JP5455875B2 - Epitaxial substrate manufacturing method - Google Patents

Description

The present invention relates to a method for manufacturing an epitaxial substrate.

  In recent years, with the development of mobile phones and optical communications, the demand for electronic devices with excellent high frequency characteristics, low power consumption and high output is rapidly increasing. Conventionally, Si devices and GaAs devices have been used for such applications. However, with higher performance of mobile phones and higher speed of optical communication, electronic devices with better high-frequency characteristics and higher output are desired.

For this reason, GaAs-based HEMTs, pseudomorphic HEMTs, GaAs-based HBTs and the like have been put into practical use. Furthermore, as an even higher performance electronic device, InP
Electronic devices such as HEMT and HBT have been actively researched and developed.

  Electronic devices using GaN have recently attracted attention. Since GaN has a large band gap of 3.39 eV, its dielectric breakdown voltage is about an order of magnitude higher than that of Si and GaAs, and its electron saturation drift velocity is higher. Therefore, the figure of merit as an electronic device is superior to Si and GaAs. As high-temperature operation devices, high-power devices, and high-frequency devices, they are promising in fields such as engine control, power conversion, and mobile communication.

In particular, since Non-Patent Document 1 has realized an electronic device having an AlGaN / GaN-based HEMT structure, development has been promoted around the world. These GaN
Conventional electronic devices have been produced by epitaxially growing a predetermined semiconductor layer on a sapphire substrate.

  In the patent document 1, the present applicant discloses a semiconductor element including a substrate, a base layer made of highly crystalline AlN on the substrate, and a conductive layer made of GaN on the base layer. Moreover, it discloses that this semiconductor element is used for a high electron mobility transistor (HEMT element).

  In addition, in the non-patent document 2, the present inventor reports that the direct current characteristics are improved in a high electron mobility transistor including a sapphire substrate / AlN underlayer / GaN layer / AlGaN layer.

JP 2003-45899 A

Khan et al. (Appl. Phys. Lett., 63 (1993), 1214) '' APPLIED PHYSICS LETTERS '' Volume 81, number 6, Aug. 2002 '' Improved dc characteristics of AlGaN / GaN high-electron-mobility transistors on AlN / sapphire templates ''

  In a high electron mobility transistor, the resistance value of the carrier mobility layer needs to be high. However, in Patent Document 1 and Non-Patent Document 2, the specific resistivity of the GaN film is not described. For this reason, there has been a problem in providing a highly efficient high electron mobility transistor suitable for practical use.

  An object of the present invention is to provide an epitaxial substrate that can be effectively used as a semiconductor element.

The present invention
Epitaxied to the underlying layer forming step by MOCVD underlayer made of aluminum nitride on a sapphire substrate; and by metal organic chemical vapor deposition semi-insulating layer made of gallium nitride on the underlying layer, trimethylgallium, ammonia And a semi-insulating layer forming step of epitaxial growth from an atmosphere comprising a carrier gas at an atmospheric pressure of 300 mbar or lower and a temperature of 1000 ° C. or higher;
The dislocation density of aluminum nitride is 10 ¹¹ / cm ² or less, the half width in the X-ray rocking curve on the (002) plane of aluminum nitride is 200 seconds or less, and the specific resistance of gallium nitride is 10 ^The present invention relates to a method for manufacturing an epitaxial substrate, which is ⁴ Ω · cm or more.

In the present invention, a dislocation density of 10 ¹¹ / cm ² or less is used, and an underlayer made of high-quality AlN having a half width of 200 seconds or less in an X-ray rocking curve on the (002) plane is used. On top, a GaN layer is generated. Here, this layer exhibits a high carrier mobility and a high resistivity of 10 ⁴ Ω · cm or more by the manufacturing method of the present invention , for example, a carrier of a high electron mobility transistor that requires a relatively high resistivity. It has been found that it is suitable as a moving layer and has reached the present invention.

It is sectional drawing which shows typically the semiconductor element 20 which concerns on one Embodiment of this invention.

BEST MODE FOR CARRYING OUT THE INVENTION

  Hereinafter, the present invention will be described in detail according to embodiments.

  FIG. 1 is a cross-sectional view schematically showing a high electron mobility transistor (HEMT) 20 using a semiconductor element according to an embodiment of the present invention. The HEMT 20 includes a substrate 1, a base layer 2, a carrier moving layer 3, and a carrier supply layer 4. On the carrier supply layer 4, a source electrode 7, a drain electrode 8, and a gate electrode 9 are formed. The sheet carrier density can be improved by increasing the resistivity of the second nitride semiconductor constituting the carrier transfer layer 3.

In the present invention, the specific resistance of the nitride semiconductor constituting the semi-insulating layer 3 is 10 ⁴ Ω · cm or more, preferably 10 ⁵ Ω · cm or more, more preferably 10 ⁶ Ω · cm or more.

  The substrate 1 is composed of a sapphire single crystal.

  When a sapphire single crystal substrate is used, it is preferable to subject the main surface on which the underlayer 2 is to be formed to surface nitridation. The surface nitriding treatment is performed by placing the substrate in an atmosphere containing nitrogen such as ammonia and heating it for a predetermined time. The thickness of the nitride layer formed on the main surface of the substrate is controlled by appropriately controlling the nitrogen concentration, nitriding temperature, and nitriding time.

  If the substrate on which the surface nitride layer is thus formed is used, the crystallinity of the underlayer 2 directly formed on the main surface can be further improved. Further, it can be made thicker, for example, up to 3 μm, which is the upper limit value of the preferable thickness described above, without setting special film forming conditions and without generating cracks or peeling.

Although dislocation density in the underlying layer 2 is ¹⁰ 11 / cm ² or less, the dislocation density is preferably as low. Particularly preferably, it is 10 ¹⁰ / cm ² or less.

  Further, the full width at half maximum in the X-ray rocking curve on the (002) plane of the first nitride semiconductor constituting the underlayer 2 is 200 seconds or less, preferably 150 seconds or less, and 100 seconds or less. Is more preferable. As a result, the crystallinity of the semi-insulating layer 3 is further improved, and the crystallinity of 200 seconds or less, preferably 150 seconds or less, is shown in the half-value width in the X-ray rocking curve.

  Therefore, the semi-insulating layer 3 has a low dislocation density, high crystallinity, and can be formed in a high quality state, and thus has extremely high mobility.

  The first nitride semiconductor constituting the underlayer 2 is AlN.

  From the viewpoint of improving the crystallinity, the film thickness of the underlayer 2 is preferably as large as possible. However, if the film thickness becomes too large, generation of cracks, peeling or the like occurs. Therefore, the film thickness of the underlayer 2 is appropriately selected depending on the material of the substrate 1, but is 0.3 μm or more, preferably 0.5 μm or more, and more preferably 1 μm to 3 μm.

The underlayer can be formed, for example, by using trimethylaluminum (TMA) and ammonia (NH ₃ ) as a feedstock, and heating by MOCVD to preferably 1100 ° C. or higher, more preferably 1200 ° C. or higher. .

  The formation temperature of the underlayer is preferably 1100 ° C. or higher as described above. The “formation temperature” in the present invention is the temperature of the substrate when the underlayer is formed.

  The upper limit of the formation temperature of the underlayer is not particularly limited, but is preferably 1300 ° C. As a result, it is possible to effectively suppress surface roughness depending on the material composition of the nitride semiconductor constituting the underlayer, and further, diffusion of composition components in the underlayer. As a result, the crystallinity of the underlayer can be maintained in a good state, and deterioration of the crystallinity of the semi-insulating layer due to surface roughness can be effectively prevented.

In this case, the crystallinity can be kept sufficiently high even when the temperature at which the underlayer 2 is formed is reduced to 1200 ° C. or lower or about 1150 ° C., for example, 10 ¹⁰ / cm ^2. The following dislocation density can be easily realized.

  Furthermore, by forming the underlayer 2 on the surface nitride layer described above, peeling and cracking are less likely to occur even if the thickness is increased. For this reason, it is possible to easily form a thick film up to, for example, about 3 μm as described above without depending on the film forming conditions. Therefore, the crystallinity is further improved and the dislocation density can be further reduced by the synergistic effect of the improvement in crystallinity due to the surface nitride layer of the underlayer 2 and the improvement in crystallinity due to the increase in thickness. .

  For example, the surface nitrided layer preferably has a nitrogen content of 2 atomic% or more at a depth of 10 angstroms from the main surface of the substrate.

  The second nitride semiconductor constituting the semi-insulating layer 3 is GaN.

The second nitride semiconductor can be formed by MOCVD using trimethyl gallium and ammonia (NH ₃ ) as feed materials. The formation temperature at this time is 1000 ° C. or higher, and preferably 1100 ° C. or lower.

  Moreover, the resistivity of the second nitride semiconductor can be increased by setting the pressure at the time of forming the second nitride semiconductor to 300 mbar or less. From this viewpoint, it is more preferable that the pressure is 200 mbar or less.

  As described above, the present invention has been described in detail according to the embodiments of the invention. However, the present invention is not limited to the above contents, and the thickness, composition, carrier concentration, and the like of each layer are described in the present invention. All modifications and changes can be made without departing from the scope of the above.

  For example, an i-AlGaN layer as a spacer layer for preventing diffusion of Si can be inserted between the carrier moving layer 3 and the carrier supply layer 4 of the HEMT shown in FIG. Further, an n-GaN layer as a contact layer for reducing the contact resistance of the electrode can be laminated on the carrier supply layer 4. Furthermore, a barrier layer may be inserted between the carrier supply layer 4 and the contact layer to prevent Si diffusion.

  The semiconductor element of the present invention can be used for an electronic device that needs to operate at high speed or high output, such as an amplifier for a mobile phone base station or a wireless LAN.

Example 1
Laminated structures 1, 2, and 3 as shown in FIG. 1 were manufactured. Specifically, the sapphire substrate 1 having a diameter of 2 inches and a thickness of 630 μm was ultrasonically cleaned with acetone and IPA, then washed with water and dried, and then placed in a MOCVD apparatus. In the MOCVD apparatus, NH ₃ , TMA, TMG, and SiH ₄ are attached as gas systems. The substrate 1 was heated to 1200 ° C. while the pressure was set to 50 mbar and H ₂ was allowed to flow at a flow rate of 1 m / sec. Thereafter, NH ₃ gas was allowed to flow along with the hydrogen carrier gas for 5 minutes to nitride the main surface of the substrate 1. As a result of ESCA analysis, it was found that a nitride layer was formed on the main surface of the substrate by this surface nitriding treatment, and the nitrogen content at a depth of 1 nm from the main surface was 7 atomic%.

Next, TMA and NH ₃ were combined and flowed at a flow rate of 5 m / sec, and the underlayer 2 (AlN layer) was epitaxially grown to a thickness of 1 μm. The dislocation density of this AlN layer 2 is 2 × 10 ¹⁰ / cm ² , and the half-value width of the X-ray diffraction rocking curve on the (002) plane is 60 seconds, indicating that the AlN layer is a good quality AlN layer. Furthermore, when the surface flatness of the AlN layer was confirmed, Ra in the 5 μm range was 1.5 Å, and it was found that the surface had an extremely flat surface.

Next, the pressure was set to 200 mbar, the temperature of the substrate was raised to 1080 ° C., TMG, NH ₃ , and H ₂ and N ₂ as carrier gases were added together and flowed at a flow rate of 1 m / sec to thicken the GaN layer 3. Epitaxially grown to a thickness of 3 μm. The dislocation density of the GaN layer 3 was 1 × 10 ⁸ / cm ² , and the half width of the X-ray rocking curve on the (002) plane was 100 seconds.

Finally, SiH ₄ was added so that the carrier density was 1 × 10 ¹⁸ / cm ^3, and n-GaN serving as a contact layer with the electrode was grown to 10 nm. After the growth is completed, a four-terminal measurement electrode made of Ti / Al / Ti / Au is formed on the surface of the GaN layer 3, and the n-GaN portion that is not covered with the electrode is removed by etching to obtain a specific resistance. When the measurement was performed, the specific resistance of the GaN layer 3 was 1 × 10 ⁶ to 1 × 10 ⁷ Ωcm.

(Comparative Example 1)
The GaN layer 3 was grown on the AlN layer 2 in the same manner as in Example 1 except that the pressure during the growth of the GaN layer 3 was changed from 200 mbar to 400 mbar. The electrode formation was performed in the same manner as in Example 1. As a result, the specific resistance of the GaN layer 3 was as low as 1 to 1000 Ωcm.

(Comparative Example 2)
The pretreatment of the substrate 1 was performed in the same manner as in Example 1. Thereafter, the substrate 1 was placed in the MOCVD apparatus, the pressure was set to 200 mbar, and the substrate was heated to 550 ° C. while H ₂ was allowed to flow at a flow rate of 1 m / sec. Thereafter, TMG and NH ₃ were combined and flowed at a flow rate of 5 m / sec, and the GaN layer 2 as a buffer layer was grown to a thickness of 20 nm.

Next, the pressure was set to 200 mbar, the temperature of the substrate was raised to 1080 ° C., TMG, NH ₃ , and H ₂ and N ₂ as carrier gases were added together and flowed at a flow rate of 1 m / sec to thicken the GaN layer 3. Epitaxially grown to a thickness of 3 μm. Next, an electrode was formed in the same manner as in Example 1, and the specific resistance was measured. As a result, the specific resistance of the GaN layer was 1 to 100 Ωcm.

As described above, according to the present invention, a substrate, a base layer made of a first nitride semiconductor epitaxially grown on the substrate, and a semi-insulating film made of a second nitride semiconductor epitaxially grown on the base layer. an epitaxial substrate and a layer, can be provided a semi-insulating layer of resistivity rather high, the epitaxial substrate can be effectively used as a semiconductor element.

(Explanation of symbols)
DESCRIPTION OF SYMBOLS 1 Substrate 2 Underlayer 3 Semi-insulating layer 4 Carrier supply layer 20 Semiconductor element

Claims (5)

An underlayer forming step of epitaxially growing an underlayer made of aluminum nitride on a sapphire substrate by metalorganic vapor phase epitaxy; and a semi-insulating layer made of gallium nitride on the underlayer by metalorganic vapor phase epitaxy ; A semi-insulating layer forming step of epitaxial growth from an atmosphere of ammonia and a carrier gas at an atmospheric pressure of 300 mbar or less and a temperature of 1000 ° C. or more, and the dislocation density of the aluminum nitride is 10 ¹¹ / cm ² or less, A method for producing an epitaxial substrate, wherein a half width in an X-ray rocking curve on the (002) plane of the aluminum nitride is 200 seconds or less, and a specific resistance of the gallium nitride is 10 ⁴ Ω · cm or more.
The method according to claim 1, further comprising a surface nitriding treatment step of forming a surface nitride layer on the sapphire substrate , and forming the underlayer on the surface nitride layer. The method according to claim 2, wherein the surface nitrided layer is formed by heating the sapphire substrate in an ammonia atmosphere. The method according to claim 1, wherein the underlayer is epitaxially grown at a temperature of 1100 ° C. or higher and 1300 ° C. or lower. The method according to claim 1, wherein the thickness of the underlayer is 1 to 3 μm.

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