Preparation of Underlying substrate Referring to Fig. 1(A), initially, GaN underlying substrate 11 having a diameter 25 of 60 mm and a thickness of 400 m was prepared. Main surface 1 1 m of underlying substrate 11 was flat and the crystal plane closest to main surface 1 1 m was the (0001) plane. The radius of curvature of the (0001) plane was 20 m as a result of measurement with X-ray diffraction. Average density of dislocation penetrating main-18-11U.500-UJ: 91UOU₁ WUU 1 surface 1 1 m of underlying substrate 11 (referred to as average dislocation density; to be understood similarly hereinafter) was 5 x 1 05 cm- 3 as a result of calculation based on measurement of dark spot density with CL (cathode luminescence).

Growth of GaN Crystal 5 Referring next to Fig. 1(A), with the H V PE method, seven GaN crystals 12 different in Si concentration were grown on underlying substrate 11 by using the SiF₄ gas as a doping gas (Experiments Nos. 1 to 7). The HVPE apparatus shown in Fig. 2 was used for growing these GaN crystals. The N H 3 gas was prepared as first source gas G 1, the HCl gas was prepared as second 10 source gas G3, the SiF₄ gas was prepared as doping gas G2, and the H 2 gas having purity of 99.999 % or higher was prepared as the carrier gas. The carrier gas was introduced into reaction tube 110 through each of first gas introduction pipe 104, second gas introduction pipe 106 and doping gas introduction pipe 105, and the temperature of heater 109 was raised to 1100 0 C. Thereafter, metal Ga was supplied 15 to source boat 107 and source boat 107 was heated. The HCl gas supplied through second gas introduction pipe 106 and Ga in source boat 107 were caused to react as Ga + HC l -+ GaC i + 1/2 H 2, to thereby generate the GaC I gas as reaction gas G7. Then, the NH₃ gas serving as first source gas G 1 supplied through first gas 20 introduction pipe 104 and the GaC i gas which is reaction gas G₇ obtained in the reaction above were fed together with the carrier gas so as to impinge the main surface of underlying substrate 11 on which the GaN crystal is to be grown, and reaction was caused on that main surface as follows: GaC l + N H 3-> GaN + HCl + H2. As a condition for growing seven GaN crystals 12 different in Si concentration 25 (Experiments Nos. 1 to 7), a supply partial pressure of the doping gas was adjusted to values shown in Table 1. Thus, seven GaN crystals each having a diameter of 60 mm and a thickness of 5 mm but different in Si concentration were grown at a crystal growth temperature of 1100 0 C in a crystal growth period of 16.67 hours. A rate of growth of-19-1 10366-US: 910601WUU₁ these GaN crystals was 300 pm/hr.

Formation of GaN Substrate Referring next to Fig. 1(B), each obtained GaN crystal 12 was sliced in a direction of thickness with the use of a slicer. Then, an outer peripheral region of sliced GaN crystal 12 was removed. Then, the sliced GaN crystal of which outer peripheral region had been removed was subjected to CMP (chemical-mechanical polishing), to thereby remove its affected layer. Thus, five GaN substrates 10 each having a diameter of 2 inches (50.8 mm) and a thickness of 400 m were obtained from each GaN crystal. 10 4. Measurement of Physical Property of GaN Substrate At five measurement points in a GaN substrate 1 0 c third from the side of underlying substrate 11 among five obtained GaN substrates 1 0 a, 1O b, 1O c, 1 O d, and 10 e, Si concentration, carrier concentration, specific resistance, and absorption coefficient for light having wavelengths of 380 nm, 500 nm to 780 nm, 440 nm to 780 15 nm, and 1500 nm were measured, and minimum values and maximum values thereof were calculated. Here, the five measurement points were set as five points in total in one GaN substrate, that is, a central point on the main surface, points distant by -2 cm and + 2 cm from the central point in a [11-20] direction respectively, and points distant by -2 cm and + 2 cm from the central point in a [10-10] direction respectively. Here, 20 Si concentration was measured with SIMS (secondary ion mass spectrometry). Carrier concentration was measured with the CV characteristics measurement method. Specific resistance was measured with the four-probe method by using a specific resistance meter. The light absorption coefficient was calculated by measuring transmittance and reflectance by using a spectrophotometer. Here, assuming that the 25 absorption coefficient within the GaN substrate is constant regardless of a depth, multiple reflection was also taken into account. Table 1 summarizes the results. Here, fluctuation in carrier concentration within the main surface of the GaN substrate was within ± 5 % from an average value, and fluctuation in carrier-20-110366-US: 910601 WO01 concentration in a direction of thickness was also within ± 5 % from the average value. In addition, average density of dislocation penetrating the main surface at the five measurement points above in third GaN substrate 1 0 c of each GaN crystal 12 (average dislocation density) was each 5 x 1 0' cm⁻², and it was as low as in underlying 5 substrate 11. In addition, the radius of curvature of the (0001) plane at the five measurement points above in third GaN substrate 1 0 c of each GaN crystal 12 was 20 m as a result of measurement with X-ray diffraction, and it was as great as in the underlying substrate. Moreover, the obtained GaN substrate had no crack. Table 1 Experiment No. 1 2 3 4 5 6 7 Partial Pressure of Doping Gas (x 10-6 atm) 2.0 1.5 1.0 0.8 0.6 0.4 0.3 Si Concentration (x 101 8 cm3) 2.2 1.6 1.1 0.85 0.69 0.45 0.36 Carrier Concentration (x 10 18 cm) 2.1 1.5 1.1 0.82 0.66 0.42 0.35 Specific Resistance (n cm) 0.095 0.011 0.014 0.017 0.02 0.031 0.035 380 nm 48 33 24 19 15 11 9 Light 500-780 Absorption n 11-15 7-10 5-7 4-5 4-5 4 3 Coefficient 440-780 (cm1) nm 11- 22 7-15 5-10 4-6 4-5 4 3 -4 1500 nm 29 16 12 10 8 6 10 Referring to Table 1, in the H V PE method, by adjusting the crystal growth temperature to a temperature not lower than 1000 ° C and not higher than 1200 ° C and the partial pressure of the doping gas containing Si to a partial pressure not lower than 0.6 x 10 ~6 atm and not higher than 1.0 x 10-6 atm and setting the carrier concentration 15 not lower than 0.66 x 10 18 cm- 3 and not higher than 1.1 x 101 8 cm3, the GaN substrate having specific resistance not higher than 0.02 Q cm, an absorption coefficient not lower-21-110366-US: 910601 WO01 than 7 cm ' for light having a wavelength of 380 nm, an absorption coefficient lower than 7 c m' for light having a wavelength from 500 nm to 780 nm, and an absorption coefficient not lower than 7 c m- 1 for light having a wavelength of 1500 nm was obtained. In addition, by adjusting the crystal growth temperature to a temperature not lower than 5 1000 0 C and not higher than 1200 ° C and the partial pressure of the doping gas containing Si to a partial pressure not lower than 0.6 x 10-6 atm and not higher than 0.8 x 10~ 6 atm and setting the carrier concentration not lower than 0.66 x 1018 cm- 3 and not higher than 0.82 x 1018 c m3, the GaN substrate having specific resistance not higher than 0.02 O₂ cm, an absorption coefficient not lower than 7 cm -1 for light having a 10 wavelength of 380 nm, an absorption coefficient lower than 7 cnf' for light having a wavelength from 440 nm to 780 nm, and an absorption coefficient not lower than 7 cm ' for light having a wavelength of 1500 nm was obtained. Regarding concentration of an impurity element other than Si in all grown crystals, concentration of O (oxygen) was not higher than 5 x 10 16 cm 3, concentration of C (carbon) was not higher than 5 x 10 16 15 c m, and concentration of other elements was again not higher than 1 x 10 16 cm, as a result of measurement with SIMS. In the examples above, a GaN substrate in which a crystal plane closest to a main surface is the (0001) plane was fabricated, however, similar results were obtained also when a GaN substrate in which a crystal plane closest to the main surface is the (10-10) 20 plane, the (11-20) plane, the (10-11) plane, the (11-22) plane, the (20-21) plane, or the (22-44) plane was fabricated. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being interpreted by the 25 terms of the appended claims. Claims What is claimed is:

366-US: 910601 WO01 WHAT IS CLAIMED IS:

Preparation of Underlying substrate Referring to Fig. 1(A), initially, GaN underlying substrate 11 having a diameter 25 of 60 mm and a thickness of 400 m was prepared. Main surface 1 1 m of underlying substrate 11 was flat and the crystal plane closest to main surface 1 1 m was the (0001) plane. The radius of curvature of the (0001) plane was 20 m as a result of measurement with X-ray diffraction. Average density of dislocation penetrating main-18-11U.500-UJ: 91UOU₁ WUU 1 surface 1 1 m of underlying substrate 11 (referred to as average dislocation density; to be understood similarly hereinafter) was 5 x 1 05 cm- 3 as a result of calculation based on measurement of dark spot density with CL (cathode luminescence).

Growth of GaN Crystal 5 Referring next to Fig. 1(A), with the H V PE method, seven GaN crystals 12 different in Si concentration were grown on underlying substrate 11 by using the SiF₄ gas as a doping gas (Experiments Nos. 1 to 7). The HVPE apparatus shown in Fig. 2 was used for growing these GaN crystals. The N H 3 gas was prepared as first source gas G 1, the HCl gas was prepared as second 10 source gas G3, the SiF₄ gas was prepared as doping gas G2, and the H 2 gas having purity of 99.999 % or higher was prepared as the carrier gas. The carrier gas was introduced into reaction tube 110 through each of first gas introduction pipe 104, second gas introduction pipe 106 and doping gas introduction pipe 105, and the temperature of heater 109 was raised to 1100 0 C. Thereafter, metal Ga was supplied 15 to source boat 107 and source boat 107 was heated. The HCl gas supplied through second gas introduction pipe 106 and Ga in source boat 107 were caused to react as Ga + HC l -+ GaC i + 1/2 H 2, to thereby generate the GaC I gas as reaction gas G7. Then, the NH₃ gas serving as first source gas G 1 supplied through first gas 20 introduction pipe 104 and the GaC i gas which is reaction gas G₇ obtained in the reaction above were fed together with the carrier gas so as to impinge the main surface of underlying substrate 11 on which the GaN crystal is to be grown, and reaction was caused on that main surface as follows: GaC l + N H 3-> GaN + HCl + H2. As a condition for growing seven GaN crystals 12 different in Si concentration 25 (Experiments Nos. 1 to 7), a supply partial pressure of the doping gas was adjusted to values shown in Table 1. Thus, seven GaN crystals each having a diameter of 60 mm and a thickness of 5 mm but different in Si concentration were grown at a crystal growth temperature of 1100 0 C in a crystal growth period of 16.67 hours. A rate of growth of-19-1 10366-US: 910601WUU₁ these GaN crystals was 300 pm/hr.

Formation of GaN Substrate Referring next to Fig. 1(B), each obtained GaN crystal 12 was sliced in a direction of thickness with the use of a slicer. Then, an outer peripheral region of sliced GaN crystal 12 was removed. Then, the sliced GaN crystal of which outer peripheral region had been removed was subjected to CMP (chemical-mechanical polishing), to thereby remove its affected layer. Thus, five GaN substrates 10 each having a diameter of 2 inches (50.8 mm) and a thickness of 400 m were obtained from each GaN crystal. 10 4. Measurement of Physical Property of GaN Substrate At five measurement points in a GaN substrate 1 0 c third from the side of underlying substrate 11 among five obtained GaN substrates 1 0 a, 1O b, 1O c, 1 O d, and 10 e, Si concentration, carrier concentration, specific resistance, and absorption coefficient for light having wavelengths of 380 nm, 500 nm to 780 nm, 440 nm to 780 15 nm, and 1500 nm were measured, and minimum values and maximum values thereof were calculated. Here, the five measurement points were set as five points in total in one GaN substrate, that is, a central point on the main surface, points distant by -2 cm and + 2 cm from the central point in a [11-20] direction respectively, and points distant by -2 cm and + 2 cm from the central point in a [10-10] direction respectively. Here, 20 Si concentration was measured with SIMS (secondary ion mass spectrometry). Carrier concentration was measured with the CV characteristics measurement method. Specific resistance was measured with the four-probe method by using a specific resistance meter. The light absorption coefficient was calculated by measuring transmittance and reflectance by using a spectrophotometer. Here, assuming that the 25 absorption coefficient within the GaN substrate is constant regardless of a depth, multiple reflection was also taken into account. Table 1 summarizes the results. Here, fluctuation in carrier concentration within the main surface of the GaN substrate was within ± 5 % from an average value, and fluctuation in carrier-20-110366-US: 910601 WO01 concentration in a direction of thickness was also within ± 5 % from the average value. In addition, average density of dislocation penetrating the main surface at the five measurement points above in third GaN substrate 1 0 c of each GaN crystal 12 (average dislocation density) was each 5 x 1 0' cm⁻², and it was as low as in underlying 5 substrate 11. In addition, the radius of curvature of the (0001) plane at the five measurement points above in third GaN substrate 1 0 c of each GaN crystal 12 was 20 m as a result of measurement with X-ray diffraction, and it was as great as in the underlying substrate. Moreover, the obtained GaN substrate had no crack. Table 1 Experiment No. 1 2 3 4 5 6 7 Partial Pressure of Doping Gas (x 10-6 atm) 2.0 1.5 1.0 0.8 0.6 0.4 0.3 Si Concentration (x 101 8 cm3) 2.2 1.6 1.1 0.85 0.69 0.45 0.36 Carrier Concentration (x 10 18 cm) 2.1 1.5 1.1 0.82 0.66 0.42 0.35 Specific Resistance (n cm) 0.095 0.011 0.014 0.017 0.02 0.031 0.035 380 nm 48 33 24 19 15 11 9 Light 500-780 Absorption n 11-15 7-10 5-7 4-5 4-5 4 3 Coefficient 440-780 (cm1) nm 11- 22 7-15 5-10 4-6 4-5 4 3 -4 1500 nm 29 16 12 10 8 6 10 Referring to Table 1, in the H V PE method, by adjusting the crystal growth temperature to a temperature not lower than 1000 ° C and not higher than 1200 ° C and the partial pressure of the doping gas containing Si to a partial pressure not lower than 0.6 x 10 ~6 atm and not higher than 1.0 x 10-6 atm and setting the carrier concentration 15 not lower than 0.66 x 10 18 cm- 3 and not higher than 1.1 x 101 8 cm3, the GaN substrate having specific resistance not higher than 0.02 Q cm, an absorption coefficient not lower-21-110366-US: 910601 WO01 than 7 cm ' for light having a wavelength of 380 nm, an absorption coefficient lower than 7 c m' for light having a wavelength from 500 nm to 780 nm, and an absorption coefficient not lower than 7 c m- 1 for light having a wavelength of 1500 nm was obtained. In addition, by adjusting the crystal growth temperature to a temperature not lower than 5 1000 0 C and not higher than 1200 ° C and the partial pressure of the doping gas containing Si to a partial pressure not lower than 0.6 x 10-6 atm and not higher than 0.8 x 10~ 6 atm and setting the carrier concentration not lower than 0.66 x 1018 cm- 3 and not higher than 0.82 x 1018 c m3, the GaN substrate having specific resistance not higher than 0.02 O₂ cm, an absorption coefficient not lower than 7 cm -1 for light having a 10 wavelength of 380 nm, an absorption coefficient lower than 7 cnf' for light having a wavelength from 440 nm to 780 nm, and an absorption coefficient not lower than 7 cm ' for light having a wavelength of 1500 nm was obtained. Regarding concentration of an impurity element other than Si in all grown crystals, concentration of O (oxygen) was not higher than 5 x 10 16 cm 3, concentration of C (carbon) was not higher than 5 x 10 16 15 c m, and concentration of other elements was again not higher than 1 x 10 16 cm, as a result of measurement with SIMS. In the examples above, a GaN substrate in which a crystal plane closest to a main surface is the (0001) plane was fabricated, however, similar results were obtained also when a GaN substrate in which a crystal plane closest to the main surface is the (10-10) 20 plane, the (11-20) plane, the (10-11) plane, the (11-22) plane, the (20-21) plane, or the (22-44) plane was fabricated. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being interpreted by the 25 terms of the appended claims. Claims What is claimed is:

366-US: 910601 WO01 WHAT IS CLAIMED IS:

Preparation of Underlying substrate Referring to Fig. 1(A), initially, GaN underlying substrate 11 having a diameter 25 of 60 mm and a thickness of 400 m was prepared. Main surface 1 1 m of underlying substrate 11 was flat and the crystal plane closest to main surface 1 1 m was the (0001) plane. The radius of curvature of the (0001) plane was 20 m as a result of measurement with X-ray diffraction. Average density of dislocation penetrating main-18-11U.500-UJ: 91UOU₁ WUU 1 surface 1 1 m of underlying substrate 11 (referred to as average dislocation density; to be understood similarly hereinafter) was 5 x 1 05 cm- 3 as a result of calculation based on measurement of dark spot density with CL (cathode luminescence).

Growth of GaN Crystal 5 Referring next to Fig. 1(A), with the H V PE method, seven GaN crystals 12 different in Si concentration were grown on underlying substrate 11 by using the SiF₄ gas as a doping gas (Experiments Nos. 1 to 7). The HVPE apparatus shown in Fig. 2 was used for growing these GaN crystals. The N H 3 gas was prepared as first source gas G 1, the HCl gas was prepared as second 10 source gas G3, the SiF₄ gas was prepared as doping gas G2, and the H 2 gas having purity of 99.999 % or higher was prepared as the carrier gas. The carrier gas was introduced into reaction tube 110 through each of first gas introduction pipe 104, second gas introduction pipe 106 and doping gas introduction pipe 105, and the temperature of heater 109 was raised to 1100 0 C. Thereafter, metal Ga was supplied 15 to source boat 107 and source boat 107 was heated. The HCl gas supplied through second gas introduction pipe 106 and Ga in source boat 107 were caused to react as Ga + HC l -+ GaC i + 1/2 H 2, to thereby generate the GaC I gas as reaction gas G7. Then, the NH₃ gas serving as first source gas G 1 supplied through first gas 20 introduction pipe 104 and the GaC i gas which is reaction gas G₇ obtained in the reaction above were fed together with the carrier gas so as to impinge the main surface of underlying substrate 11 on which the GaN crystal is to be grown, and reaction was caused on that main surface as follows: GaC l + N H 3-> GaN + HCl + H2. As a condition for growing seven GaN crystals 12 different in Si concentration 25 (Experiments Nos. 1 to 7), a supply partial pressure of the doping gas was adjusted to values shown in Table 1. Thus, seven GaN crystals each having a diameter of 60 mm and a thickness of 5 mm but different in Si concentration were grown at a crystal growth temperature of 1100 0 C in a crystal growth period of 16.67 hours. A rate of growth of-19-1 10366-US: 910601WUU₁ these GaN crystals was 300 pm/hr.

Formation of GaN Substrate Referring next to Fig. 1(B), each obtained GaN crystal 12 was sliced in a direction of thickness with the use of a slicer. Then, an outer peripheral region of sliced GaN crystal 12 was removed. Then, the sliced GaN crystal of which outer peripheral region had been removed was subjected to CMP (chemical-mechanical polishing), to thereby remove its affected layer. Thus, five GaN substrates 10 each having a diameter of 2 inches (50.8 mm) and a thickness of 400 m were obtained from each GaN crystal. 10 4. Measurement of Physical Property of GaN Substrate At five measurement points in a GaN substrate 1 0 c third from the side of underlying substrate 11 among five obtained GaN substrates 1 0 a, 1O b, 1O c, 1 O d, and 10 e, Si concentration, carrier concentration, specific resistance, and absorption coefficient for light having wavelengths of 380 nm, 500 nm to 780 nm, 440 nm to 780 15 nm, and 1500 nm were measured, and minimum values and maximum values thereof were calculated. Here, the five measurement points were set as five points in total in one GaN substrate, that is, a central point on the main surface, points distant by -2 cm and + 2 cm from the central point in a [11-20] direction respectively, and points distant by -2 cm and + 2 cm from the central point in a [10-10] direction respectively. Here, 20 Si concentration was measured with SIMS (secondary ion mass spectrometry). Carrier concentration was measured with the CV characteristics measurement method. Specific resistance was measured with the four-probe method by using a specific resistance meter. The light absorption coefficient was calculated by measuring transmittance and reflectance by using a spectrophotometer. Here, assuming that the 25 absorption coefficient within the GaN substrate is constant regardless of a depth, multiple reflection was also taken into account. Table 1 summarizes the results. Here, fluctuation in carrier concentration within the main surface of the GaN substrate was within ± 5 % from an average value, and fluctuation in carrier-20-110366-US: 910601 WO01 concentration in a direction of thickness was also within ± 5 % from the average value. In addition, average density of dislocation penetrating the main surface at the five measurement points above in third GaN substrate 1 0 c of each GaN crystal 12 (average dislocation density) was each 5 x 1 0' cm⁻², and it was as low as in underlying 5 substrate 11. In addition, the radius of curvature of the (0001) plane at the five measurement points above in third GaN substrate 1 0 c of each GaN crystal 12 was 20 m as a result of measurement with X-ray diffraction, and it was as great as in the underlying substrate. Moreover, the obtained GaN substrate had no crack. Table 1 Experiment No. 1 2 3 4 5 6 7 Partial Pressure of Doping Gas (x 10-6 atm) 2.0 1.5 1.0 0.8 0.6 0.4 0.3 Si Concentration (x 101 8 cm3) 2.2 1.6 1.1 0.85 0.69 0.45 0.36 Carrier Concentration (x 10 18 cm) 2.1 1.5 1.1 0.82 0.66 0.42 0.35 Specific Resistance (n cm) 0.095 0.011 0.014 0.017 0.02 0.031 0.035 380 nm 48 33 24 19 15 11 9 Light 500-780 Absorption n 11-15 7-10 5-7 4-5 4-5 4 3 Coefficient 440-780 (cm1) nm 11- 22 7-15 5-10 4-6 4-5 4 3 -4 1500 nm 29 16 12 10 8 6 10 Referring to Table 1, in the H V PE method, by adjusting the crystal growth temperature to a temperature not lower than 1000 ° C and not higher than 1200 ° C and the partial pressure of the doping gas containing Si to a partial pressure not lower than 0.6 x 10 ~6 atm and not higher than 1.0 x 10-6 atm and setting the carrier concentration 15 not lower than 0.66 x 10 18 cm- 3 and not higher than 1.1 x 101 8 cm3, the GaN substrate having specific resistance not higher than 0.02 Q cm, an absorption coefficient not lower-21-110366-US: 910601 WO01 than 7 cm ' for light having a wavelength of 380 nm, an absorption coefficient lower than 7 c m' for light having a wavelength from 500 nm to 780 nm, and an absorption coefficient not lower than 7 c m- 1 for light having a wavelength of 1500 nm was obtained. In addition, by adjusting the crystal growth temperature to a temperature not lower than 5 1000 0 C and not higher than 1200 ° C and the partial pressure of the doping gas containing Si to a partial pressure not lower than 0.6 x 10-6 atm and not higher than 0.8 x 10~ 6 atm and setting the carrier concentration not lower than 0.66 x 1018 cm- 3 and not higher than 0.82 x 1018 c m3, the GaN substrate having specific resistance not higher than 0.02 O₂ cm, an absorption coefficient not lower than 7 cm -1 for light having a 10 wavelength of 380 nm, an absorption coefficient lower than 7 cnf' for light having a wavelength from 440 nm to 780 nm, and an absorption coefficient not lower than 7 cm ' for light having a wavelength of 1500 nm was obtained. Regarding concentration of an impurity element other than Si in all grown crystals, concentration of O (oxygen) was not higher than 5 x 10 16 cm 3, concentration of C (carbon) was not higher than 5 x 10 16 15 c m, and concentration of other elements was again not higher than 1 x 10 16 cm, as a result of measurement with SIMS. In the examples above, a GaN substrate in which a crystal plane closest to a main surface is the (0001) plane was fabricated, however, similar results were obtained also when a GaN substrate in which a crystal plane closest to the main surface is the (10-10) 20 plane, the (11-20) plane, the (10-11) plane, the (11-22) plane, the (20-21) plane, or the (22-44) plane was fabricated. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being interpreted by the 25 terms of the appended claims. Claims What is claimed is:

366-US: 910601 WO01 WHAT IS CLAIMED IS:

Preparation of Underlying substrate Referring to Fig. 1(A), initially, GaN underlying substrate 11 having a diameter 25 of 60 mm and a thickness of 400 m was prepared. Main surface 1 1 m of underlying substrate 11 was flat and the crystal plane closest to main surface 1 1 m was the (0001) plane. The radius of curvature of the (0001) plane was 20 m as a result of measurement with X-ray diffraction. Average density of dislocation penetrating main-18-11U.500-UJ: 91UOU₁ WUU 1 surface 1 1 m of underlying substrate 11 (referred to as average dislocation density; to be understood similarly hereinafter) was 5 x 1 05 cm- 3 as a result of calculation based on measurement of dark spot density with CL (cathode luminescence).

Growth of GaN Crystal 5 Referring next to Fig. 1(A), with the H V PE method, seven GaN crystals 12 different in Si concentration were grown on underlying substrate 11 by using the SiF₄ gas as a doping gas (Experiments Nos. 1 to 7). The HVPE apparatus shown in Fig. 2 was used for growing these GaN crystals. The N H 3 gas was prepared as first source gas G 1, the HCl gas was prepared as second 10 source gas G3, the SiF₄ gas was prepared as doping gas G2, and the H 2 gas having purity of 99.999 % or higher was prepared as the carrier gas. The carrier gas was introduced into reaction tube 110 through each of first gas introduction pipe 104, second gas introduction pipe 106 and doping gas introduction pipe 105, and the temperature of heater 109 was raised to 1100 0 C. Thereafter, metal Ga was supplied 15 to source boat 107 and source boat 107 was heated. The HCl gas supplied through second gas introduction pipe 106 and Ga in source boat 107 were caused to react as Ga + HC l -+ GaC i + 1/2 H 2, to thereby generate the GaC I gas as reaction gas G7. Then, the NH₃ gas serving as first source gas G 1 supplied through first gas 20 introduction pipe 104 and the GaC i gas which is reaction gas G₇ obtained in the reaction above were fed together with the carrier gas so as to impinge the main surface of underlying substrate 11 on which the GaN crystal is to be grown, and reaction was caused on that main surface as follows: GaC l + N H 3-> GaN + HCl + H2. As a condition for growing seven GaN crystals 12 different in Si concentration 25 (Experiments Nos. 1 to 7), a supply partial pressure of the doping gas was adjusted to values shown in Table 1. Thus, seven GaN crystals each having a diameter of 60 mm and a thickness of 5 mm but different in Si concentration were grown at a crystal growth temperature of 1100 0 C in a crystal growth period of 16.67 hours. A rate of growth of-19-1 10366-US: 910601WUU₁ these GaN crystals was 300 pm/hr.

Formation of GaN Substrate Referring next to Fig. 1(B), each obtained GaN crystal 12 was sliced in a direction of thickness with the use of a slicer. Then, an outer peripheral region of sliced GaN crystal 12 was removed. Then, the sliced GaN crystal of which outer peripheral region had been removed was subjected to CMP (chemical-mechanical polishing), to thereby remove its affected layer. Thus, five GaN substrates 10 each having a diameter of 2 inches (50.8 mm) and a thickness of 400 m were obtained from each GaN crystal. 10 4. Measurement of Physical Property of GaN Substrate At five measurement points in a GaN substrate 1 0 c third from the side of underlying substrate 11 among five obtained GaN substrates 1 0 a, 1O b, 1O c, 1 O d, and 10 e, Si concentration, carrier concentration, specific resistance, and absorption coefficient for light having wavelengths of 380 nm, 500 nm to 780 nm, 440 nm to 780 15 nm, and 1500 nm were measured, and minimum values and maximum values thereof were calculated. Here, the five measurement points were set as five points in total in one GaN substrate, that is, a central point on the main surface, points distant by -2 cm and + 2 cm from the central point in a [11-20] direction respectively, and points distant by -2 cm and + 2 cm from the central point in a [10-10] direction respectively. Here, 20 Si concentration was measured with SIMS (secondary ion mass spectrometry). Carrier concentration was measured with the CV characteristics measurement method. Specific resistance was measured with the four-probe method by using a specific resistance meter. The light absorption coefficient was calculated by measuring transmittance and reflectance by using a spectrophotometer. Here, assuming that the 25 absorption coefficient within the GaN substrate is constant regardless of a depth, multiple reflection was also taken into account. Table 1 summarizes the results. Here, fluctuation in carrier concentration within the main surface of the GaN substrate was within ± 5 % from an average value, and fluctuation in carrier-20-110366-US: 910601 WO01 concentration in a direction of thickness was also within ± 5 % from the average value. In addition, average density of dislocation penetrating the main surface at the five measurement points above in third GaN substrate 1 0 c of each GaN crystal 12 (average dislocation density) was each 5 x 1 0' cm⁻², and it was as low as in underlying 5 substrate 11. In addition, the radius of curvature of the (0001) plane at the five measurement points above in third GaN substrate 1 0 c of each GaN crystal 12 was 20 m as a result of measurement with X-ray diffraction, and it was as great as in the underlying substrate. Moreover, the obtained GaN substrate had no crack. Table 1 Experiment No. 1 2 3 4 5 6 7 Partial Pressure of Doping Gas (x 10-6 atm) 2.0 1.5 1.0 0.8 0.6 0.4 0.3 Si Concentration (x 101 8 cm3) 2.2 1.6 1.1 0.85 0.69 0.45 0.36 Carrier Concentration (x 10 18 cm) 2.1 1.5 1.1 0.82 0.66 0.42 0.35 Specific Resistance (n cm) 0.095 0.011 0.014 0.017 0.02 0.031 0.035 380 nm 48 33 24 19 15 11 9 Light 500-780 Absorption n 11-15 7-10 5-7 4-5 4-5 4 3 Coefficient 440-780 (cm1) nm 11- 22 7-15 5-10 4-6 4-5 4 3 -4 1500 nm 29 16 12 10 8 6 10 Referring to Table 1, in the H V PE method, by adjusting the crystal growth temperature to a temperature not lower than 1000 ° C and not higher than 1200 ° C and the partial pressure of the doping gas containing Si to a partial pressure not lower than 0.6 x 10 ~6 atm and not higher than 1.0 x 10-6 atm and setting the carrier concentration 15 not lower than 0.66 x 10 18 cm- 3 and not higher than 1.1 x 101 8 cm3, the GaN substrate having specific resistance not higher than 0.02 Q cm, an absorption coefficient not lower-21-110366-US: 910601 WO01 than 7 cm ' for light having a wavelength of 380 nm, an absorption coefficient lower than 7 c m' for light having a wavelength from 500 nm to 780 nm, and an absorption coefficient not lower than 7 c m- 1 for light having a wavelength of 1500 nm was obtained. In addition, by adjusting the crystal growth temperature to a temperature not lower than 5 1000 0 C and not higher than 1200 ° C and the partial pressure of the doping gas containing Si to a partial pressure not lower than 0.6 x 10-6 atm and not higher than 0.8 x 10~ 6 atm and setting the carrier concentration not lower than 0.66 x 1018 cm- 3 and not higher than 0.82 x 1018 c m3, the GaN substrate having specific resistance not higher than 0.02 O₂ cm, an absorption coefficient not lower than 7 cm -1 for light having a 10 wavelength of 380 nm, an absorption coefficient lower than 7 cnf' for light having a wavelength from 440 nm to 780 nm, and an absorption coefficient not lower than 7 cm ' for light having a wavelength of 1500 nm was obtained. Regarding concentration of an impurity element other than Si in all grown crystals, concentration of O (oxygen) was not higher than 5 x 10 16 cm 3, concentration of C (carbon) was not higher than 5 x 10 16 15 c m, and concentration of other elements was again not higher than 1 x 10 16 cm, as a result of measurement with SIMS. In the examples above, a GaN substrate in which a crystal plane closest to a main surface is the (0001) plane was fabricated, however, similar results were obtained also when a GaN substrate in which a crystal plane closest to the main surface is the (10-10) 20 plane, the (11-20) plane, the (10-11) plane, the (11-22) plane, the (20-21) plane, or the (22-44) plane was fabricated. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being interpreted by the 25 terms of the appended claims. Claims What is claimed is:

366-US: 910601 WO01 WHAT IS CLAIMED IS: