JP2009078971A - Nitride semiconductor single crystal substrate and method for synthesizing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To decrease an absorption coefficient of a nitride semiconductor single crystal substrate itself. <P>SOLUTION: The nitride semiconductor single crystal substrate has a composition of AlN, a total impurity density of not more than 1×10<SP>17</SP>cm<SP>-3</SP>, and an absorption coefficient of not more than 50 cm<SP>-1</SP>in the full wavelength range from 350 to 780 nm. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a nitride semiconductor single crystal that can be used as a substrate for various electronic devices, and more particularly to improvement of fracture toughness and translucency of a nitride semiconductor single crystal substrate.

  When a nitride semiconductor single crystal wafer is used as a substrate of a semiconductor electronic device, it is natural that the nitride semiconductor single crystal wafer should not be broken during the process of manufacturing the semiconductor electronic device. This is because if the nitride semiconductor single crystal wafer is cracked during the process, the subsequent process cannot be performed, and the wafer is wasted.

In recent years, various electronic devices have been fabricated using not only silicon single crystal wafers but also nitride semiconductor single crystal wafers. Among the nitride semiconductor single crystal wafers, hexagonal Al _x Ga _1-x N (0 ≦ x ≦ 1) semiconductor wafers can be preferably used for manufacturing various electronic devices. In the present specification, an Al _x Ga _1-x N (0 ≦ x ≦ 1) semiconductor may be abbreviated as an AlGaN-based semiconductor.

  By the way, as described in Jpn. J. Appl. Phys. Vol. 40 (2001) pp. L426-L427 of Non-Patent Document 1, AlGaN-based single crystals have lower fracture toughness than silicon single crystals. It has a tendency to break. In particular, an AlN substrate has a fracture toughness that is about a fraction of that of a SiC substrate or a sapphire substrate, and is easily broken during handling.

  On the other hand, a nitride semiconductor single crystal wafer is often used for manufacturing a light emitting device, and is often used as a substrate of a nitride semiconductor light emitting device capable of emitting light of a short wavelength. In this case, short-wavelength light easily excites electrons in the semiconductor substrate, that is, is easily absorbed in the semiconductor substrate. Thus, if light having a short wavelength is absorbed by the nitride semiconductor substrate, the efficiency of extracting light from the light emitting element to the outside of the element is reduced. Therefore, it is desired that the nitride semiconductor single crystal substrate used for manufacturing the light emitting element has a small absorption coefficient even for light having a short wavelength as much as possible.

  In J. Appl. Phys., Vol. 44, 1973, pp. 292-296 of Non-Patent Document 2, an epitaxial AlN film having a relatively small absorption coefficient from the visible region to the ultraviolet region is HVPE (hydride vapor phase epitaxy). It is reported that it can be nurtured by law. However, it cannot be said that the AlN film in Non-Patent Document 2 also has a sufficiently small absorption coefficient in a short wavelength region (particularly in the ultraviolet region). Therefore, even if the AlN layer is grown thicker by HVPE and used as a nitride semiconductor single crystal substrate, further reduction of the absorption coefficient in the short wavelength region of the AlN substrate is desired.

Jpn.J.Appl.Phys.Vol.40 (2001) pp.L426-L427 J.Appl.Phys., Vol.44,1973, pp.292-296

  As described above, nitride semiconductor single crystal wafers are used as substrates for various electronic devices, and in recent years, the demand for AlGaN-based single crystal substrates is increasing. However, this AlGaN-based single crystal wafer is easily broken, and this can be a factor that reduces the productivity of electronic devices. Therefore, if possible, it is desired to improve the fracture toughness of the AlGaN single crystal wafer itself.

  In addition, a nitride semiconductor single crystal wafer is often used as a substrate for a light emitting element having a short wavelength in recent years. In this case, if the nitride semiconductor substrate absorbs light having a short wavelength, the light extraction efficiency of the short wavelength light emitting element is lowered. Therefore, if possible, it is desirable to reduce the absorption coefficient of the AlGaN single crystal substrate itself.

  In view of this situation, one object of the present invention is to improve the fracture toughness of the AlGaN single crystal substrate itself. Another object of the present invention is to reduce the absorption coefficient of the AlGaN single crystal substrate itself.

Nitride semiconductor single crystal substrate according to the invention has a composition of AlN, and the total impurity concentration 1 × 10 ¹⁷ cm ^-3 or less, and an absorption coefficient in our Keru 50 cm ^-1 or less in the entire wavelength range of 350~780nm It is a feature that.

  The nitride semiconductor single crystal substrate as described above can be preferably synthesized by the HVPE method. In the crystal growth furnace used in this HVPE method, the inner wall of the region where the source gas contacts at a temperature of 800 ° C. or higher is formed of pBN (pyrolytic boron nitride), or any of nitride, carbide, and oxide It is preferable that the sintered body is formed of a member that is surface-coated with any of pBN, nitride, carbide, and oxide.

  According to the present invention, a nitride semiconductor single crystal substrate can have improved fracture toughness. Therefore, in the manufacturing process of a semiconductor electronic device using the substrate, the substrate is not easily damaged, and the productivity of the device is reduced. Can be increased.

  Furthermore, according to the present invention, since the nitride semiconductor single crystal substrate can have improved translucency, the light extraction efficiency of a semiconductor light emitting device using the substrate can be increased.

1 is a single crystal growth apparatus that can be used for synthesizing an AlGaN-based single crystal substrate according to the present invention by HVPE. It is a graph which shows the wavelength dependence of the absorption coefficient in the AlN single crystal substrate by this invention.

  As described above, considering the use of an AlGaN single crystal substrate for the production of various semiconductor electronic devices, it is desired that the substrate is difficult to break. The physical property value that defines the resistance to cracking is the fracture toughness value. Here, the present inventors have found that the fracture toughness value decreases as the impurities of the AlGaN-based single crystal substrate increase, and the substrate easily breaks. That is, it has been found that it is important to reduce the impurity density in order to improve the toughness of the AlGaN single crystal substrate.

Therefore, the inventors have grown the AlN single crystal and the GaN single crystal by removing the impurity source as much as possible. In the growth of the AlN crystal, an AlN single crystal having a diameter of 51 mm having a (0001) principal surface was used as a seed crystal substrate, and AlCl ₃ or AlCl and HN ₃ were used as source gases. On the other hand, in growing a GaN crystal, a GaN single crystal having a diameter of 51 mm having a (0001) principal surface was used as a seed crystal substrate, and GaCl and HN ₃ were used as source gases.

  FIG. 1 is a schematic cross-sectional view illustrating a single crystal growth furnace used in synthesizing an AlN single crystal and a GaN single crystal according to the present invention by HVPE. As shown in this figure, the reaction tube 1 made of quartz glass has an exhaust port 1a, and a heater 2 is disposed on the outer periphery thereof.

  Here, quartz glass can be a source of contamination of Si and O at high temperatures (particularly at high temperatures of 800 ° C. or higher). Further, even if a graphite inner tube is disposed in a region where the temperature in the reaction tube 1 becomes high, it can become a C contamination source at a high temperature.

  Therefore, in the reaction tube 1, the pBN inner tube 3 was arranged in a region where the temperature became 800 ° C. or higher. However, the inner tube 3 is not limited to pBN, and may be formed of a sintered body of nitride, carbide, or oxide (preferably not using a binder), and may be formed of nitride, carbide, or oxide. It may be formed of a coated member.

In the inner tube 3, an AlN or GaN seed crystal substrate 5 was disposed on the pBN base 4. Then, the group 3 source gas (AlCl ₃ , AlCl, or GaCl) was introduced into the inner pipe 3 from the first gas introduction pipe 6, and NH ₃ gas was introduced from the second gas introduction pipe 7.

As the carrier gas, any one of high purity H ₂ , N ₂ and Ar, or a mixed gas thereof was used. The supply ratio of the Group 3 source gas and NH ₃ gas was set within the range of 1:10 to 1: 1000. The temperature of the substrate was set within the range of 900 to 1100 ° C. The synthesis conditions were adjusted so that the growth rate of the AlN or GaN crystal was 10 to 50 μm / h, and an AlN or GaN single crystal having a thickness of 5 mm was grown on the substrate 5. If an Al source gas and a Ga source gas are introduced into the simultaneous inner tube 3, an AlGaN mixed crystal single crystal can be grown.

  From the obtained GaN crystal and AlN crystal, an AlN substrate and a GaN substrate having a main surface of (0001) plane and a diameter of 51 mm and a thickness of 0.5 mm were cut out. Both surfaces of these substrates were mirror-polished and then etched to obtain a double-sided mirror AlN substrate and GaN substrate having a thickness of 0.4 mm.

When the impurity density of these AlN substrate and GaN substrate was measured by SIMS (secondary ion mass spectrometry), the most impurity in any substrate was O, but the density was 5 × 10 ¹⁶ cm ^−3. The total impurity density was 1 × 10 ¹⁷ cm ⁻³ or less.

  Furthermore, fracture toughness values were measured for these AlN substrates and GaN substrates. In this case, the fracture toughness was evaluated by the following equations (1) and (2) based on the length of the crack formed on the substrate when the indenter was pressed by the Vickers hardness measurement method using a diamond square pyramid indenter. .

K _C = ξ (E / H _V ) ^1/2 (P / c ^3/2 ) (1)
H _V = P / (2a ^3/2 ) (2)
Here, K _C is the fracture toughness, H _V is the Vickers hardness, E is Young's modulus, xi] configuration constants (= 0.016), P is the indenter load (0.5 to 5 N), 2a indentations length, C represents the crack length.

As a result of the evaluation by the above formulas (1) and (2), the fracture toughness value of the AlN substrate was 0.5 MPa · m ^1/2 and the fracture toughness value of the GaN substrate was 1.2 MPa · m ^1/2 . .

In addition, the GaN substrate in which impurities such as O and C are mixed at about 1 × 10 ¹⁸ cm ⁻³ without using the inner tube 3 has a fracture toughness value of about 1.0 MPa · m ^1/2, which is highly purified. It has been found that the fracture toughness value is improved. Thus, an AlGaN-based substrate showing a high fracture toughness value was successfully produced.

When the outer peripheral grinding process (with a diameter of 2 inches) of the obtained GaN substrate was performed, the low-purity GaN substrate having a low fracture toughness value was frequently cracked, and the yield was about 20%. On the other hand, in the GaN substrate whose fracture toughness value was increased by high purity, the yield was improved to 80%. Since fracture becomes a larger problem as the substrate becomes larger in diameter, an improvement in fracture toughness value is not so strongly desired for a small substrate of less than about 20 cm ² .

  By the way, among nitride semiconductors, AlN and AlGaN (high Al concentration) have a wide energy band gap and are expected as light emitting materials in the ultraviolet region. More specifically, an attempt has been made to fabricate an ultraviolet light emitting device by forming a pn junction with a similar group III nitride on an AlN or AlGaN substrate. At this time, if ultraviolet rays generated in the light emitting element are absorbed by the substrate, the efficiency of extracting ultraviolet rays from the light emitting element to the outside is lowered.

  Basically, light having energy lower than the band gap of the substrate is transmitted through the substrate, so that it is considered that AlN or AlGaN (Al composition is sufficiently large) may be used. However, it is generally known that AlN and AlGaN absorb light with energy considerably lower than the band gap. The origin of this absorption is not necessarily clear, but can be considered to be due to impurities.

  Also for the high purity AlN substrate obtained by the present invention, the absorption coefficient was measured in order to know its optical characteristics. In this case, the absorption coefficient was calculated by measuring transmittance and reflectance. Further, the absorption coefficient in the substrate was assumed to be constant regardless of the substrate depth, and was calculated in consideration of multiple reflections.

FIG. 2 shows the absorption coefficient of the AlN substrate thus measured. That is, in the graph of FIG. 2, the horizontal axis represents the wavelength of the excitation light, and the range from 300 nm to 800 nm is displayed. On the other hand, the vertical axis represents the absorption coefficient, and the range from 0 cm ⁻¹ to 80 cm ⁻¹ is displayed.

As shown in FIG. 2, in the high-purity AlN substrate according to the present invention, the absorption coefficient starts to increase rapidly as the wavelength decreases in the wavelength region of 350 nm or less, but in the wavelength region of 350 nm or more, the absorption coefficient is 50 cm. ^-1 or less. Here, the absorption coefficient of 50 cm ⁻¹ means that the amount of transmitted light attenuates to 1 / e at a transmission distance of (1/50) cm = 200 μm. And since the typical board | substrate thickness of light emitting elements, such as LED (light emitting diode), is about 200 micrometers, it is preferable that the board | substrate for light emitting elements has an absorption coefficient of 50 cm <^-1> or less.

  In addition, when a wafer for a substrate is cut out from the obtained AlGaN-based single crystal, the main surface of the substrate to be sliced is not only the (0001) plane, but also the (11-20) plane, the (10-12) plane, The (10-10) plane, the (10-11) plane, or a plane inclined in any direction from those planes can be used. The plane orientation of the seed crystal substrate can also be set to an arbitrary plane orientation. However, it is preferable from the viewpoint of productivity to use a seed crystal substrate having the same principal plane orientation as that of the substrate to be cut.

  In the above-described embodiment, a seed crystal substrate having a diameter of 51 mm is used. However, if a seed crystal substrate having a larger diameter is available, it can be used as well. The thickness of the single crystal grown by the HVPE method is not limited to 5 mm in the above example, and it goes without saying that a thicker AlN crystal may be grown.

  Further, the high-purity AlGaN-based substrate obtained by the present invention includes light-emitting elements such as light-emitting diodes and laser diodes; electronic elements such as rectifiers, bipolar transistors, field-effect transistors, and HEMTs (high electron mobility transistors); temperature sensors , Semiconductor sensors such as pressure sensors, radiation sensors, and visible / ultraviolet light detection sensors; and if used for the fabrication of SAW (surface acoustic wave) devices, etc., it is less likely to break during the process and improve production efficiency. it can.

  According to the present invention, since the nitride semiconductor single crystal substrate can have improved fracture toughness, it is possible to increase the productivity by reducing the damage in the manufacturing process of the semiconductor electronic device using the substrate.

  According to the present invention, since the nitride semiconductor single crystal substrate can have improved translucency, it is possible to provide a semiconductor light emitting device with improved light extraction efficiency by using the substrate.

  1 quartz glass reaction tube, 1a exhaust port, 2 heater, 3 pBN inner tube, 4 pBN base, 5 AlGaN seed crystal substrate, 6 first gas introduction tube, 7 second gas introduction tube.

Claims (3)

A nitride semiconductor single crystal substrate having a composition of AlN, a total impurity density of 1 × 10 ¹⁷ cm ⁻³ or less, and an absorption coefficient of 50 cm ⁻¹ or less in a total wavelength range of 350 to 780 nm.   2. The method for synthesizing a nitride semiconductor single crystal substrate according to claim 1, wherein the substrate is synthesized by an HVPE method.   In the crystal growth furnace used for the HVPE method, the inner wall of the region where the source gas contacts at a temperature of 800 ° C. or higher is formed of pBN, or formed of a sintered body of any one of nitride, carbide, and oxide. 3. The method for synthesizing a nitride semiconductor single crystal substrate according to claim 2, wherein the nitride semiconductor single crystal substrate is formed of a member coated with a surface of any one of pBN, nitride, carbide, and oxide.

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