Semiconductor device

Abstract

The present invention comprises a buffer layer (102) that is composed of V-polar (N-polar) AlGaN and is formed upon a substrate (101), a barrier layer (103) that is formed upon the buffer layer (102), a channel layer (104), a gate electrode (105), a source electrode (106), and a drain electrode (107). The barrier layer (103) and the channel layer (104) are composed of a V-polar (N-polar) nitride semiconductor. The buffer layer (102) is composed of AlxGa1-xN (0 < x ≤ 0.8) and has a thickness of 500 nm or greater.

Description

Semiconductor Devices

[0001] The present invention relates to a semiconductor device.

[0002] Transistors such as heterojunction field effect transistors (HFETs) and high electron mobility transistors (HEMTs) are turned on and off by changing the carrier density in the channel layer using an electric field generated by a gate voltage.

[0003] When this type of transistor is constructed from a nitride semiconductor, a heterostructure such as AlGaN / GaN is used, in which a two-dimensional electron gas (2DEG) is used as a channel. The two-dimensional electron gas (2DEG) is formed by electrons gathering at the interface between AlGaN and GaN to compensate for the difference in polarization between the two.

[0004] In a typical GaN-based HEMT, a gate electrode is formed on an AlGaN layer several nanometers to several tens of nanometers thick to control the 2DEG concentration at the AlGaN / GaN interface. For high-frequency applications of these transistors, it is important to confine carriers to the thin region at the AlGaN / GaN interface and eliminate other leakage paths. This speeds up the response to voltage applied to the gate electrode and achieves stable operation.

[0005] Meanwhile, HEMTs using group III nitride semiconductors formed by crystal growth in the +c-axis direction on a substrate have a structure in which an AlGaN layer is stacked on a GaN layer. This results in the AlGaN layer, which has a large bandgap, being located on the side where the source and drain electrodes (ohmic electrodes) are formed, resulting in issues such as high contact resistance and the inability to thin the AlGaN layer to maintain carrier density, which leads to a short channel effect. These issues hinder the improvement of the high-frequency characteristics of HEMTs using nitride semiconductors such as GaN.

[0006] To solve the above-mentioned problems, several techniques have been investigated, such as first, regrowth of the region directly under the ohmic electrode to reduce the contact resistance, and second, increasing the Al composition to make the AlGaN layer thinner to suppress the short channel effect. However, these techniques have limitations in reducing the ohmic contact resistance.

[0007] Here, a nitride semiconductor with group V polarity formed by crystal growth in the −c-axis direction on a substrate, for example, a GaN layer with group V polarity, is a crystal obtained by inverting a GaN layer with group III polarity, and a HEMT constructed using such a nitride semiconductor with group V polarity has the following three advantages:

[0008] First, the AlGaN layer, which requires a high Al composition and a thickness of about 20 nm to supply carriers and has high resistance, is located below the GaN layer and is not disposed between the ohmic electrode and the channel (2DEG), thereby reducing the contact resistance.

[0009] Secondly, the thickness of the GaN layer on the side where the ohmic electrode is formed does not have a large effect on the carrier density, so that the short channel effect can be suppressed by making the layer thin.

[0010] Third, the AlGaN layer directly below the channel acts as a back barrier, suppressing the short channel effect.

[0011] Due to these advantages, further improvement in the high frequency characteristics of HEMTs can be expected by fabricating HEMTs using GaN layers with group V polarity (Non-Patent Document 1).

[0012] T. Ishiguro et al., "New Model of Fe Diffusion in Highly Resistive Fe-Doped Buffer Layer for GaN High-Electron-Mobility Transistor," Japanese Journal of Applied Physics, vol. 52, 08JB17, 2013. A. Hayasaka et al., "Reduced Buffer Leakage and Improvement of Three-Terminal Characteristics in Group V Polarity GaN HEMTs," Proceedings of the 84th Autumn Meeting of the Japan Society of Applied Physics, 23a-B201-7.

[0013] However, the above-mentioned technology has a problem called buffer leakage. When group V polarity GaN is epitaxially grown, the oxygen concentration increases, unlike in the case of group III polarity. Furthermore, because these conditions are prone to the formation of pits, a type of crystal defect, the crystal quality is inferior to that of HEMTs using group III polarity GaN. In particular, a thick buffer layer introduced to reduce crystal defects is more likely to cause buffer leakage than in the case of group III polarity (Non-Patent Document 5).

[0014] As a method for improving this type of buffer leakage, a method of doping the buffer layer with Fe or C to increase its resistance has been proposed and adopted. In the case of Fe, doping is often carried out using the organic metal raw material Cp2-Fe as the raw material, and in the case of C, doping is often carried out using C contained in the methyl group of the Ga raw material. These function to form a level in the midgap of the nitride semiconductor and deactivate carriers, thereby increasing the resistance.

[0015] However, increasing the resistance of the buffer layer by doping with impurities also presents challenges. For example, Fe diffusion into the device layer is problematic. Furthermore, C doping increases the on-resistance during HEMT operation, and introduces new challenges, such as current collapse, which reduces the drain current.

[0016] The present invention has been made to solve the above problems, and has as its object to suppress buffer leakage in semiconductor devices such as transistors that use nitride semiconductors of group V polarity.

[0017] The semiconductor device according to the present invention includes a buffer layer made of AlGaN with group V polarity formed on a substrate, a barrier layer made of AlGaN with group V polarity formed on the buffer layer, a channel layer made of a nitride semiconductor with group V polarity formed on the barrier layer, a gate electrode formed on the channel layer, and source and drain electrodes formed on the channel layer.

[0018] As described above, according to the present invention, the buffer layer is made of AlGaN with group V polarity, so that buffer leakage can be suppressed in semiconductor devices such as transistors that use nitride semiconductors with group V polarity.

[0019] Fig. 1 is a cross-sectional view showing the structure of a semiconductor device according to an embodiment of the present invention. Fig. 2 is a characteristic diagram showing the electron mobility in an AlGaN layer with respect to changes in the flow rate of an Al source material used in crystal growth of an AlGaN layer and changes in the flow rate of a Ga source material. Fig. 3 is a characteristic diagram showing the electron mobility in an AlGaN layer with respect to changes in the flow rate of a completely lattice-relaxed Al source material. x Ga 1-x The critical thickness of the GaN layer in the structure in which the GaN layer is sandwiched between N layers is x Ga 1-x FIG. 10 is a characteristic diagram showing the change in N relative to the Al composition x.

[0020] A semiconductor device according to an embodiment of the present invention will be described below with reference to FIG.

[0021] This semiconductor device includes a buffer layer 102 made of AlGaN with group V polarity (N polarity) formed on a substrate 101. The buffer layer 102 is made of Al x Ga 1-x N (0<x≦0.8) and has a thickness of 500 nm or more. The semiconductor device also includes a barrier layer 103, a channel layer 104, a gate electrode 105, a source electrode 106, and a drain electrode 107.

[0022] The barrier layer 103 is formed on the buffer layer 102. The barrier layer 103 is made of AlGaN with group V polarity (N polarity). The barrier layer 103 can be made of AlGaN with a high Al composition, InAlN, InAlGaN, AlScN, or the like, in order to form a sheet carrier (2DEG) in the channel layer 104 by the polarization effect.

[0023] If there is a difference in Al composition between the buffer layer 102 and the barrier layer 103 (i.e., the Al composition of the barrier layer 103 is greater than the Al composition of the buffer layer 102), a two-dimensional hole gas (2DHG) may form at the interface between the buffer layer 102 and the barrier layer 103 due to polarization effects, potentially impairing device characteristics. Even if a 2DHG is not formed, the band may shift to a higher energy side, which may lead to increased buffer leakage. To avoid this, the barrier layer 103 can be made of compositionally graded AlGaN. This structure gradually increases the Al composition from the buffer layer 102 side to the channel layer 104 side, preventing the formation of a 2DHG.

[0024] Doping the buffer layer 102 and the barrier layer 103 with Si is also effective. Making these layers n-type can suppress the formation of 2DHG and also makes them effective as carrier supply layers to the channel layer 104. When doping with Si to make them n-type, buffer leakage is likely to increase in the region under the channel, so it is desirable to introduce an appropriate element isolation structure.

[0025] Furthermore, a very thin spacer layer made of AlN can be formed at the interface between the channel layer 104 and the barrier layer 103. This configuration can suppress the influence of mixed crystal scattering between the channel layer 104 and the barrier layer 103 (at the interface), thereby improving mobility.

[0026] The channel layer 104 is formed on the barrier layer 103. The channel layer 104 is composed of a nitride semiconductor with group V polarity (N polarity). The channel layer 104 can be composed of GaN, InN, InGaN, or AlGaN, which has a lower Al composition than the buffer layer 102. AlGaN can be used to increase the breakdown voltage, and InN or InGaN can be used to increase the speed. In either case, it is important to make the Al composition lower than that of the back barrier layer. The thickness of the channel layer 104 affects the control by the gate electrode 105, so it is important to make it thinner if the goal is to increase the speed.

[0027] The gate electrode 105 is formed on the channel layer 104. In this example, the gate electrode 105 is formed on the top barrier layer 108. When the channel layer 104 is exposed, gate leakage may be large and uncontrollable. By inserting the top barrier layer 108 between the gate electrode 105 and the channel layer 104, it is possible to suppress the gate leakage.

[0028] The top barrier layer 108 must be designed to an appropriate thickness, since if it is too thick, it may impair control by the gate electrode 105. Furthermore, since the top barrier layer 108 is introduced to suppress gate leakage, it is desirable for the top barrier layer 108 to be composed of a material with a larger band gap than the channel layer 104. The top barrier layer 108 can be composed of, for example, AlGaAs. Materials such as AlGaN can be formed on the channel layer 104 by epitaxial growth. The top barrier layer 108 can also be composed of various dielectric materials, such as SiN, SiO 2 , and Al 2 O 3 . In particular, constructing the top barrier layer 105 from a dielectric material with a high dielectric constant increases the controllability of the gate electrode 105, enabling higher performance.

[0029] The source electrode 106 and the drain electrode 107 are formed on the channel layer 104. In this example, the source electrode 106 and the drain electrode 107 are formed on the channel layer 104 via a contact layer 109. The contact layer 109 can be made of, for example, GaN that has been made n-type by doping it with a high concentration of Si.

[0030] In this semiconductor device, electrons gather in the channel layer 104 near the heterojunction interface between the barrier layer 103 and the channel layer 104 to compensate for the difference in polarization between these layers, forming a two-dimensional electron gas (2DEG). This semiconductor device is a field-effect transistor with the above-mentioned 2DEG as a channel. Furthermore, by constructing this field-effect transistor from an N-polarity nitride semiconductor, the channel layer 104 is disposed on the side of the source electrode 106 and the drain electrode 107, thereby reducing the contact resistance between the source electrode 106 and the drain electrode 107 and the channel formed by the 2DEG.

[0031] Here, the substrate 101 is desired to enable crystal growth of an N-polarity nitride semiconductor, and further, to enable crystal growth of the buffer layer 102 made of AlGaN with a high Al composition, to have a smaller lattice constant than GaN. Note that the lattice constant is measured in a direction parallel to the plane of the substrate 101 (in-plane direction). Examples of substrates that satisfy the above-mentioned required characteristics include a sapphire substrate, a C-polarity hexagonal SiC substrate, and an N-polarity AlN substrate.

[0032] Furthermore, in order to perform high-quality crystal growth of an N-polar nitride semiconductor, the substrate 101 may have a main surface tilted (off-angled) from the C-plane. For example, it may be possible to use a substrate 101 with an off-angle of 0.5 degrees or more and 5 degrees or less. The off-angle may be formed in a direction that is off with respect to the m-plane of the GaN or AlGaN crystal nuclei.

[0033] For example, on a sapphire substrate, the crystals of AlGaN or GaN grow rotated by 30 degrees relative to the sapphire crystal, so that the off-axis can be formed in the a-axis direction of the sapphire crystal. Also, a hexagonal SiC substrate and an N-polar AlN substrate can have an off-axis in the m-axis direction.

[0034] Next, the formation of the buffer layer 102 will be described. As is well known, nitride semiconductors typically grow with group III polarity on a substrate 101 made of sapphire, a non-polar crystal, with the C-plane as the main surface. For this reason, the surface of the sapphire substrate 101 is first nitrided. For example, the surface of the substrate 101 can be nitrided by heating the substrate 101 to 950 to 1100°C in an ammonia atmosphere (for a treatment time of several minutes).

[0035] A semiconductor layer made of AlN or AlGaN is formed on the surface of the nitrided sapphire substrate 101. For example, by growing AlN or AlGaN at a low temperature of, for example, about 500 to 800°C, a semiconductor layer can be formed in which crystalline regions are dispersed in a small portion of the amorphous material. The semiconductor layer thus formed has an N-polarity. A buffer layer 102 made of N-polar AlGaN can be formed by subsequently growing AlGaN crystals on the semiconductor layer thus grown, for example, by metalorganic chemical vapor deposition. The buffer layer 102 is formed on and in contact with the semiconductor layer described above. The AlN mole fraction (Al composition) of the AlGaN constituting the buffer layer 102 is set to be greater than 0. The buffer layer 102 can include the semiconductor layer described above.

[0036] By forming the buffer layer 102 as described above, it is possible to form the buffer layer 102 in a strain-free or small-strain state. Furthermore, since the lattice constant is fixed to the buffer layer 102, the channel layer 104 grown on the buffer layer 102 is formed in a state in which compressive strain is applied in the in-plane direction due to its larger lattice constant than AlGaN. In addition, since the thickness of the channel layer 104 is set to a maximum of 100 nm or less, if the buffer layer 102 is made of strain-free AlGaN, it can be formed without introducing crystal defects due to strain relaxation.

[0037] Here, forming a GaN layer between the substrate 101 and the buffer layer 102 is undesirable because it can cause problems such as cracks in the buffer layer 102. When a GaN layer is formed on the substrate 101, the lattice constants of the barrier layer 103 and the channel layer 104 are fixed to the lattice constant of the GaN layer. When a buffer layer 102 made of AlGaN, which has a smaller lattice constant than GaN, is formed on a GaN layer fixed to the lattice constant of unstrained GaN, an in-plane tensile strain is applied to the buffer layer 102. Therefore, if the buffer layer 102 is formed thicker than the critical thickness, cracks will occur due to relaxation of the tensile strain.

[0038] It is important to form the buffer layer 102 thick in order to suppress buffer leakage. Therefore, as described above, if a GaN layer is formed between the substrate 101 and the buffer layer 102, the buffer layer 102 cannot be formed thick enough to have high resistance, and a sufficient buffer leakage suppression effect cannot be obtained.

[0039] Next, the effect of the buffer layer 102 will be described with reference to FIG. 2. FIG. 2 shows the electron mobility in the AlGaN layer relative to changes in the flow rate of the Al source material and the Ga source material used in crystal growth of the AlGaN layer. Here, trimethylaluminum (TMA) was used as the Al source material, and trimethylgallium (TMGa) was used as the Ga source material. A sapphire substrate was used as the substrate 101, and the AlGaN layer was formed via the low-temperature grown semiconductor layer described above. Mobility values ​​are shown in FIG. 2. The Al composition in the AlGaN layer increases from the upper left to the lower right of the graph in FIG. 2.

[0040] As shown in FIG. 2, when the GaN layer was formed without supplying TMA, the mobility was about 500 cm 2 On the other hand, the AlGaN layer formed with the supply amount of TMA set to 10 cc and the supply amount of TMG set to 24 cc has a mobility of about 50 cm 2 / Vs. It can be seen that the mobility decreases as the Al composition in the AlGaN layer increases. This is thought to be due to the increase in the band gap of the material, the increase in residual carrier density (described later), and the decrease in mobility due to intercrystallization.

[0041] The thickness of the buffer layer 102 can be any thickness as long as it achieves high resistance. However, in the case of a substrate made of a heterogeneous material such as sapphire or SiC, crystal defects are introduced due to relaxation of the large lattice mismatch. Therefore, in order to reduce crystal defects, it is preferable to grow the crystal to a certain thickness and promote a reduction in defect density through pair annihilation. Furthermore, by growing the buffer layer 102 to a thickness of at least 500 nm, the strain in the buffer layer 102 can be almost completely relaxed, which facilitates the design of strain in layers above the buffer layer 102 and ensures the reliability of the semiconductor device. Therefore, the buffer layer 102 can be made 500 nm or thicker in order to function as a high-resistance buffer layer while reducing crystal defect density.

[0042] Next, the allowable Al composition and thickness will be described. x Ga 1-x As long as N (0<x≦0.8) is satisfied, the semiconductor device according to the embodiment can be realized regardless of the Al composition.

[0043] On the other hand, GaN constituting the channel layer 104 has a larger lattice constant than AlGaN and is subjected to compressive strain. Therefore, in order to avoid impairing the crystal quality of the channel layer 104, the channel layer 104 must be designed to have a thickness equal to or less than the critical thickness.

[0044] Fully relaxed Al x Ga 1-x The critical thickness of the GaN layer in the structure in which the GaN layer is sandwiched between the N layers is defined as Al x Ga 1-x The results of plotting N against the Al composition x are shown in Figure 3. x Ga 1-xAs the Al composition of the N layer increases, the critical thickness, i.e., the thickness of the GaN layer that can be grown without introducing crystal defects, decreases. In an actual HEMT structure, materials with even higher Al compositions are used for the barrier layer 103 and the top barrier layer 108, so it is necessary to set an even stricter (smaller) channel layer thickness.

[0045] The thickness of the channel layer 104 may be set to a small value of about several nm in the region directly under the gate when a recess structure is formed, but by setting the thickness to about 10 nm in other regions, the effects of point defects and electron scattering from the surface side can be suppressed and high mobility can be ensured. Many N-polarity HEMTs have a channel layer thickness of 10 nm or more (References 1, 2, and 3). From this perspective, it is desirable for the thickness of the channel layer 104 to be 10 nm.

[0046] As can be seen from Figure 3, the fully lattice-relaxed Al x Ga 1-x When the Al composition of the N layer is 0.8, the critical thickness of the sandwiched GaN layer is 9.6 nm. If the thickness of the sandwiched GaN layer is greater than this, the critical thickness will be exceeded, introducing crystal defects and degrading device characteristics such as mobility, reliability, and breakdown voltage. For this reason, the Al composition of the buffer layer 102 in the semiconductor device according to the embodiment can be set to a maximum of 0.8.

[0047] As described above, according to the present invention, the buffer layer is made of AlGaN with group V polarity, so that buffer leakage can be suppressed in semiconductor devices such as transistors that use nitride semiconductors with group V polarity.

[0048] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention.

[0049] [Reference 1] E. Akso et al., "First Demonstration of Four-Finger N-polar GaN HEMT Exhibiting Record 712-mW Output Power With 31.7% PAE at 94 GHz", IEEE Microwave and Wireless Technology Letters, vol. 33, no. 6, pp. 683-686, 2023. [Reference 2] AA Purdue et al., "N-Polar GaN HEMTs in a High-Uniformity 100-mm Wafer Process With 43.6% Power-Added Efficiency and 2 W / mm at 94 GHz", IEEE Microwave and Wireless Technology Letters, vol. 33, no. 7, pp. 1011-1014, 2023. [Reference 3] W. Li et al., "Record RF Power Performance at 94 GHz From Millimeter-Wave N-Polar "GaN-on-Sapphire Deep-Recess HEMTs", IEEE Transactions on Electron Devices, vol. 70, no. 4, pp. 2075-2080, 2023.

[0050] 101...substrate, 102...buffer layer, 103...barrier layer, 104...channel layer, 105...gate electrode, 106...source electrode, 107...drain electrode, 108...top barrier layer, 109...contact layer.

Claims

1. A semiconductor device comprising: a buffer layer made of AlGaN with group V polarity formed on a substrate; a barrier layer made of AlGaN with group V polarity formed on the buffer layer; a channel layer made of a nitride semiconductor with group V polarity formed on the barrier layer; a gate electrode formed on the channel layer; and a source electrode and a drain electrode formed on the channel layer.

2. The semiconductor device according to claim 1, wherein the buffer layer is Al x Ga 1-x N (0<x≦0.8) and has a thickness of 500 nm or more.

3. A semiconductor device according to claim 1 or 2, wherein the channel layer is made of any of GaN, InN, InGaN, and AlGaN having a smaller Al composition than the buffer layer.

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