The present disclosure relates to a process for fabricating a substrate for the epitaxial growth of a layer of a III-N alloy based on gallium (e.g., a layer of gallium nitride (GaN), of aluminum gallium nitride (AlGaN) or of indium gallium nitride (InGaN)), to a process for fabricating such a layer of III-N alloy, and to a process for fabricating a high-electron-mobility transistor (HEMT) in such a layer of III-N alloy.
III-N semiconductors, in particular, gallium nitride (GaN), aluminum gallium nitride (AlGaN) or indium gallium nitride (InGaN), appear to be particularly promising, in particular, as regards formation of high-power light-emitting diodes (LEDs) and of electronic devices operating at high frequency, i.e., devices such as high-electron-mobility transistors (HEMTs) or other field-effect transistors (FETs).
In so far as these III-N alloys are difficult to find in the form of bulk substrates of large size, they are generally formed by heteroepitaxy, i.e., by epitaxy on a substrate made of a different material.
The selection of such a substrate, in particular, takes into account the difference in lattice parameter and the difference in coefficient of thermal expansion between the material of the substrate and the III-N alloy. Specifically, the larger these differences, the greater the risk of formation, in the gallium nitride, of crystal defects such as dislocations, and the greater the risk of generation of high mechanical stresses, liable to cause excessive strains.
The materials most frequently considered for the heteroepitaxy of III-N alloys are sapphire and silicon carbide (SiC).
Apart from its small difference in lattice parameter with gallium nitride, silicon carbide is particularly preferred for high-power electronic applications because of its thermal conductivity, which is clearly higher than that of sapphire, and which therefore allows the thermal energy generated during component operation to be more easily dissipated.
In radiofrequency (RF) applications, it is sought to use semi-insulating silicon carbide, i.e., silicon carbide that has an electrical resistivity higher than or equal to 105 Ω·cm, to minimize parasitic losses (generally called RF losses) in the substrate. However, this material is particularly expensive and currently is available only in the form of substrates of limited size.
Silicon would allow fabricating costs to be drastically decreased and substrates of large size to be accessed, but structures of III-N-alloy-on-silicon type are penalized by RF losses and by a poor dissipation of heat.
Composite structures, such as SopSiC or SiCopSiC structures, have also been investigated [1] but have not proved to be entirely satisfactory. These structures comprise a layer of single-crystal silicon or a layer of single-crystal SiC (intended to form a seed layer for the epitaxial growth of the gallium nitride) on a polycrystalline SiC substrate, respectively. Although polycrystalline SiC is a material that is inexpensive, that is available in the form of substrates of large size and that dissipates heat well, these composite structures are penalized by the presence of a layer of silicon oxide at the interface between the layer of single-crystal silicon or SiC and the polycrystalline SiC substrate, which forms a thermal barrier hindering the dissipation of heat from the layer of III-N alloy to the polycrystalline SiC substrate.
One aim of the present disclosure is therefore to remedy the aforementioned drawbacks and, in particular, limitations related to the size and cost of semi-insulating SiC substrates.
The aim of the present disclosure is therefore to provide a process for fabricating a substrate for the epitaxial growth of a III-N alloy based on gallium, in particular, with a view to forming HEMTs or other high-frequency, high-power electronic devices in which RF losses are minimized and the dissipation of heat is maximized.
To this end, the present disclosure provides a process for fabricating a substrate for epitaxial growth of a layer of gallium nitride (GaN), of aluminum gallium nitride (AlGaN) or of indium gallium nitride (InGaN), comprising the following successive steps:
By “high-frequency,” what is meant in the present text is a frequency higher than 3 kHz.
By “high-power,” what is meant in the present text is a power density higher than 0.5 W/mm injected through the gate of the transistor.
By “high electrical resistivity,” what is meant in the present text is an electrical resistivity higher than or equal to 100 Ω·cm.
By “semi-insulating SiC,” what is meant in the present text is silicon carbide having an electrical resistivity higher than or equal to 105 Ω·cm.
This process allows a base substrate of high electrical resistivity and of high thermal conductivity comprising a layer of semi-insulating SiC having a crystal quality suitable for the subsequent epitaxial growth of a layer of gallium nitride to be formed and the final structure to benefit from the good properties thereof as regards the dissipation of heat and the limitation of RF losses. Since the layer of semi-insulating SiC makes direct contact with the substrate of high electrical resistivity and of high thermal conductivity, the structure further contains no thermal barrier.
A process that consisted in forming the layer of semi-insulating SiC by epitaxy directly on a substrate of high electrical resistivity would lead to the formation of a high number of dislocations in the semi-insulating SiC because of the insufficient crystal quality of the substrate of high electrical resistivity or the difference in lattice parameter between the material of the substrate and silicon carbide. In contrast, the process according to the present disclosure makes it possible to use, as a seed for the growth of the semi-insulating SiC, a layer of single-crystal SiC, the quality of which is optimal because it was obtained via transfer from the donor substrate.
According to advantageous but optional features of the present disclosure, which may be implemented alone or in combination when this is technically possible:
Another subject of the present disclosure relates to a process for fabricating a layer of a III-N alloy based on gallium on a substrate obtained using the process that has just been described.
The process comprises:
The layer of gallium nitride, of aluminum gallium nitride (AlGaN) or of indium gallium nitride (InGaN) has a thickness typically between 1 and 2 μm.
Another subject of the present disclosure relates to a process for fabricating a high-electron-mobility transistor (HEMT) in such a layer of III-N alloy based on gallium.
The process comprises:
Further features and advantages of the present disclosure will become apparent from the following detailed description, with reference to the appended drawings, in which:
For the sake of legibility of the figures, the various layers have not necessarily been shown to scale.
The present disclosure provides a process for fabricating substrates for the epitaxial growth of binary or ternary III-N alloys based on gallium. The alloys comprise gallium nitride (GaN), aluminum gallium nitride (AlxGa1-xN, where 0<x<1, designated in abbreviated form by AlGaN below) and indium gallium nitride (InxGa1-xN, where 0<x<1, designated in abbreviated form by InGaN below). For the sake of conciseness, in the rest of the text the fabrication of a substrate for the epitaxial growth of a layer of GaN will be described; however, a person skilled in the art will be able to tailor the growth conditions to form a layer of AlGaN or InGaN, the substrate serving for this epitaxial growth remaining the same.
The process uses a base substrate of single-crystal silicon carbide (SiC) that serves as seed for the growth of a layer of semi-insulating SiC, to form a donor substrate. A thin layer of semi-insulating SiC of the donor substrate is then transferred using the Smart Cut™ process to a receiver substrate, having a high electrical resistivity.
To this end, a base substrate made of single-crystal SiC having an excellent crystal quality, i.e., in particular, SiC free of dislocations, will be chosen.
In certain embodiments, the base substrate may be a bulk substrate of single-crystal SiC. In other embodiments, the base substrate may be a composite substrate, comprising a superficial layer of single-crystal SiC and at least one other layer of another material. In this case, the layer of single-crystal SiC will have a thickness larger than or equal to 0.5 μm.
There are various crystal forms (also called polytypes) of silicon carbide. The most common are the forms 4H, 6H and 3C. Preferably, the single-crystal silicon carbide is chosen from the 4H and 6H polytypes, but any polytype may be used to implement embodiments of the present disclosure.
In the figures, a bulk base substrate 10 made of single-crystal SiC has been shown.
As known per se, as illustrated in
At the present time, processes of epitaxy of GaN are mainly implemented on the silicon face of the SiC. However, it is not impossible to grow GaN on the carbon face of the SiC. The orientation of the base substrate (silicon face/carbon face) and therefore of the donor substrate during the implementation of the method is chosen depending on the face of the SiC intended for the growth of the layer of GaN.
With reference to
Advantageously, the growth of the layer 11 is performed on the carbon face 10-C of the substrate 10. It is therefore the carbon face 11-C of the semi-insulating SiC that is located on the surface of the donor substrate.
There are various techniques for forming semi-insulating SiC. According to one embodiment, the layer of SiC is doped with vanadium during its epitaxial growth. According to another embodiment, silicon, carbon and vanadium are simultaneously deposited using suitable precursors in an epitaxial reactor.
The layer of semi-insulating SiC advantageously has a thickness larger than the thickness of the layer to be subsequently transferred to the receiver substrate. Preferably, the layer of semi-insulating SiC has a thickness larger that a plurality of times the thickness of the layer to be transferred. Thus, the donor substrate will possibly be used a plurality of times to transfer a layer of semi-insulating SiC, this making the process more economical. For example, the epitaxial layer of semi-insulating SiC preferably has a thickness larger than 3 μm, more preferably larger than or equal to 5 μm, and even larger than or equal to 10 μm.
Since semi-insulating SiC is a rare material, the proposed production process allows the lack of availability of semi-insulating SiC substrates on the market to be overcome.
With reference to
With reference to
In the illustrated embodiment, on account of the initial orientation of the base substrate, the ionic species are implanted through the carbon face 11-C of the donor substrate.
Preferably, the thin layer 12 of single-crystal semi-insulating SiC has a thickness smaller than 1 μm. Specifically, such a thickness is accessible on an industrial scale with the Smart Cut™ process. In particular, the implantation devices available on industrial fabrication lines allow such an implantation depth to be obtained.
With reference to
The main function of the receiver substrate is to form, with the layer 12 of semi-insulating SiC transferred to the receiver substrate, a substrate suitable for the epitaxial growth of GaN.
Since the epitaxy is performed at high temperatures, the receiver substrate is preferably chosen to have a coefficient of thermal expansion substantially equal to that of SiC, in order not to generate stress or strain during the epitaxy of the GaN. Thus, particularly advantageously, the receiver substrate has, with SiC, a difference in coefficient of thermal expansion smaller than or equal to 3×10−6 K−1 in absolute value.
Moreover, apart from its high electrical resistivity, the receiver substrate advantageously contributes to the dissipation of heat within the final structure. A material having a high thermal conductivity is therefore advantageously chosen for the receiver substrate.
Thus, preferred materials for the receiver substrate are: ceramics (for example, but non-limitingly polycrystalline SiC (pSiC), polycrystalline aluminum nitride (pAlN), beryllium oxide (BeO)), diamond, or, to a lesser extent, silicon of electrical resistivity higher than or equal to 100 Ω·cm (the thermal conductivity of the latter being lower than that of the other materials mentioned).
The layer 11 of semi-insulating SiC of the donor substrate is bonded to the receiver substrate 20. It is a question of direct bonding, i.e., bonding without use of a bonding layer—which would be liable to form a thermal barrier—interposed between the substrates.
With reference to
The effect of this detachment is to transfer the layer 12 of semi-insulating SiC to the receiver substrate 20.
As illustrated in
The remainder of the donor substrate, which comprises the base substrate 10 and the segment 11′ of the layer 11 of semi-insulating SiC that was not transferred to the receiver substrate 20 (see
The mode of recycling may vary depending on the thickness of the residual segment 11′.
In the case where this thickness is very small, in particular, smaller than the thickness of a new layer of semi-insulating SiC to be transferred (i.e., typically smaller than 1 μm), the entirety of this segment may be removed to keep only the base substrate 10. The base substrate 10 may thus be reused in the process described starting from
In the case where the thickness of the residual segment 11′ of semi-insulating SiC is significant (i.e., typically larger than 1 μm), the segment 11′ may be kept on the base substrate 10, after a polish of its surface.
If the thickness of the segment after polishing is larger than the thickness of the layer 12 to be transferred to a new receiver substrate, the structure comprising the base substrate 10 and of the segment 11′ of semi-insulating SiC may be used as a new donor substrate in the process described above starting from the step described with reference to
Optionally, especially if the thickness of the segment 11′ of semi-insulating SiC after polishing is smaller than the thickness of the layer 12 to be transferred to a new receiver substrate, a new thickness of semi-insulating SiC may be grown by epitaxial regrowth on the segment 11′ after polishing, to obtain a layer of semi-insulating SiC having a sufficient thickness for the implementation of the process starting from the step described with reference to
Returning to the substrate of
With reference to
Next, as illustrated in
It is thus possible to continue the fabrication of transistors, in particular, HEMTs, from this heterojunction, using processes known to those skilled in the art, the channel of the transistor being formed level with the heterojunction, and the source, drain and gate of the transistor being formed on the channel.
On account of the initial orientation of the base substrate 10 (the carbon face 10-C of which received the implantation and was bonded to the receiver substrate), it is the silicon face 12-Si of the layer of semi-insulating SiC that is uncovered on the final substrate, this being particularly favorable to the growth of GaN, of AlGaN or of InGaN.
One variant of the process described above, which especially allows a more conventional orientation of the single-crystal SiC, and in which it is the silicon face that receives the implantation and is bonded to the receiver substrate, will now be described.
To this end, the base substrate is formed by transferring a layer of single-crystal SiC from an initial substrate to an intermediate carrier, then a layer of semi-insulating SiC is grown by epitaxy on the transferred layer of SiC to form the donor substrate.
With reference to
In certain embodiments, the initial substrate may be a bulk substrate of single-crystal SiC. In other embodiments, the initial substrate may be a composite substrate, comprising a superficial layer of single-crystal SiC and at least one other layer of another material. In this case, the layer of single-crystal SiC will have a thickness larger than or equal to 0.5 μm.
There are various crystal forms (also called polytypes) of silicon carbide. The most common are the forms 4H, 6H and 3C. Preferably, the single-crystal silicon carbide is chosen from the 4H and 6H polytypes, but any polytype may be used to implement embodiments of the present disclosure.
In the figures, a bulk initial substrate 50 made of single-crystal SiC has been shown.
As known per se, as illustrated in
The orientation of the initial substrate (silicon face/carbon face) and therefore of the donor substrate during the implementation of the method is chosen depending on the face of the SiC intended for the growth of the layer of GaN.
Particularly advantageously, it is the silicon face 50-Si of the initial substrate 50 that is chosen for the implementation of the steps of the process. Specifically, this is the most conventional orientation in industrial processes involving single-crystal silicon carbide.
With reference to
The implanted species typically comprise hydrogen and/or helium. A person skilled in the art will be able to define the required implantation dose and energy.
Preferably, the thin layer 51 of single-crystal semi-insulating SiC has a thickness smaller than 1 μm. Specifically, such a thickness is accessible on an industrial scale with the Smart Cut™ process. In particular, the implantation devices available on industrial fabrication lines allow such an implantation depth to be obtained.
With reference to
The main function of the intermediate carrier 40 is to temporarily hold the layer 51 of single-crystal SiC between its transfer from the initial substrate and the growth of a layer of semi-insulating SiC on the layer of single-crystal SiC.
To this end, the intermediate carrier 40 is chosen to have a coefficient of thermal expansion substantially equal to that of the SiC, in order not to generate stresses or strains during the epitaxy of the semi-insulating SiC. Thus, particularly advantageously, the intermediate carrier and the initial substrate (or the layer of single-crystal SiC in the case of a composite initial substrate) have a difference in coefficient of thermal expansion smaller than or equal to 3×10−6 K−1 in absolute value.
Preferably, the intermediate carrier is also made of SiC so as to minimize the difference in coefficient of thermal expansion. Particularly advantageously, the intermediate carrier 40 is an SiC substrate having a crystal quality lower than that of the initial substrate. What is meant by that is that the intermediate carrier may be a polycrystalline SiC substrate, or indeed a substrate of single-crystal SiC but that may comprise dislocations of all types (contrary to the single-crystal SiC of the initial substrate which is chosen for an excellent crystal quality to ensure the quality of the epitaxial layer of semi-insulating SiC). Such a substrate of lower crystal quality has the advantage of being less expensive than a substrate of same quality as the initial substrate, while being perfectly adapted to the function of temporary carrier.
The bonding of the initial substrate to the intermediate carrier is advantageously direct, i.e., without use of a bonding layer at the interface between the initial substrate and the intermediate carrier. Optionally, at least one of the surfaces to be brought into contact may be cleaned and/or activated, for example, via bombardment with neutral species, to increase bonding energy.
Alternatively, the initial substrate may be bonded to the intermediate carrier via a bonding layer (not shown) made of a refractory material, able to withstand the temperature of epitaxy of the semi-insulating SiC without degrading.
With reference to
The effect of this detachment is to transfer the layer 51 of single-crystal SiC to the intermediate carrier 40.
As illustrated in
The remainder 50′ of the initial substrate (see
The rest of the process comprises similar steps to the steps described with reference to
With reference to
Since the growth of the layer 11 is performed on the carbon face 51-C of the base substrate, it is the carbon face 11-C of the semi-insulating SiC that is located on the surface of the donor substrate.
The layer of semi-insulating SiC advantageously has a thickness larger than the thickness of the layer to be subsequently transferred to the receiver substrate.
With reference to
With reference to
On account of the initial orientation of the base substrate, the ionic species are implanted through the carbon face 51-C of the donor substrate.
Preferably, the thin layer 12 of single-crystal semi-insulating SiC has a thickness smaller than 1 μm, which is accessible on an industrial scale with the Smart Cut™ process.
With reference to
The main function of the receiver substrate 20 is to form, with the layer 12 of semi-insulating SiC transferred to the receiver substrate, a substrate suitable for the epitaxial growth of GaN.
Since the epitaxy is performed at high temperatures, the receiver substrate is preferably chosen to have a coefficient of thermal expansion substantially equal to that of SiC, in order not to generate stress or strain during the epitaxy of the GaN. Thus, particularly advantageously, the receiver substrate has, with SiC, a difference in coefficient of thermal expansion smaller than or equal to 3×10−6 K−1 in absolute value.
Moreover, apart from its high electrical resistivity, the receiver substrate advantageously contributes to the dissipation of heat within the final structure. A material having a high thermal conductivity is therefore advantageously chosen for the receiver substrate.
Thus, preferred materials for the receiver substrate are: ceramics (for example, but non-limitingly polycrystalline SiC (pSiC), polycrystalline aluminum nitride (pAlN), beryllium oxide (BeO)), diamond, or, to a lesser extent, silicon of electrical resistivity higher than or equal to 100 Ω·cm (the thermal conductivity of the latter being lower than that of the other materials mentioned).
The layer 11 of semi-insulating SiC of the donor substrate is bonded to the receiver substrate 20. It is a question of direct bonding, i.e., bonding without use of a bonding layer—which would be liable to form a thermal barrier—interposed between the substrates.
With reference to
The effect of this detachment is to transfer the layer 12 of semi-insulating SiC to the receiver substrate 20.
As illustrated in
The remainder of the donor substrate, which comprises the base substrate and the segment 11′ of the layer 11 of semi-insulating SiC that was not transferred to the receiver substrate 20 (see
The various modes of recycling have already been described above.
Returning to the substrate of
With reference to
Next, as illustrated in
It is thus possible to continue the fabrication of transistors, in particular, HEMTs, from this heterojunction, using processes known to those skilled in the art, the channel of the transistor being formed level with the heterojunction, and the source, drain and gate of the transistor being formed on the channel.
Whatever the embodiment, the structure thus obtained is particularly advantageous in that it comprises a layer of semi-insulating SiC, which on the one hand serves as seed for the epitaxial growth of the layer of III-N alloy and on the other hand both dissipates heat well and limits RF losses, and which is obtained at lower cost. Moreover, the receiver substrate, which bears the layer of semi-insulating SiC, and which has both a high electrical resistivity and a high thermal conductivity, makes direct contact with the layer, so that the structure does not comprise any thermal barrier.
Number | Date | Country | Kind |
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2010208 | Oct 2020 | FR | national |
This application is a national phase entry under 35 U.S.C. § 371 of International Patent Application PCT/FR2021/051710, filed Oct. 4, 2021, designating the United States of America and published as International Patent Publication WO 2022/074319 A1 on Apr. 14, 2022, which claims the benefit under Article 8 of the Patent Cooperation Treaty to French Patent Application Serial No. FR2010208, filed Oct. 6, 2020.
Filing Document | Filing Date | Country | Kind |
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PCT/FR2021/051710 | 10/4/2021 | WO |