JP2015115429A - Nitride semiconductor epitaxial substrate and nitride semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nitride semiconductor epitaxial substrate and a nitride semiconductor device, which can improve device properties.SOLUTION: A semiconductor epitaxial substrate comprises: a substrate (1); an initial growth layer (2) formed on the substrate (1); a buffer layer (3) formed on the initial growth layer (2); a superlattice buffer layer (4) which is formed on the buffer layer 3 and in which high-Al-containing layers each composed of AlGaN(0.5≤x≤1.0) and low-Al-containing layers each composed of AlGaN(0≤y≤0.3) are alternately laminated one on the other; a channel layer (5) which is formed on the superlattice buffer layer (4) and composed of nitride semiconductor layers; and a barrier layer (6) formed on the channel layer (5). The channel layer 5 near a boundary face with the superlattice buffer layer (4) is subjected to δ doping with an n-type dopant.

Description

  The present invention relates to a nitride semiconductor epitaxial substrate using a nitride semiconductor, and also relates to a nitride semiconductor device.

  As an electronic device using a nitride semiconductor, a structure having a heterojunction made of AlGaN and GaN is generally used.

  More specifically, the electronic device includes a substrate such as sapphire or Si, a buffer layer formed on the substrate and made of a nitride semiconductor, and generally formed on the buffer layer. And a barrier layer made of AlGaN formed on the channel layer. A two-dimensional electron gas is formed at the interface between the AlGaN barrier layer and the GaN channel layer.

  In the electronic device, the source electrode and the drain electrode are formed so as to be in ohmic contact with the two-dimensional electron gas, and the gate electrode is formed so as to be positioned between the source electrode and the drain electrode.

  By the way, when forming a nitride semiconductor on a sapphire substrate or SiC substrate, it is not a big problem, but when a Si substrate having a thermal expansion coefficient smaller than that of a nitride semiconductor is used, The Si substrate warps in a downwardly convex shape, and furthermore, a crack is formed in the nitride semiconductor crystal itself by stress. Therefore, the Si substrate is not suitable for forming an electronic device using a nitride semiconductor.

  As a method of reducing the difference in thermal expansion coefficient between the Si substrate and the nitride semiconductor multilayer body, it is possible to use a buffer layer in which AlGaN layers having different compositions are alternately grown. Proposed in 1).

JP-A-2005-85852

  Here, a mechanism of two-dimensional electron gas generation in the nitride semiconductor will be described with reference to FIGS. 6A and 6B.

  First, as shown in FIG. 6A, when a thin AlGaN layer having a thin lattice constant that does not cause stress relaxation is formed on GaN having a substantially bulk lattice constant with the stress relaxed, the GaN and AlGaN layers Piezopolarization occurs due to the difference in spontaneous polarization and the in-plane distortion of the AlGaN layer on GaN, and a two-dimensional electron gas is formed at the interface between GaN and the AlGaN layer.

  On the other hand, when a buffer layer in which AlGaN layers having different compositions are alternately grown is considered as one average lump, this buffer layer can be considered equivalent to an AlGaN layer with relaxed stress. Since GaN formed on the AlGaN layer has a larger lattice constant than the AlGaN layer, a two-dimensional hole gas is formed at the interface between the AlGaN layer and GaN, contrary to FIG. 6A.

  The two-dimensional hole gas generated in this manner causes a leak current and causes a decrease in device characteristics.

  Therefore, the method disclosed in Japanese Patent Application Laid-Open No. 2005-85852 has a problem that it causes deterioration of device characteristics.

  An object of the present invention is to provide a nitride semiconductor epitaxial substrate and a nitride semiconductor device that can improve device characteristics.

In order to solve the above problems, the nitride semiconductor epitaxial substrate of the present invention is
A substrate,
An initial growth layer formed on the substrate;
A buffer layer formed on the initial growth layer;
A high Al content layer formed on the buffer layer and having a composition of Al _x Ga _1-x N (0.5 ≦ x ≦ 1.0), and Al _y Ga _1-y N (0 ≦ y ≦ 0). .3) a superlattice buffer layer formed by alternately laminating low Al-containing layers having the composition;
A channel layer formed on the superlattice buffer layer and made of a nitride semiconductor layer;
A barrier layer formed on the channel layer,
The channel layer in the vicinity of the interface with the superlattice buffer layer is characterized in that an n-type dopant is δ-doped.

In the nitride semiconductor epitaxial substrate of one embodiment,
The n-type dopant is Si or Ge.

In the nitride semiconductor epitaxial substrate of one embodiment,
The δ-doped portion is within a range of 50 nm from the interface between the superlattice buffer layer and the channel layer.

In the nitride semiconductor epitaxial substrate of one embodiment,
The concentration of the δ doping is in the range of 95% to 105% of the carrier concentration obtained by CV measurement at the interface between the superlattice buffer layer and the channel layer when the channel layer is not subjected to the δ doping. is there.

The electronic device of the present invention is
A substrate,
An initial growth layer formed on the substrate;
A buffer layer formed on the initial growth layer;
A high Al content layer formed on the buffer layer and having a composition of Al _x Ga _1-x N (0.5 ≦ x ≦ 1.0), and Al _y Ga _1-y N (0 ≦ y ≦ 0). .3) a superlattice buffer layer formed by alternately laminating low Al-containing layers having the composition;
A channel layer formed on the superlattice buffer layer and made of a nitride semiconductor layer;
A barrier layer formed on the channel layer,
The channel layer in the vicinity of the interface with the superlattice buffer layer is characterized in that an n-type dopant is δ-doped.

  According to the nitride semiconductor epitaxial substrate of the present invention, the channel layer in the vicinity of the interface with the superlattice buffer layer is doped with n-type dopant by δ doping, so that electrons are introduced into the interface between the superlattice buffer layer and the channel layer. Since it is possible to sufficiently compensate the holes at the interface, the formation of a two-dimensional hole gas at the interface can be prevented.

  Therefore, when the nitride semiconductor epitaxial substrate is used for an electronic device, leakage current can be reduced and device characteristics can be improved.

  The nitride semiconductor device of the present invention supplies electrons to the interface between the superlattice buffer layer and the channel layer by δ-doping the n-type dopant in the channel layer near the interface with the superlattice buffer layer, Since holes at this interface can be sufficiently compensated, it is possible to prevent the formation of two-dimensional hole gas at the interface.

  Therefore, the nitride semiconductor device can reduce leakage current and improve device characteristics.

FIG. 1 is a schematic sectional view of a nitride semiconductor epitaxial substrate according to a first embodiment of the present invention. FIG. 2A is a graph of the results of CV measurement when δ doping is not performed. FIG. 2B is a graph of the results of CV measurement when δ doping is performed. FIG. 3 is a diagram for explaining the CV measurement method. FIG. 4A is a graph showing the relationship between the position of δ doping and the carrier concentration at the interface. FIG. 4B is a graph showing the relationship between carrier concentration and distance. FIG. 5 is a graph showing the relationship between the concentration of δ doping and the carrier concentration at the interface. FIG. 6A is a diagram for explaining generation of a two-dimensional electron gas in a nitride semiconductor. FIG. 6B is a diagram for explaining generation of a two-dimensional hole gas in a nitride semiconductor. FIG. 7 is a schematic configuration diagram of a nitride semiconductor device according to an embodiment of the present invention.

  The nitride semiconductor epitaxial substrate of the present invention will be described in detail below with reference to the illustrated embodiments.

[First Embodiment]
FIG. 1 is a schematic sectional view of a nitride semiconductor epitaxial substrate (wafer) according to a first embodiment of the present invention.

In the nitride semiconductor epitaxial substrate, an initial growth layer 2 made of undoped AlN having a thickness of 100 nm and a buffer layer 3 made of Al _0.2 Ga _0.8 N having a thickness of 20 nm are sequentially epitaxially grown on the Si substrate 1. Then, the superlattice buffer layer 4 is epitaxially grown. The superlattice buffer layer 4 is formed by alternately laminating an undoped AlN layer having a thickness of 4 nm and an undoped Al _0.1 Ga _0.9 N layer having a thickness of 23 nm. Of the AlN layer and the Al _0.1 Ga _0.9 N layer, the Al _0.1 Ga _0.9 N layer is the uppermost layer and is in contact with the superlattice buffer layer 4 described later. In addition, for example, 100 layers of the AlN layer and the Al _0.1 Ga _0.9 N layer are formed. The AlN layer is an example of a high Al-containing layer, and the Al _0.1 Ga _0.9 N layer is an example of a low Al-containing layer.

Subsequently, a channel layer 5 made of, for example, GaN having a thickness of 1 μm is epitaxially grown on the superlattice buffer layer 4. At this time, δ is doped with Si as an example of an n-type dopant. More specifically, the Si is doped at a position of 20 nm above the interface between the superlattice buffer layer 4 and the channel layer 5 (hereinafter simply referred to as “interface”). The Si doping concentration was 5 × 10 ¹⁵ cm ⁻³ . The position where Si is doped is indicated by a dotted line in FIG. The GaN is an example of a nitride semiconductor.

Thereafter, a barrier layer 6 made of undoped Al _0.2 Ga _0.8 N, for example, having a thickness of 25 nm is epitaxially grown on the channel layer 5.

  The Si doping concentration is set based on the structure of the superlattice buffer layer 4. More specifically, a charge substantially equal to the piezoelectric charge resulting from the lattice constant difference between AlGaN and GaN equivalent to the average Al composition of the superlattice buffer layer 4 plus the difference in the spontaneous polarization charge between AlGaN and GaN. An amount of doping that provides is desirable.

  Actually, the carrier concentration at the interface is obtained by CV measurement in the absence of the δ doping, and the Si doping concentration is set so as to substantially match this carrier concentration.

  2A is a result of CV measurement of a HEMT (High Electron Mobility Transistor) formed using a nitride semiconductor epitaxial substrate that is different from the nitride semiconductor epitaxial substrate of FIG. 1 only in that δ doping is not performed. Indicates. FIG. 2B shows the results of CV measurement of the HEMT formed using the nitride semiconductor epitaxial substrate of FIG. The HEMT is an example of an electronic device.

  2A and 2B, a bias voltage is applied by the LCR meter 12 between the gate electrode G of the HEMT 10 and the stage 11, as shown in FIG. 2A and 2B represent relative distances in the substrate direction with respect to the surface of the nitride semiconductor epitaxial substrate.

  As shown in FIG. 2A, when δ doping is not performed, carriers are observed between the channel layer and the superlattice buffer layer, suggesting the presence of a two-dimensional hole gas. On the other hand, as shown in FIG. 2B, when δ doping is performed, carriers due to the two-dimensional hole gas are not seen, indicating that the two-dimensional hole gas is compensated by electrons from Si.

  Accordingly, it is possible to prevent the formation of a two-dimensional hole gas at the interface, thereby reducing the HEMT leakage current and improving the device characteristics.

  In addition, the HEMT can improve the device characteristics, so that the lifetime can be extended.

  Moreover, since the Si is used as an example of an n-type dopant, electrons can be effectively supplied to the interface.

  Further, since the activation rate of Si is almost 100%, δ doping can be easily controlled.

  In the first embodiment, Si is doped at a position of 20 nm upward from the interface between the superlattice buffer layer 4 and the channel layer 5. However, Si may not be doped at that position. In this case, the Si doping position may be set within a range of up to 50 nm from the interface between the superlattice buffer layer 4 and the channel layer 5.

  In the said 1st Embodiment, although said Si was used as an example of an n-type dopant, you may use Ge as an example of an n-type dopant.

  In the first embodiment, the initial growth layer 2 made of undoped AlN is used. However, for example, an initial growth layer made of AlGaN or GaN depending on conditions may be used.

  In the first embodiment, the channel layer 5 made of GaN is used. However, for example, a channel layer made of InGaN, AlGaN, or the like may be used. The channel layer may be composed of a single layer or a plurality of layers.

In the first embodiment, the barrier layer 6 made of Al _0.2 Ga _0.8 N is used. However, for example, a barrier layer made of AlInN, AlGaInN, or the like may be used. The barrier layer may be composed of a single layer or a plurality of layers.

  In the first embodiment, the barrier layer 6 is in contact with the channel layer 5, but the barrier layer 6 may not be in contact with the channel layer 5. In this case, an improvement in mobility may be improved by providing an intermediate layer made of, for example, Al between the barrier layer 6 and the channel layer 5.

  In the first embodiment, a cap layer made of GaN may be provided on the barrier layer 6.

[Second Embodiment]
The nitride semiconductor epitaxial substrate of the second embodiment of the present invention has the same structure as the nitride semiconductor epitaxial substrate of the first embodiment.

  In the second embodiment, the carrier concentration at the interface between the superlattice buffer layer and the channel layer (hereinafter simply referred to as “interface”) was measured by CV measurement using the Si concentration and position of δ doping as parameters.

  FIG. 4A shows the relationship between the position of δ doping and the carrier concentration at the interface.

  Here, the Si concentration of the δ doping is matched with the peak carrier concentration of the interface measured by CV when δ doping is not performed in the nitride semiconductor epitaxial substrate of the second embodiment. Although the approximate doping concentration can be obtained by calculation, the reliability of the approximate doping concentration obtained by calculation is not necessarily high, so the result of CV measurement is used.

As shown in FIG. 4A, when Si is δ-doped within 50 nm from the interface, the two-dimensional hole gas near the interface can be compensated (0 cm ⁻³ on the plot, but the measured value is When about 2 × 10 ¹⁴ cm ⁻³ ) is far away from the position of the two-dimensional hole gas, as shown in FIG. 4B, a new peak carrier concentration due to electrons appears, and the leakage current increases.

  Therefore, if Si is doped at a position of 50 nm from the interface, holes at the interface can be reliably compensated.

  FIG. 4B shows the result of CV measurement of HEMT formed using a nitride semiconductor epitaxial substrate that is δ-doped at 100 nm from the interface.

  FIG. 5 shows the relationship between the concentration of δ doping and the carrier concentration at the interface.

  Here, the δ doping is performed at a position of 30 nm from the interface.

First, the carrier concentration at the interface decreases as the concentration of δ doping increases. In the nitride semiconductor epitaxial substrate of the second embodiment, when the δ doping concentration is within the range of 95% to 105% of the carrier concentration of the interface measured by CV when δ doping is not performed, the carrier concentration of the interface is Becomes about 2 × 10 ¹⁴ cm ⁻³ which is considered to be close to the measurement limit. When the concentration of δ doping exceeds the above range, the carrier concentration at the interface increases, and an electronic layer serving as a new leak source is formed.

  Therefore, when the δ doping concentration is not δ doped in the nitride semiconductor epitaxial substrate of the second embodiment, the carrier concentration obtained by CV measurement of the interface between the superlattice buffer layer and the channel layer is 95. If it is in the range of% to 105%, it is possible to prevent formation of an electronic layer that becomes a new leak source while compensating for holes.

  The present invention is not limited to the nitride semiconductor epitaxial substrate described in the first embodiment and the second embodiment, but also nitride semiconductors obtained by dividing each nitride semiconductor epitaxial substrate into a plurality of pieces. Includes devices.

  An example of the nitride semiconductor device is a HEMT shown in FIG. In FIG. 7, the same components as those shown in FIG. 1 are denoted by the same reference numerals as those of the components shown in FIG.

  The HEMT includes a source electrode 11 formed on the barrier layer 6, a drain electrode 12 formed on the barrier layer 6 so as to have a predetermined distance from the source electrode 11, a source electrode 11 and a drain electrode. 12 and a gate electrode 13 formed between the two.

  Although specific first and second embodiments of the present invention have been described, the present invention is not limited to the first and second embodiments, and various modifications may be made within the scope of the present invention. Can do. For example, what combined suitably the content of the said 1st, 2nd embodiment is good also as one Embodiment of this invention.

  That is, the present invention and the embodiment are summarized as follows.

The nitride semiconductor epitaxial substrate of the present invention is
Substrate 1;
An initial growth layer 2 formed on the substrate 1;
A buffer layer 3 formed on the initial growth layer 2;
A high Al content layer formed on the buffer layer 3 and having a composition of Al _x Ga _1-x N (0.5 ≦ x ≦ 1.0), and Al _y Ga _1-y N (0 ≦ y ≦ Superlattice buffer layer 4 formed by alternately laminating low Al-containing layers having a composition of 0.3),
A channel layer 5 formed on the superlattice buffer layer 4 and made of a nitride semiconductor layer;
A barrier layer 6 formed on the channel layer 5;
The channel layer 5 in the vicinity of the interface with the superlattice buffer layer 4 is characterized in that an n-type dopant is δ-doped.

  According to the above configuration, since the channel layer 5 is δ-doped with n-type dopant, electrons can be supplied to the interface between the superlattice buffer layer 4 and the channel layer 5 to sufficiently compensate the holes at this interface. . Thereby, it is possible to prevent the two-dimensional hole gas from being formed at the interface.

  Therefore, when the nitride semiconductor epitaxial substrate is used for an electronic device, leakage current can be reduced and device characteristics (for example, life) can be improved.

In the nitride semiconductor epitaxial substrate of one embodiment,
The n-type dopant is Si or Ge.

  According to the embodiment, since the n-type dopant is Si or Ge, electrons can be effectively supplied to the interface between the superlattice buffer layer 4 and the channel layer 5.

  Moreover, since the activation rate of Si and Ge is almost 100%, it is easy to control.

In the nitride semiconductor epitaxial substrate of one embodiment,
The δ-doped portion is within a range of 50 nm from the interface between the superlattice buffer layer 4 and the channel layer 5.

  According to the above embodiment, since the δ-doped portion is within a range of 50 nm from the interface between the superlattice buffer layer 4 and the channel layer 5, the positive interface between the superlattice buffer layer 4 and the channel layer 5 is correct. The hole can be reliably compensated.

  Although the density of the two-dimensional hole gas has a local peak, even if δ doping with an n-type dopant is performed away from the position where this peak occurs, not only the hole at the above position can be compensated but also a new electron. A channel is formed.

In the nitride semiconductor epitaxial substrate of one embodiment,
The concentration of the δ doping is 95% to 105% of the carrier concentration obtained by CV measurement of the interface between the superlattice buffer layer 4 and the channel layer 5 when the channel layer 5 is not subjected to the δ doping. Within range.

  According to the embodiment, the δ doping concentration is 95% of the carrier concentration obtained by CV measurement of the interface between the superlattice buffer layer 4 and the channel layer 5 when the channel layer 5 is not subjected to δ doping. Since it is in the range of ˜105%, it is possible to prevent formation of an electronic layer that becomes a new leak source while compensating for holes.

The nitride semiconductor device of the present invention is
Substrate 1;
An initial growth layer 2 formed on the substrate 1;
A buffer layer 3 formed on the initial growth layer 2;
A high Al content layer formed on the buffer layer 3 and having a composition of Al _x Ga _1-x N (0.5 ≦ x ≦ 1.0), and Al _y Ga _1-y N (0 ≦ y ≦ Superlattice buffer layer 4 formed by alternately laminating low Al-containing layers having a composition of 0.3),
A channel layer 5 formed on the superlattice buffer layer 4 and made of a nitride semiconductor layer;
A barrier layer 6 formed on the channel layer 5;
The channel layer 5 in the vicinity of the interface with the superlattice buffer layer 4 is characterized in that an n-type dopant is δ-doped.

  According to the above configuration, the channel layer 5 in the vicinity of the interface with the superlattice buffer layer 4 is supplied with electrons to the interface between the superlattice buffer layer 4 and the channel layer 5 by being n-doped with δ doping. And since the hole of this interface can fully be compensated, it can prevent that two-dimensional hole gas is formed in the said interface.

  Therefore, the nitride semiconductor device can reduce leakage current and improve device characteristics.

  According to the present invention, it is possible to suppress the generation of two-dimensional hole gas due to the superlattice buffer layer, and for example, it is possible to improve the leakage characteristics of the transistor.

1 Si substrate 2 Initial growth layer 3 Buffer layer 4 Superlattice buffer layer 5 Channel layer 6 Barrier layer

Claims (5)

A substrate,
An initial growth layer formed on the substrate;
A buffer layer formed on the initial growth layer;
A high Al content layer formed on the buffer layer and having a composition of Al _x Ga _1-x N (0.5 ≦ x ≦ 1.0), and Al _y Ga _1-y N (0 ≦ y ≦ 0). .3) a superlattice buffer layer formed by alternately laminating low Al-containing layers having the composition;
A channel layer formed on the superlattice buffer layer and made of a nitride semiconductor layer;
A barrier layer formed on the channel layer,
A nitride semiconductor epitaxial substrate, wherein the channel layer in the vicinity of the interface with the superlattice buffer layer is doped with n-type dopant by δ. The nitride semiconductor epitaxial substrate according to claim 1,
The nitride semiconductor epitaxial substrate, wherein the n-type dopant is Si or Ge. The nitride semiconductor epitaxial substrate according to claim 1 or 2,
The nitride semiconductor epitaxial substrate, wherein the δ-doped portion is within a range of 50 nm from the interface between the superlattice buffer layer and the channel layer. In the nitride semiconductor epitaxial substrate according to any one of claims 1 to 3,
The concentration of the δ doping is in the range of 95% to 105% of the carrier concentration obtained by CV measurement at the interface between the superlattice buffer layer and the channel layer when the channel layer is not subjected to the δ doping. There is provided a nitride semiconductor epitaxial substrate. A substrate,
An initial growth layer formed on the substrate;
A buffer layer formed on the initial growth layer;
A high Al content layer formed on the buffer layer and having a composition of Al _x Ga _1-x N (0.5 ≦ x ≦ 1.0), and Al _y Ga _1-y N (0 ≦ y ≦ 0). .3) a superlattice buffer layer formed by alternately laminating low Al-containing layers having the composition;
A channel layer formed on the superlattice buffer layer and made of a nitride semiconductor layer;
A barrier layer formed on the channel layer,
A nitride semiconductor device, wherein the channel layer in the vicinity of the interface with the superlattice buffer layer is doped with an n-type dopant by δ.

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