There is provided a nitride semiconductor substrate, including:
There is provided the nitride semiconductor substrate of the supplementary description 1, wherein the concentration of the donors in the drift layer is ⅓ times or more of a total concentration of the carbons in the drift layer over the entire area of the drift layer.
(Supplementary Description 3)
There is provided the nitride semiconductor substrate of the supplementary description 1 or 2, wherein the drift layer contains hydrogen, and a concentration of the hydrogen in the drift layer is 5.0×1016/cm3 or less.
(Supplementary Description 4)
There is provided the nitride semiconductor substrate of any one of the supplementary descriptions 1 to 3, wherein the substrate is configured as a monocrystalline gallium nitride substrate.
(Supplementary Description 5)
There is provided the nitride semiconductor substrate of the supplementary description 4, wherein a dislocation density on a main surface of the substrate is 1×107/cm2 or less.
(Supplementary Description 6)
There is provided the nitride semiconductor substrate of any one of the supplementary descriptions 1 to 5, wherein the concentration of the donors in the drift layer is gradually decreased from the substrate side toward the surface side of the drift layer, and
There is provided the nitride semiconductor substrate of any one of the supplementary descriptions 1 to 5, wherein the concentration of the donors in the drift layer is gradually decreased from the substrate side toward the surface side of the drift layer, and
There is provided the nitride semiconductor substrate of any one of the supplementary descriptions 1 to 5, wherein the concentration of the donors in the drift layer is gradually decreased from the substrate side toward the surface side of the drift layer, and the total concentration of the carbons in the drift layer is gradually decreased from the substrate side toward the surface side of the drift layer.
(Supplementary Description 9)
There is provided the nitride semiconductor substrate of any one of the supplementary descriptions 1 to 8, wherein the difference obtained by subtracting the concentration of the carbons that function as acceptors in the drift layer from the concentration of the donors in the drift layer, is linearly decreased from the substrate side toward the surface side of the drift layer.
(Supplementary Description 10)
There is provided the nitride semiconductor substrate of any one of the supplementary descriptions 1 to 8, wherein the difference obtained by subtracting the concentration of the carbons that function as acceptors in the drift layer from the concentration of the donors in the drift layer, is decreased stepwise from the substrate side toward the surface side of the drift layer.
(Supplementary Description 11)
There is provided the nitride semiconductor substrate of any one of the supplementary descriptions 1 to 8, wherein the difference obtained by subtracting the concentration of the carbons that function as acceptors in the drift layer from the concentration of the donors in the drift layer, is nonlinearly decreased from the substrate side toward the surface side of the drift layer.
(Supplementary Description 12)
There is provided a semiconductor device, including:
There is provided a method for manufacturing a nitride semiconductor substrate, including:
There is provided a method for manufacturing a semiconductor device, including:
10, 12 Nitride semiconductor substrate
20, 22 Semiconductor device
100, 102 Substrate
140, 142 Drift layer
| Number | Date | Country | Kind |
|---|---|---|---|
| 2016-070543 | Mar 2016 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2017/004983 | 2/10/2017 | WO | 00 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2017/169176 | 10/5/2017 | WO | A |
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| 20180350965 | Shibata | Dec 2018 | A1 |
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| Entry |
|---|
| International Search Report for PCT/JP2017/004983 (dated Mar. 21, 2017). |
| Written Opinion for PCT/JP2017/004983 (dated Mar. 21, 2017). |
| Notification of Reasons for Refusal issued in corresponding Japanese Patent Application No. JP2016-070543 drafted on May 28, 2018, and an English translation dated Aug. 31, 2018. |
| English Translation of the International Preliminary Report on Patentability and Written Opinion for PCT/JP2017/004983 (dated Oct. 11, 2018). |
| Number | Date | Country | |
|---|---|---|---|
| 20200127101 A1 | Apr 2020 | US |
