JP7779207B2 - Nitride semiconductor wafer and manufacturing method thereof - Google Patents

Description

The present invention relates to nitride semiconductor wafers, and in particular to nitride semiconductor wafers with reduced warpage.

Nitride semiconductor wafers, which have an initial AlN layer, a buffer layer, and a GaN-HEMT structure epitaxial layer stacked in that order on a silicon single crystal substrate, are used as epitaxial substrates for power devices and RF devices.

Wafer fabricated by epitaxially growing nitride semiconductors on SOI substrates are also used, but epitaxial growth on SOI substrates results in greater wafer warpage than on silicon single crystal substrates. To suppress warpage, it is necessary to devise a buffer layer and reduce warpage while monitoring in-situ warpage data (Patent Document 1).

Silicon single crystal substrates can be made with relatively little warping, but when epitaxially growing nitride semiconductors on SOI substrates, it is difficult to suppress warping.

Patent No. 6473017

The present invention has been made to solve the above problems, and aims to provide a nitride semiconductor wafer with reduced warpage.

In order to solve the above problems, the present invention provides:
A nitride semiconductor wafer comprising: a silicon-based substrate; a buffer layer made of a nitride semiconductor laminated on the silicon-based substrate; and a functional layer including at least a GaN layer laminated on the buffer layer,
the buffer layer is doped with Fe, and the Fe concentration distribution in the stacking direction of the buffer layer has a point where the Fe concentration is maximum and decreases from the point where the Fe concentration is maximum toward the functional layer;
The nitride semiconductor wafer has an Fe concentration of 2.5×10 ¹⁸ atoms/cm ³ or more and 6.0×10 ¹⁸ atoms/cm ³ or less at the point where the Fe concentration is maximum, and an Fe concentration of 4.0×10 ¹⁷ atoms/cm ³ or less on the upper surface of the buffer layer facing the functional layer.

Doping with Fe with such a concentration distribution makes it possible to suppress warpage without changing the structure of the nitride semiconductor wafer. If the highest Fe concentration in the buffer layer is 2.5×10 ¹⁸ atoms/cm ³ or more, warpage can be significantly suppressed. Furthermore, by setting the highest Fe concentration in the buffer layer to 6.0×10 ¹⁸ atoms/cm ³ or less, the Fe concentration in the upper surface of the buffer layer can be set to 4.0×10 ¹⁷ atoms/cm ³ or less. If the Fe concentration in the upper surface of the buffer layer is 4.0×10 ¹⁷ atoms/cm ³ or less, it is possible to prevent the Fe concentration in the device surface from becoming high due to the memory effect of Fe, which would deteriorate the device characteristics.

Furthermore, it is preferable that the silicon-based substrate is a silicon single crystal substrate or an SOI (silicon-on-insulator) substrate.

The present invention can be used with such silicon-based substrates, but the present invention is particularly effective when using SOI substrates, as these substrates tend to warp significantly.

Furthermore, it is preferable that the buffer layer consists of an AlGaN layer and a superlattice layer in which GaN layers and AlN layers are alternately stacked.

This type of structure is more effective in suppressing warping.

The present invention also provides a method for producing a nitride semiconductor wafer, comprising the steps of:
(1) a step of stacking a buffer layer made of a nitride semiconductor on a silicon-based substrate; and (2) a step of stacking a functional layer including at least a GaN layer on the buffer layer to produce a nitride semiconductor wafer,
In the step (1), a doping gas for doping with Fe is supplied, and a flow rate of the doping gas is adjusted to make the Fe concentration distribution in the stacking direction in the buffer layer have a point where the Fe concentration is maximum and decrease from the point where the Fe concentration is maximum toward the functional layer;
The present invention provides a method for manufacturing a nitride semiconductor wafer, in which the Fe concentration at the point where the Fe concentration is maximum is set to 2.5×10 ¹⁸ atoms/cm ³ or more and 6.0×10 ¹⁸ atoms/cm ³ or less, and the Fe concentration on the upper surface of the buffer layer facing the functional layer is set to 4.0×10 ¹⁷ atoms/cm ³ or less.

The nitride semiconductor wafer of the present invention with reduced warpage can be manufactured in this manner.

Furthermore, in step (1), the silicon-based substrate is preferably a silicon single crystal substrate or an SOI (silicon-on-insulator) substrate.

The present invention can be used with such silicon-based substrates, but the present invention is particularly effective when using SOI substrates, as these substrates tend to warp significantly.

Furthermore, in step (1), it is preferable that the buffer layer be composed of an AlGaN layer and a superlattice layer in which GaN layers and AlN layers are alternately stacked.

This type of structure is more effective in suppressing warping.

As described above, the present invention can provide a nitride semiconductor wafer in which a nitride semiconductor is epitaxially grown on a silicon single crystal substrate or an SOI substrate, and which suppresses warping without changing the structure of the buffer layer, etc., as well as a method for manufacturing the same.

1 is a schematic diagram showing an example of a nitride semiconductor wafer of the present invention. 1 shows the results of measuring the Fe concentration distribution in the stacking direction of the nitride semiconductor wafers (silicon-based substrates: silicon single crystal substrates) produced in Examples 1 and 2 by SIMS. 1 shows the results of measuring the Fe concentration distribution in the stacking direction of nitride semiconductor wafers (silicon-based substrates: SOI substrates) produced in Examples 1 and 2 by SIMS.

As mentioned above, there was a need to develop nitride semiconductor wafers with reduced warpage.

As a result of extensive research into the above-mentioned issues, the inventors discovered that warpage of nitride semiconductor wafers can be suppressed by appropriately controlling the Fe concentration distribution in the buffer layer of the nitride semiconductor wafer, leading to the completion of the present invention.

That is, the present invention provides a nitride semiconductor wafer including a silicon-based substrate, a buffer layer made of a nitride semiconductor stacked on the silicon-based substrate, and a functional layer including at least a GaN layer stacked on the buffer layer, wherein the buffer layer is doped with Fe, the Fe concentration distribution in the stacking direction in the buffer layer has a point at which the Fe concentration is maximum, and is a concentration distribution such that the Fe concentration decreases from the point at which the Fe concentration is maximum towards the functional layer, the Fe concentration at the point at which the Fe concentration is maximum is 2.5× ¹⁰ atoms/cm ^or more and 6.0× ¹⁰ atoms/cm ^or less, and the Fe concentration of the upper surface of the buffer layer facing the functional layer is 4.0× ¹⁰ atoms/cm ^or less.

The present invention is described in detail below, but is not limited to these.

[Nitride semiconductor wafer]
The nitride semiconductor wafer of the present invention will be described with reference to Fig. 1. Note that the structure of the nitride semiconductor wafer shown in Fig. 1 is an example, and the present invention is not limited to this.

The nitride semiconductor wafer 10 shown in FIG. 1 has a silicon substrate 1, a buffer layer 3 made of a nitride semiconductor layered on the silicon substrate 1, and a functional layer 4 including at least a GaN layer layered on the buffer layer 3. The functional layer 4 is composed of, for example, a channel layer (C-GaN) made of GaN and a barrier layer (not shown) made of AlGaN with a different band gap from that of the channel layer. It is also preferable to form a high-resistivity GaN layer (breakdown layer: R-GaN) between the buffer layer 3 and the channel layer.

Here, the silicon-based substrate 1 is not particularly limited, but is preferably a silicon single crystal substrate or an SOI (silicon-on-insulator) substrate, such as a 150 mm diameter, 675 μm, (111) Si substrate or a 150 mm diameter, 675 μm, (111) SOI substrate.

An initial layer 2 made of AlN and having a thickness of 100 to 200 nm may be provided between the silicon substrate 1 and the buffer layer 3.

The buffer layer 3 is not particularly limited, but is preferably composed of an AlGaN layer and a superlattice layer in which GaN and AlN layers are alternately stacked. For example, it can be composed of superlattice layers (SLs) in which, for example, 23 pairs of GaN layers with thicknesses of 5 to 30 nm and AlN layers with thicknesses of 3 to 10 nm are alternately stacked on an AlGaN layer with thicknesses of 100 to 200 nm.

In the nitride semiconductor wafer of the present invention, the buffer layer 3 is doped with Fe, and the Fe concentration distribution in the stacking direction in the buffer layer 3 has a point where the Fe concentration is maximum and decreases from the point where the Fe concentration is maximum toward the functional layer 4, with the Fe concentration at the point where the Fe concentration is maximum being 2.5× ¹⁰ atoms/cm ^or more and 6.0× ¹⁰ atoms/cm ^or less, and the Fe concentration on the upper surface of the buffer layer 3 facing the functional layer 4 being 4.0× ¹⁰ atoms/cm ^or less. In the present invention, the Fe concentration distribution can be determined from the results of SIMS measurement.

The Fe concentration distribution in the buffer layer will be described more specifically with reference to Figures 2 and 3, which show the results of the Examples described below. Figure 2 shows the Fe concentration distribution measured by SIMS for a nitride semiconductor wafer epitaxially grown on a silicon single crystal substrate. Figure 3 shows the Fe concentration distribution when epitaxially grown on an SOI substrate under similar conditions. In both Figures 2 and 3, the buffer layer has a point where the Fe concentration is maximum (near a depth of 1.2 μm on the horizontal axis in the figure). From this point, the Fe concentration decreases (gradually decreases) toward the functional layer side, and the Fe concentration at the upper surface of the buffer layer on the functional layer side (near a depth of 0.55 μm on the horizontal axis in the figure) is 4.0 × 10 ¹⁷ atoms/cm ³ or less. As can be seen, the Fe concentration peaks in the buffer layer and decreases (gradually decreases) toward the device layer.

If the maximum Fe concentration is lower than 2.5×10 ¹⁸ atoms/cm ³ , no significant warpage suppression effect is observed, and if it exceeds 6.0×10 ¹⁸ atoms/cm ³ , it becomes difficult to keep the concentration at the top surface of the buffer layer at 4.0×10 ¹⁷ atoms/cm ³ or less. If the concentration at the top surface of the buffer layer exceeds 4.0×10 ¹⁷ atoms/cm ³ , the Fe memory effect increases the Fe concentration at the device surface, deteriorating the device characteristics.

To achieve such an Fe concentration distribution, a doping gas such as _Cp2Fe (biscyclopentadienyl iron) can be flowed in an MOCVD apparatus during epitaxial growth of, for example, the lower third of the superlattice layer and the underlying AlGaN layer, and the flow rate can be adjusted to dope Fe to a desired concentration. Doping with Fe in this way can suppress warpage without changing the structure of the buffer layer not only when an SOI substrate is used, but also when a silicon single crystal substrate is used, but a more significant effect is obtained when an SOI substrate is used.

As the functional layer 4, a high-resistivity GaN layer (breakdown layer) with a thickness of 300 to 900 nm is preferably provided on the buffer layer 3. By providing such a high-resistivity layer between the device layer and the buffer layer, it is possible to more reliably suppress the deterioration of the current collapse phenomenon and lateral leakage current at high temperatures, and it is also possible to more reliably suppress the incorporation of Fe into the channel layer, thereby preventing degradation of forward characteristics such as a decrease in mobility. A channel layer made of a GaN layer, which serves as the device layer, is formed on the high-resistivity GaN layer. Although not shown in Figure 1, a barrier layer made of an AlGaN layer can be formed on the channel layer made of GaN, and by providing a source electrode, drain electrode, and gate electrode, for example, a high electron mobility transistor (HEMT) can be created.

[Method for manufacturing nitride semiconductor wafer]
The present invention also provides a method for producing a nitride semiconductor wafer, comprising: (1) a step of stacking a buffer layer made of a nitride semiconductor on a silicon-based substrate; and (2) a step of stacking a functional layer including at least a GaN layer on the buffer layer to produce a nitride semiconductor wafer, wherein in the step (1), a doping gas for doping with Fe is flowed and a flow rate of the doping gas is adjusted to have an Fe concentration distribution in the stacking direction in the buffer layer that has a point at which the Fe concentration is maximum and that decreases from the point at which the Fe concentration is maximum toward the functional layer, so that the Fe concentration at the point at which the Fe concentration is maximum is 2.5× ¹⁰ atoms/cm ^or more and 6.0× ¹⁰ atoms/cm ^or less, and the Fe concentration on the upper surface of the buffer layer facing the functional layer is 4.0× ¹⁰ atoms/cm ^or less.

The nitride semiconductor wafer of the present invention can be manufactured in this manner. The method for manufacturing the nitride semiconductor wafer of the present invention is described in detail below.

[Step (1)]
Step (1) is a step of depositing a buffer layer made of a nitride semiconductor on a silicon-based substrate.

In this process, first, a silicon-based substrate such as a silicon single crystal substrate or an SOI (silicon-on-insulator) substrate is prepared. Next, a buffer layer made of a nitride semiconductor is epitaxially grown on the silicon-based substrate in an MOCVD apparatus. Alternatively, an initial layer made of AlN may be epitaxially grown on the silicon-based substrate, and then a buffer layer made of a nitride semiconductor may be epitaxially grown on top of that. As described above, the buffer layer preferably consists of an AlGaN layer and a superlattice layer in which GaN and AlN layers are alternately stacked.

During epitaxial growth, trimethylaluminum (TMAl) can be used as an Al source, TMGa can be used as a Ga source, and _NH can be used as an N source, but is not limited to these. The carrier gas can be _N and/or _H , and the process temperature is preferably set to, for example, about 900 to 1200°C.

In the present invention, the Fe concentration distribution in the stacking direction in the buffer layer has a point where the Fe concentration is maximum and decreases from the point where the Fe concentration is maximum toward the functional layer, and the Fe concentration at the point where the Fe concentration is maximum is 2.5× ¹⁰ atoms/cm ^or more and 6.0× ¹⁰ atoms/cm or ^less , and the Fe concentration on the upper surface of the buffer layer facing the functional layer is 4.0× ¹⁰ atoms/cm ^or less.

To achieve such an Fe concentration distribution, a doping gas such as _Cp2Fe (biscyclopentadienyl iron) is flowed in an MOCVD apparatus during epitaxial growth of, for example, the lower third of the superlattice layer and the underlying AlGaN layer, and the flow rate is adjusted to dope Fe to the desired concentration.

[Step (2)]
Step (2) is a step of manufacturing a nitride semiconductor wafer by stacking a functional layer including at least a GaN layer on the buffer layer.

In this process, appropriate functional layers can be epitaxially grown depending on the intended use of the nitride semiconductor wafer. For example, in an MOCVD apparatus, a high-resistivity GaN layer such as the one described above can be epitaxially grown, and then a GaN layer that will serve as the device layer can be epitaxially grown on top of that, and then a barrier layer made of an AlGaN layer can be epitaxially grown on top of that. By providing a source electrode, a drain electrode, and a gate electrode on top of that, it can be made into, for example, a high electron mobility transistor (HEMT).

Nitride semiconductor wafers manufactured in this way have reduced warpage due to the appropriate control of the Fe concentration distribution in the buffer layer.

The present invention will be explained in detail below using examples and comparative examples, but the present invention is not limited to these.

Example 1
As silicon-based substrates, the following silicon single crystal substrates and SOI substrates were prepared.
(1) Silicon single crystal substrate 150 mmφ p-Type (111) 675 μm Oi: 25.6 ppma ASTM 79 5000 Ω cm
(2) SOI substrate 150mmφ SOI layer: 100nm/Box layer: 200nm/Si substrate: 675μm
SOI layer: (111) CZ 1 kΩcm N (nitrogen): 5e14 atoms/cm ³ Oi: 25.6 ppma ASTM 79
Si substrate: (100) 8mΩcm Oi: 16ppma ASTM79

Next, these two wafers were subjected to epitaxial growth in the same batch in an MOCVD reactor, producing nitride semiconductor wafers under the following conditions.

First, a 150 nm thick initial layer of AlN was formed, followed by a 160 nm thick AlGaN layer. Fe doping was initiated by flowing _Cp2Fe at a flow rate of 50 sccm. Next, 25 nm thick GaN layers and 4.2 nm thick AlN layers were grown alternately. The supply of _Cp2Fe was stopped after eight pairs of GaN and AlN layers had been grown. GaN and AlN layers were then grown alternately to form a total of 23 pairs of superlattice layers (SLs), forming a buffer layer consisting of an AlGaN layer and a superlattice layer. Next, a 670 nm thick high-resistivity GaN layer was grown, followed by a 200 nm thick GaN layer, which would become the channel layer.

Thereafter, two wafers were removed and the amount of warpage was measured. The results are shown in Table 1. The Fe concentration in the depth direction was also measured by SIMS. The Fe concentration distribution of the wafer using a silicon single crystal substrate is shown in FIG. 2, and the Fe concentration distribution of the wafer using an SOI substrate is shown in FIG. 3. The maximum Fe concentration in the buffer layer of both wafers was 6.0×10 ¹⁸ atoms/cm ³ , and the Fe concentration on the upper surface of the buffer layer was 4.0×10 ¹⁷ atoms/cm ^3. As is clear from Table 1, the amount of warpage of both wafers in Example 1 was suppressed compared to Comparative Examples 1 and 2, which will be described later.

In this invention, the amount of warpage was measured by placing a flat silicon single crystal substrate as a reference on a three-point support stage, aligning the horizontal surface, and measuring the difference from that plane.

Example 2
Nitride semiconductor wafers were manufactured under the same conditions as in Example 1, except that the flow rate of _Cp2Fe was set to 20 sccm. As shown in Figures 2 and 3, the maximum Fe concentration in the buffer layer of each wafer was 2.5 x ¹⁰¹⁸ atoms/ ^cm3 , and the Fe concentration on the upper surface of the buffer layer was 2.5 x ¹⁰¹⁷ atoms/ ^cm3 . As shown in Table 1, it can be seen that in Example 2, the warpage of each wafer was suppressed compared to Comparative Examples 1 and 2, although not as much as in Example 1.

(Comparative Example 1)
Nitride semiconductor wafers were manufactured in the same manner as in Example 1, except that the flow rate of _Cp2Fe was further reduced to set the maximum Fe concentration in the buffer layer to 2.0 x ¹⁰¹⁸ atoms/ ^cm3 . As shown in Table 1, in Comparative Example 1, all wafers showed improved warpage compared to Comparative Example 2, but the effect as significant as in Examples 1 and 2 was not observed.

(Comparative Example 2)
Except for not flowing Cp ₂ Fe, nitride semiconductor wafers were manufactured under the same conditions as in Example 1. As shown in Table 1, it can be seen that all wafers in Comparative Example 2 had larger warpage than those in Examples 1 and 2.

As described above, it has become clear that the present invention can provide a nitride semiconductor wafer in which a nitride semiconductor is epitaxially grown on a silicon single crystal substrate or an SOI substrate, and in which warping is suppressed without changing the structure of the buffer layer, etc., as well as a method for manufacturing the same.

The present invention is not limited to the above-described embodiments. The above-described embodiments are merely examples, and anything that has substantially the same configuration as the technical concept described in the claims of the present invention and exhibits similar effects is within the technical scope of the present invention.

1... Silicon-based substrate, 2... Initial layer, 3... Buffer layer, 4... Functional layer,
10...Nitride semiconductor wafer.

Claims (4)

A nitride semiconductor wafer comprising: a silicon-based substrate; a buffer layer made of a nitride semiconductor laminated on the silicon-based substrate; and a functional layer including at least a GaN layer laminated on the buffer layer,
the buffer layer is doped with Fe, and the Fe concentration distribution in the stacking direction of the buffer layer has a point where the Fe concentration is maximum and decreases from the point where the Fe concentration is maximum toward the functional layer;
the Fe concentration at the point where the Fe concentration is maximum is 2.5×10 ¹⁸ atoms/cm ³ or more and 6.0×10 ¹⁸ atoms/cm ³ or less, and the Fe concentration of the upper surface of the buffer layer on the functional layer side is 4.0×10 ¹⁷ atoms/cm ³ or less ,
The nitride semiconductor wafer is characterized in that the silicon-based substrate is an SOI (silicon-on-insulator) substrate . 2. The nitride semiconductor wafer according to claim 1 , wherein the buffer layer comprises an AlGaN layer and a superlattice layer in which GaN layers and AlN layers are alternately stacked. A method for manufacturing a nitride semiconductor wafer, comprising:
(1) a step of stacking a buffer layer made of a nitride semiconductor on a silicon-based substrate; and (2) a step of stacking a functional layer including at least a GaN layer on the buffer layer to produce a nitride semiconductor wafer,
In the step (1), a doping gas for doping with Fe is supplied, and a flow rate of the doping gas is adjusted to make the Fe concentration distribution in the stacking direction in the buffer layer have a point where the Fe concentration is maximum and decrease from the point where the Fe concentration is maximum toward the functional layer;
The Fe concentration at the point where the Fe concentration is maximum is set to 2.5×10 ¹⁸ atoms/cm ³ or more and 6.0×10 ¹⁸ atoms/cm ³ or less, and the Fe concentration of the upper surface of the buffer layer on the functional layer side is set to 4.0×10 ¹⁷ atoms/cm ³ or less , and
A method for producing a nitride semiconductor wafer , wherein in the step (1), the silicon-based substrate is an SOI (silicon-on-insulator) substrate . 4. The method for producing a nitride semiconductor wafer according to claim 3 , wherein in the step (1), the buffer layer comprises an AlGaN layer and a superlattice layer in which GaN layers and AlN layers are alternately stacked.