Silicon carbide epitaxial substrate and silicon carbide semiconductor device

2:

• The silicon carbide substrate according to claim 1, wherein the third impurity concentration is 2×1019 cm−3 or less.

3:

• The silicon carbide substrate according to claim 1, wherein the third impurity concentration is 5×1018 cm−3 or more.

4:

• The silicon carbide substrate according to claim 1, wherein the second impurity concentration is 5×1016 cm−3 or more and 1×1019 cm−3 or less.

5:

• The silicon carbide substrate according to claim 1, wherein the fourth impurity concentration is 1×1014 cm−3 or more and 5×1016 cm−3 or less.

6:

• The silicon carbide substrate according to claim 1, wherein an impurity concentration between the silicon carbide single-crystal substrate and the first silicon carbide layer transitions monotonically from said first impurity concentration to said second impurity concentration.

8:

• The silicon carbide substrate according to claim 7, wherein the silicon carbide single-crystal substrate, the first silicon carbide layer, the second silicon carbide layer, and the third silicon carbide layer are directly formed on each other.

9:

• The silicon carbide substrate according to claim 1, wherein a difference between the second impurity concentration and the third impurity concentration and a difference between the third impurity concentration and the fourth impurity concentration are given by a concentration difference of impurity atoms different from protons.

10:

• The silicon carbide substrate according to claim 1, wherein a difference between the second impurity concentration and the third impurity concentration and a difference between the third impurity concentration and the fourth impurity concentration are given by a concentration difference of impurities including atoms of at least one of nitrogen, phosphorus, aluminum and boron.

13:

• The silicon carbide semiconductor device according to claim 12, wherein the first electrode is brought into an ohmic contact with the silicon carbide single-crystal substrate, and the second electrode is brought into an ohmic contact with the fourth silicon carbide layer.

14:

• The silicon carbide semiconductor device according to claim 11, comprising an electrode directly connected to the silicon carbide single-crystal substrate.

15:

• The silicon carbide semiconductor device according to claim 11, comprising no intervening layer being present between the silicon carbide single-crystal substrate and the first silicon carbide layer.

16:

• The silicon carbide semiconductor device according to claim 11, wherein the second and fourth impurity concentrations are both lower than the first impurity concentration.

17:

• The silicon carbide substrate according to claim 1, comprising an electrode directly connected to the silicon carbide single-crystal substrate.

18:

• The silicon carbide substrate according to claim 1, comprising no intervening layer being present between the silicon carbide single-crystal substrate and the first silicon carbide layer.

19:

• The silicon carbide substrate according to claim 1, wherein the second and fourth impurity concentrations are both lower than the first impurity concentration.

20:

• The silicon carbide substrate according to claim 1, wherein the second silicon carbide layer has a thickness of 500 nm or more.