JP2007311810A - Epitaxial substrate, epitaxial substrate for electronic device, and electronic device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a means for obtaining an electronic device at practical level formed from GaN based materials by improving properties such as high-speed operation. <P>SOLUTION: A first group III nitride base layer 2 and a second group III nitride base layer 3 including at least Al are formed on a base material 1 in order. A region A where an acceptor impurity exists is then formed so as to be extended from a boundary face 9 between the base layer 2 and the base layer 3 to the base layer 3 to form an epitaxial substrate 5, and the substrate 5 is used as a substrate when the electronic device is manufactured. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an epitaxial substrate, an epitaxial substrate for an electronic device, and an electronic device.

  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. In addition, electronic devices such as InP-based HEMTs and HBTs have been actively researched and developed as higher-performance electronic devices.

  However, in the manufacture of these higher-performance electronic devices, the structure of the epitaxially grown semiconductor layer for manufacturing the electronic device becomes more complicated, the device process becomes finer, and the manufacturing cost increases. Since the material system constituting the semiconductor layer is also more expensive, a new material system that replaces these material systems has been desired.

  Electronic devices using GaN as such new materials 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.

  FIG. 1 is a diagram showing an example of a conventional HEMT device. The HEMT shown in FIG. 1 includes a first underlayer 2 made of AlN or the like formed on a base material 1 made of sapphire single crystal or the like at a low temperature of 600 ° C. or less by MOCVD, and MOCVD formed on the underlayer 2. A second underlayer 3 made of GaN epitaxially grown by the method or the MBE method and a semiconductor layer 4 made of AlGaN epitaxially grown on the underlayer 3 are provided. A gate electrode 6, a source electrode 7, and a drain electrode 8 are provided on the semiconductor layer 4. In FIG. 1, the base material 1 and the foundation layers 2 and 3 constitute an epitaxial substrate 5.

  When a predetermined voltage is applied to the gate electrode 6, a current flows between the source electrode 7 and the drain electrode 8 so as to pass through the interface between the base layer 3 and the semiconductor layer 4 as indicated by arrows in the figure. To function as a device.

  However, in the HEMT using the GaN-based material as shown in FIG. 1 and other electronic devices, the high-speed operability is not sufficient, and it is insufficient for use as a practical electronic device.

  An object of the present invention is to provide means for improving characteristics such as high-speed operability and obtaining a practical-level electronic device made of a GaN-based material.

  In order to achieve the above object, the present invention provides a substrate made of a single crystal material, a first group III nitride underlayer containing at least Al formed on the substrate, and the first group III nitride. A second group III nitride underlayer formed on the material underlayer, and acceptor impurities at the interface between the first group III nitride underlayer and the second group III nitride underlayer. And the acceptor impurity is 0.01 to 0 in the second group III nitride underlayer from the interface between the first group III nitride underlayer and the second group III nitride underlayer. The invention relates to an epitaxial substrate characterized in that it penetrates and exists at a depth of 5 μm.

  As described above, according to the epitaxial substrate of the present invention, characteristics such as high-speed operability can be improved, and a practical electronic device made of a GaN-based material can be provided.

  The present inventors diligently studied to achieve the above object, and have found the following facts. That is, at the interface 9 between the underlayer 2 and the underlayer 3, a relatively large amount of n-type carriers are generated along with the dislocation disappearance process, and a leak current due to the carriers is generated at the interface 9. I found it. Therefore, it has been found that when a predetermined voltage is applied to the gate electrode 6, a part of the voltage is used for generating a leakage current, so that sufficient high-speed operability cannot be realized.

  Therefore, the inventors conducted further studies to suppress the leakage current. As a result, since the leakage current is generated from n-type carriers, an acceptor impurity is added to the interface 9 to capture the n-type carriers, resulting in the present invention. As a result, it is possible to provide an electronic device excellent in high-speed operability by suppressing the leakage current.

  The acceptor impurity in the present invention is an impurity that acts as an acceptor for the first group III nitride underlayer and the second group III nitride underlayer, and is a group II element in the periodic table. For example, Mg, Be, Zn, and C can be exemplified.

In a preferred embodiment of the present invention, the acceptor impurity concentration is set to 10 ¹⁰ / cm ² to 10 ²⁰ / cm ² , further 10 ¹⁴ / cm ² to 10 ¹⁸ / cm ² . As a result, n-type carriers present at the interface between the underlying layers can be sufficiently captured, and the leakage current can be sufficiently suppressed.

  Further, in the present invention, the acceptor impurity is introduced from the interface between the first group III nitride underlayer and the second group III nitride underlayer into the second group III nitride underlayer. It penetrates to a depth of 0.01 to 0.5 μm and exists. As a result, n-type carriers present at the interface between the underlying layers can be sufficiently captured, and the leakage current can be sufficiently suppressed.

  Furthermore, in a preferred aspect of the present invention, the acceptor impurity concentration is set below the second group III nitride from the interface between the first group III nitride underlayer and the second group III nitride underlayer. Decrease in the thickness direction of the formation. Even in this case, n-type carriers existing at the interface between the underlying layers can be sufficiently captured, and the leakage current can be sufficiently suppressed. Furthermore, since the acceptor impurity does not exist at a high concentration in the second group III nitride underlayer, it is possible to suppress deterioration in carrier mobility caused by the acceptor impurity.

  Specifically, it can be decreased continuously or stepwise from the interface toward the thickness direction of the second group III nitride underlayer. Furthermore, a locally high concentration portion can be formed inside the second group III nitride underlayer as necessary in a state where the concentration is reduced overall in the thickness direction. .

  The acceptor impurity is not only present at the interface between the above-described underlayer and the second group III nitride underlayer, but also penetrates into the first group III nitride underlayer from the interface by diffusion or the like. May exist.

  Hereinafter, the present invention will be described in detail based on embodiments of the invention.

  FIG. 2 is a diagram showing a configuration of a HEMT using the epitaxial substrate of the present invention. The same reference numerals are used for the same components as the HEMT 10 shown in FIG. In the HEMT 20 shown in FIG. 2, a first group III nitride underlayer 2 containing at least Al is formed on a base material 1 from a predetermined single crystal material, and the underlayer 2 is epitaxially grown on the underlayer 2. The second group III nitride underlayer 3 containing Al with a small content and the semiconductor layer 4 epitaxially grown on the underlayer 3 are provided. A gate electrode 6, a source electrode 7, and a drain electrode 8 are provided on the semiconductor layer 4. In FIG. 2, the base material 1 and the foundation layers 2 and 3 constitute an epitaxial substrate 5.

  Further, an acceptor impurity is contained in the region A extending from the interface 9 to the foundation layer 3 between the foundation layer 2 and the foundation layer 3. Examples of the acceptor impurities include Group II Mg, Be, and Zn that function as acceptors for the Group III nitrides constituting the foundation layer 2 and the foundation layer 3 as described above.

Further, as described above, the acceptor impurity concentration is preferably 10 ¹⁰ / cm ² to 10 ²⁰ / cm ² , more preferably 10 ¹⁴ / cm ² to 10 ¹⁸ / cm ² . Furthermore, in the present invention, the thickness t of the region A, that is, the depth t at which acceptor impurities enter the underlayer 3 from the interface 9 is set to 0.01 to 0.5 μm as described above. It is necessary. As a result, n-type carriers generated at the interface 9 can be sufficiently captured, and the occurrence of leakage current at the interface 9 can be sufficiently reduced.

Region A containing acceptor impurities can be formed as follows. That is, when the underlayer 3 is formed by the MOCVD method, trimethylaluminum (TMA), triethylaluminum (TEA), other group III element supply gas, which is a raw material of the underlayer 3, and nitrogen such as ammonia (NH ₃ ) In addition to the source gas, acceptor impurity gases such as Cp ₂ Mg, DEZn, Cp ₂ Be, and CH ₄ are used only in the initial stage of formation. In addition, regarding the doping of C, a methyl group in the organometallic can be used. Then, after forming the base layer 3 to a certain thickness, the supply of the impurity gas is stopped to form the region A where the acceptor impurity exists so as to extend from the interface 9 to the base layer 3. Can do.

  Note that not only the MOCVD method but also the MBE method can be used to form a similar structure.

  In the HEMT 20 shown in FIG. 2, the first group III nitride underlayer 2 needs to contain Al, and preferably contains 50 atomic% or more of Al with respect to all the group III elements, and further includes group III elements. It is preferable that all are made of Al and the underlayer 2 is made of AlN. As a result, misfit dislocations generated at the interface between the base material 1 and the underlayer 2 are entangled at the interface and do not propagate into the underlayer 2, so that the dislocation density in the layer is reduced. As a result, the dislocation density in the second group III nitride underlayer 3 and the semiconductor layer 4 formed on the underlayer 2 is also reduced, and the crystal quality is improved.

  For this reason, the mobility of carriers due to the operation of the HEMT increases, and a current flows through the interface between the base layer 3 and the semiconductor layer 4 in FIG. Further increase. Further, since carrier capture due to dislocation is also suppressed, the carrier density is increased, a larger current can be passed, and the signal intensity can be increased.

  The half width of the X-ray rocking curve at the (002) plane of the first group III nitride underlayer 2 is preferably 100 seconds or less, and more preferably 60 seconds or less. Even in this case, due to the high crystallinity of the underlayer 2, the crystallinity of the underlayer 3 and the semiconductor layer 4 epitaxially grown on the underlayer 2 is also improved, so that the mobility and density of carriers are improved, Since a larger current can flow at a high speed, the high-speed response can be improved and the signal intensity can be increased.

  Such a highly crystalline underlayer 2 can be formed by setting the temperature of the substrate 1 to 1100 ° C. or higher by, for example, MOCVD. In addition, it is preferable to set the temperature of the base material 1 to 1250 ° C. or less from the viewpoint of suppressing surface roughness.

  The first group III nitride underlayer 2 can also contain additive elements such as B, Si, Ge, Zn, Be, and Mg in addition to group III elements such as Ga and In other than Al. Furthermore, it is possible to include not only elements added intentionally but also trace elements that are inevitably taken in depending on the film forming conditions and the like, as well as trace impurities contained in the raw materials and reaction tube materials.

  The thickness of the underlayer 2 is preferably set to 2 μm to 3 μm or more from the viewpoint of improving crystallinity.

  The second group III nitride underlayer 3 can have any composition depending on the composition of the underlayer 2 and the composition of the semiconductor layer 4. In addition to group III elements such as Al, Ga, and In, additional elements such as B, Si, Ge, Zn, Be, and Mg can also be included. Furthermore, it is possible to include not only elements added intentionally but also trace elements that are inevitably taken in depending on the film forming conditions and the like, as well as trace impurities contained in the raw materials and reaction tube materials.

  The semiconductor layer 4 can be made of any semiconductor material depending on the purpose. However, when the underlayer 2 and the underlayer 3 are made of a group III nitride as described above, the epitaxial growth is facilitated, and the interface In order to obtain an HEMT with good characteristics by arbitrarily controlling the atomic arrangement state and energy state in the structure, it is also preferable to use a group III nitride.

Single crystal materials constituting the substrate 1 are sapphire single crystals, ZnO single crystals, LiAlO ₂ single crystals, LiGaO ₂ single crystals, MgAl ₂ O ₄ single crystals, oxide single crystals such as MgO single crystals, Si single crystals, Group IV or IV-IV single crystal such as SiC single crystal, GaAs single crystal, AlN single crystal, GaN single crystal, III-V single crystal such as AlGaN single crystal, boride single crystal such as ZrB _2, etc. A known substrate material can be used.

  Hereinafter, the present invention will be described specifically by way of examples.

(Example)
In this example, a HEMT 20 as shown in FIG. 2 was produced. A sapphire single crystal base material was used as the base material 1, and this was placed on a susceptor installed in an MOCVD apparatus. Next, the substrate was heated to 1200 ° C. with a heater in the susceptor, and the pressure was set to 25 Torr.

Then, using trimethyl aluminum (TMA) as Al feedstock, using ammonia gas (NH ₃₎ as a nitrogen feedstock, these raw material gases together with hydrogen carrier _gas, a gas supply amount so that the NH 3 / TMA = 400 It was set, introduced into the reaction tube, and supplied onto the substrate to form an AlN underlayer film as a first group III nitride underlayer 2 having a thickness of 1 μm.

Next, after setting the pressure to atmospheric pressure, the base material temperature was set to 1050 ° C., trimethylgallium (TMG) was used as a Ga feedstock, and the gas supply amount was set to NH ₃ / TMA = 3000, A GaN underlayer as the second group III nitride underlayer 3 was formed to a thickness of 3 μm by supplying the AlN film. In the initial stage of the formation of the GaN base film, Cp ₂ Mg is flowed at a flow rate that gives a concentration of 3 × 10 ¹⁶ / cm ² , and the presence region A of Mg as an acceptor impurity has a thickness of 0.2 μm. Formed.

Next, these source gases are supplied onto the GaN base film at a flow rate ratio of NH ₃ /TMA/TMG=3000/0.3/1, and SiH ₄ gas is supplied at a concentration of 4 × 10 ¹⁸ / cm ² . Thus, an n-Al _0.3 Ga _0.7 N film as the semiconductor layer 4 was formed to a thickness of 25 nm. Next, a predetermined electrode was formed, and Hall effect measurement was performed. Next, the n-Al _0.3 Ga _0.7 N film was removed by etching, a predetermined electrode was formed with a gap of 10 μm, and leakage current was evaluated. As a result, a two-dimensional electron gas concentration of 1.0 × 10 ¹³ / cm ² and a mobility of 1200 cm ² / Vs were realized, and the leakage current value when 20 V was applied was 0.1 μA or less.

(Comparative example)
In this comparative example, a HEMT 10 as shown in FIG. 1 was produced. The production conditions were the same as in Example 1 except that when forming the GaN underlayer which was the second group III nitride underlayer 3, Cp ₂ Mg was not supplied and the region A where acceptor impurities were present was not formed. It implemented like. Although the two-dimensional electron gas concentration is 1.0 × 10 ¹³ / cm ² and the mobility is 1200 cm ² / Vs, the leakage at the time of applying 20 V evaluated by removing the n-Al _0.3 Ga _0.7 N film by etching. The current value was 10 μA.

  As is apparent from the examples and comparative examples, the AlN underlayer film and the GaN underlayer film constituting the first group III nitride underlayer 2 and the second group III nitride underlayer 3 according to the present invention. When the region A where the impurity is present is formed so that the acceptor impurity Mg is present at the interface, the leakage current is suppressed, the current blocking characteristic is excellent, and the capacitance due to the presence of unnecessary carriers is also reduced. Since it is suppressed, it can be seen that the high frequency characteristics are excellent.

  The present invention has been described in detail based on the embodiments of the invention with specific examples. However, the present invention is not limited to the embodiments of the invention, and does not depart from the scope of the invention. All changes and modifications are possible.

  For example, in the above specific example, the case where the region A where the acceptor impurity exists is present in the second group III nitride underlayer 3 has been shown. However, the region A exists in the first group III nitride underlayer 2. It can also be done. In addition, after the first group III nitride underlayer 2 is formed, acceptor impurities are adsorbed only on the extreme surface of the underlayer 2 by ion irradiation or the like so that the acceptor impurities are present almost only at the interface 9. You can also. Further, in the second group III nitride underlayer 3, the acceptor impurity concentration can be decreased, for example, continuously or stepwise from the interface 9 in the film thickness direction.

  In addition, although the case where the epitaxial substrate of this invention was used about HEMT was demonstrated in the above, it can be used also for another electronic device, for example, HBT etc.

  In addition, it is possible to improve the crystal quality of each layer by inserting a multilayer laminated film such as a buffer layer or a strained superlattice at any interface between the substrate 1 and the semiconductor layer 4 or by making the growth conditions multi-stage. it can.

  As described above, according to the epitaxial substrate of the present invention, characteristics such as high-speed operability can be improved, and a practical electronic device made of a GaN-based material can be provided.

It is a block diagram of the conventional HEMT. It is a block diagram of HEMT using the epitaxial substrate of this invention.

Explanation of symbols

1 Substrate 2 First Group III Nitride Underlayer 3 Second Group III Nitride Underlayer 4 Semiconductor Layer 5 Epitaxial Substrate 6 Gate Electrode 7 Source Electrode 8 Drain Electrode 9 Interface 10, 20 HEMT

Claims (8)

  A substrate made of a single crystal material, a first group III nitride underlayer containing at least Al formed on the substrate, and a second group formed on the first group III nitride underlayer A group III nitride underlayer including an acceptor impurity at least at an interface between the first group III nitride underlayer and the second group III nitride underlayer. From the interface between the group III nitride underlayer of the first layer and the second group III nitride underlayer to penetrate into the second group III nitride underlayer at a depth of 0.01 to 0.5 μm. An epitaxial substrate characterized in that: The epitaxial substrate according to claim 1, wherein the acceptor impurity concentration is 10 ¹⁰ / cm ² to 10 ²⁰ / cm ² .   The acceptor impurity concentration decreases in the thickness direction of the second group III nitride underlayer from the interface between the first group III nitride underlayer and the second group III nitride underlayer. The epitaxial substrate according to claim 1, wherein the epitaxial substrate is characterized in that:   4. The epitaxial substrate according to claim 1, wherein the Al content in the first group III nitride underlayer is 50 atomic% or more with respect to all group III elements. 5.   The epitaxial substrate according to claim 4, wherein the first group III nitride underlayer is made of AlN.   The epitaxial substrate according to any one of claims 1 to 5, wherein a half width in an X-ray rocking curve of the (002) plane of the first group III nitride underlayer is 100 seconds or less. .   The epitaxial substrate for electronic devices containing the epitaxial substrate as described in any one of Claims 1-6.   An electronic device comprising the epitaxial substrate for an electronic device according to claim 7.

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