Rare earth group III nitrides, N-polarity HEMT

Abstract

The high electron mobility transistor (HEMT) heterostructure includes a substrate (210), an N-polar channel layer (220), and an N-polar barrier layer (218) disposed between the substrate and the channel layer, the barrier layer comprising a rare earth III-nitride material, which may be ScAlN.

Description

Background Art

[0001] The present disclosure generally relates to high electron mobility transistors (HEMTs), and more particularly to a nitrogen-polarity (N-polarity) configuration of a HEMT including a rare-earth group III nitride barrier layer.

[0002] HEMT devices are semiconductor devices that have many applications, particularly high-frequency or high-speed applications.

[0003] Many known group III nitride HEMT devices utilize a metal-polarity orientation that indicates the direction of the semiconductor crystal in a heterostructure. Metal polarity is a more standard approach for RF applications. In a metal-polarity heterostructure, a metal-like atomic plane (such as gallium, aluminum, and / or indium, etc.) in the group III nitride material terminates on the upper surface. Figures 1A and 1B illustrate a metal-polarity configuration and an N-polarity configuration, respectively. Although the metal-polarity configuration has several advantages, it also has some limitations.

[0004] The materials for the various layers within a HEMT heterostructure can significantly affect the performance of the device, and one of the recently developed and notable group III nitride materials for use as a charge generation layer or a barrier layer in a HEMT high-power transistor is ScAlN. ScAlN has several advantages compared to conventional materials such as AlGaN. For example, significantly higher intrinsic polarization and the ability to match the lattice constant of GaN on the {0001} plane, etc. However, there are still issues regarding the use of this material within a metal-polarity HEMT device, particularly with respect to leakage through the barrier and leakage from the backside of the channel into the buffer. The present disclosure addresses these problems.

Summary of the Invention

[0005] In one non-limiting configuration, a high electron mobility transistor heterostructure includes a substrate, an N-polarity channel layer, and an N-polarity barrier layer disposed between the substrate and the channel layer, wherein the barrier layer includes a rare-earth group III nitride material.

[0006] In non-restrictive configurations, the rare earth group III nitride material is selected from the group consisting of ScAlN, YAlN, LaAlN, PrAlN, GdAlN, ErAlN, and combinations thereof.

[0007] In more non-restrictive configurations, the rare earth group III nitride material is selected from the group consisting of ScAlN, YAlN, and combinations thereof.

[0008] In a more non-limiting configuration, the heterostructure further includes at least one of a nucleation layer, a buffer layer, and a charge relaxation transition layer between the substrate and the barrier layer.

[0009] Other non-limiting configurations, heterostructures, further include a nucleation layer between the substrate and the barrier layer.

[0010] Furthermore, in other non-restrictive configurations, the nucleation layer includes an AlN nucleation layer.

[0011] In a more unrestricted configuration, the heterostructure further includes a buffer layer between the nucleation layer and the barrier layer.

[0012] Furthermore, in a less restrictive configuration, the buffer layer may include a GaN buffer layer.

[0013] In other non-restrictive configurations, the heterostructure further includes a charge relaxation transition layer between the buffer layer and the barrier layer.

[0014] In a further less restrictive configuration, the charge relaxation transition layer includes a graded rare-earth group III nitride alloy layer.

[0015] In other non-restrictive configurations, the charge relaxation transition layer includes a doped GaN layer.

[0016] In a further less restrictive configuration, the heterostructure further includes at least one capping layer located on the opposite side of the channel layer from the barrier layer.

[0017] Furthermore, in non-limiting configurations, at least one capping layer is selected from the group consisting of etching stop layers, intermediate layers, upper barriers, and combinations thereof.

[0018] In other non-restrictive configurations, the channel layer includes a GaN channel layer.

[0019] Furthermore, in other non-restrictive configurations, the rare-earth group III nitride material in the barrier layer is N-polarized.

[0020] In other non-restrictive configurations, the channel layer is N-polarized.

[0021] A detailed description of one or more embodiments of this disclosure is given below with reference to the accompanying drawings. [Brief explanation of the drawing]

[0022] [Figure 1] A and B are crystal orientation diagrams of metallic and N polarity of Group III nitrides. [Figure 2] This diagram illustrates a conventional metallic polarity HEMT heterostructure configuration. [Figure 2A] This diagram illustrates a conventional metallic polarity HEMT heterostructure configuration. [Figure 3] This figure illustrates a conventional metallic polar HEMT heterostructure configuration with an additional AlGaN back barrier layer. [Figure 3A] This figure illustrates a conventional metallic polar HEMT heterostructure configuration with an additional AlGaN back barrier layer. [Figure 4] This figure illustrates the N-polar HEMT heterostructure configuration according to this disclosure. [Figure 4A] This figure illustrates the N-polar HEMT heterostructure configuration according to this disclosure. [Figure 5] This figure illustrates different, non-limiting embodiments of the heterostructure configuration disclosed herein. [Figure 6] This figure illustrates different, non-limiting embodiments of the heterostructure configuration disclosed herein. [Figure 7] These are diagrams illustrating non-limiting embodiments of different heterostructure configurations disclosed in this specification. [Figure 8] These are diagrams illustrating non-limiting embodiments of different heterostructure configurations disclosed in this specification.

Embodiments for Carrying Out the Invention

[0023] Like reference numerals and designations in the various drawings indicate like components.

[0024] This disclosure relates to the N-polarity configuration of a HEMT heterostructure that utilizes a rare-earth group III nitride material for a charge generation layer. In one particularly suitable non-limiting configuration, ScAlN is utilized as the charge generation layer. This helps to obtain all the advantages of using ScAlN as a material for the charge generation layer in the N-polarity orientation, while at the same time solving the leakage problems described above.

[0025] Figures 2 and 2A show a typical arrangement of a metal-polarity HEMT heterostructure. Figure 2 shows a substrate 10 on which a GaN buffer layer 12 is deposited. A channel layer 16, such as an unintentionally doped (UID) GaN channel layer, can be deposited on the buffer layer 12. Finally, a barrier layer 18 can be deposited on the channel layer 16, as schematically illustrated in Figure 2. The barrier layer 18 can be fabricated from various group III nitride materials (IIIA and IIIB, e.g., AlGaN, InAlN, ScAlN). In this configuration, as described above, when the barrier layer results in a very high two-dimensional electron gas (2DEG) charge density (about ~1E13 cm , ,

[0026] ,

[0025] , , , , , -2 ,

[0024] ,

[0023] , , , , exceeded), due to insufficient carrier confinement, the unintentional leakage path from the 2DEG into the buffer becomes an increasingly problematic issue. Additionally, since a high electric field is induced between the 2DEG and the surface, breakdown can occur prematurely.

[0026] Figures 3 and 3A show alternative arrangements of metallic polar HEMT heterostructures. Figure 3 shows a substrate 10 on which a GaN buffer layer 12 is deposited. An AlGaN back barrier layer 14 can typically be deposited on the buffer layer 12. A channel layer 16, such as a UID GaN channel layer, can be deposited on the back barrier 14. Finally, a barrier layer 18 can be deposited on the channel layer 16, as schematically illustrated in Figure 3. The barrier layer 18 can be fabricated from various group III nitride materials (IIIA and IIIB, e.g., AlGaN, InAlN, ScAlN). In this configuration, as shown in Figure 3A, leakage can occur due to unintended further 2DEGs that may be established near the interface between the buffer layer 12 and the back barrier layer 14, as previously described.

[0027] Figures 4 and 4A illustrate one non-restrictive configuration of the N-polar heterostructure 100. This configuration includes a substrate 110, a GaN buffer layer 112, a charge relaxation transition layer 114, a rare-earth group III nitride (IIIA, IIIB, lanthanide, e.g., ScAlN, YAlN, etc.) barrier layer 116, followed by a channel layer 118. In this configuration (referred to as the N-polar configuration), the crystal orientation of the heterostructure profile layer is such that the nitrogen atomic plane terminates on the upper surface of the material heterostructure, as illustrated in Figure 1B, for example.

[0028] In one configuration, the charge relaxation transition layer 114 may be, for example, a graded rare-earth group III nitride alloy and / or highly doped GaN. The purpose of this transition layer is to manipulate the electronic structure of the resulting heterostructure to prevent unintended 2D electron gas or 2D hole gas from forming near the lower interface of the barrier layer 116. Thus, in the case of highly doped GaN, suitable dopants include silicon, carbon, iron, beryllium, magnesium, etc. (which are known in the art to cause band bending and / or Fermi level pinning in GaN), depending on the specific buffer structure. A more aggressive approach using a transition layer may be employed to prevent unintended 2D gas formation due to the fact that, unlike in metallic polar configurations, the intended 2DEG is located on the opposite side of the barrier layer, since the barrier 116 and channel 118 have an N-polar crystal orientation.

[0029] This orientation and configuration significantly reduces the leakage caused by the configurations in Figures 2 and 3, and as a result, it has been found to be manageable. In both N-polar and metallic polar orientations, a pair of layers, more specifically, a GaN channel layer and a charge generation layer, are used to form a 2DEG. In the case of metallic polar HEMTs, the charge generation layer is closer to the surface, while in N-polar HEMTs, the GaN channel layer is closer to the surface. Due to its greater electron affinity, the GaN channel layer lowers the barrier and prevents electron leakage. In the metallic polar configuration of Figure 2, 2A, the GaN channel layer is on the same side as the barrier buffer layer, and leakage into the buffer is possible unless a back barrier is added between the channel and the buffer. Furthermore, in the metallic polar configuration of Figure 3, 3A, the commonly used back barrier material is the same as the charge generation layer material. As a result, the introduction of a back barrier layer can easily induce further unintended 2DEGs (as shown in Figure 3A) near the interface between the back barrier and the buffer. If present, these unintended 2DEGs function as leakage paths, degrading device performance in metallic polar configurations. Conversely, in the N-polar orientation (Figure 4, 4A), the charge generation layer exists between the intended 2DEG and the buffer, thus acting as an enhanced leak barrier to the buffer.

[0030] The combination of layers and crystal orientations in these different configurations creates energy wells that trap electrons within the 2D sheet. These 2DEGs exist in both metallic and N-polar configurations. As mentioned earlier, the GaN channel side of the energy well has a shorter energy barrier, allowing some electrons to escape into adjacent layers in that direction.

[0031] In a metallic polarity configuration, the GaN channel side of the energy well faces the buffer and substrate. While a back barrier can be added to reduce this leakage, unfortunately, the most convenient barrier to add is the same material used to create the energy well. This is undesirable as it could create a second energy well that traps electrons.

[0032] In the case of N polarity, conversely, the GaN channel side of the energy well faces the surface. As a result, adding an electron barrier on top of the GaN channel does not require any further GaN layer on top of that new barrier. Therefore, there are no further material pairs where GaN is above the barrier. Consequently, when attempting to reduce leakage by adding a barrier against electron leakage, no further channels are formed. Furthermore, the electric field within the GaN layer can retain electrons in the well as long as it remains below a certain thickness, thus reducing the need to add a specific barrier layer.

[0033] The heterostructure illustrated in Figure 4 has at least an N-polar channel layer 118 and a barrier layer 116. In the simplest case, the entire heterostructure has an N-polar structure because this is carried over during the generation of the various layers of the heterostructure. However, not all layers need to be N-polar, and under some circumstances, it is desirable to have one or more layers between the substrate 110 and the barrier layer 116, which may not be N-polar.

[0034] Figures 2A, 3A, and 4A, aligned with Figures 2, 3, and 4, show the calculated free carrier charge density distribution along with 1D Schrödinger-Poisson band structure simulations for each of the aligned heterostructures. These figures highlight the favorable band structures generated by the heterostructures disclosed herein (Figure 4A).

[0035] Figures 5-8 illustrate different, non-limiting configurations of the N-polar heterostructure according to this disclosure.

[0036] It becomes clear that each of these different configurations has different layers. As is well known in the art, these layers are generally grown from a substrate using epitaxial fabrication processes that deposit or grow various semiconductor layers on a crystalline substrate to produce a desired material heterostructure. This heterostructure then becomes a device through subsequent processing steps such as lithography and / or etching. Such devices typically include gates and ohmic contacts, as well as other layers or components, all of which are well known to those skilled in the art.

[0037] Referring to the configuration in Figure 5, the heterostructure 200 is schematically illustrated as having a substrate 210. A nucleation layer 212, such as an AlN nucleation layer, can be formed on the substrate 210. Following the nucleation layer 212, a buffer layer, such as a GaN buffer 214, can be grown on the nucleation layer 212. Next, an optional charge relaxation transition layer 216 can be grown on the buffer layer 214.

[0038] Following this layer, an N-polarity barrier (charge generation) layer 218 can be deposited on the transition layer 216, and then an N-polarity UID GaN channel layer 220 can be deposited.

[0039] In particular, the N-polarity configuration is advantageous because it can be used in combination with a rare-earth group III nitride barrier layer 218 (ScAlN, YAlN, etc.). This configuration provides all the desired advantages of the ScAlN material while significantly reducing the leakage problems encountered in Figures 2 and 3.

[0040] Suitable materials for rare earth group III nitride materials include ScAlN, YAlN, LaAlN, PrAlN, GdAlN, ErAlN, and combinations thereof. Particularly useful rare earth group III nitride materials are ScAlN, YAlN, and combinations thereof.

[0041] In the configurations shown in Figures 4-8, it is no longer necessary to add further embedding layers within the heterostructure (as required in metallic polar configurations such as those in Figure 2 or 3), making it considerably easier to incorporate enhanced charge confinement into the heterostructure. Instead, an optional capping layer can be added to the surface (e.g., Figure 8). Therefore, for HEMT heterostructures with very high 2DEG charge densities, the N-polar orientation offers considerable advantages over the metallic polar configuration.

[0042] A further advantage of the configuration shown in Figures 4-8 is that the ScAlN (or other rare-earth group III nitride material) barrier layer is no longer exposed to air and is instead located within the heterostructure. This helps prevent accidental processing or surface degradation of the ScAlN charge generation layer.

[0043] Furthermore, ohmic contacts can be formed without passing through the barrier layer, which could potentially disrupt the 2DEG that is normally formed by contact between the channel layer and the barrier layer.

[0044] Figure 6 shows a simplified, non-limiting configuration of device 300 having a substrate 310, a nucleation layer 312, a ScAlN barrier layer 314, and a channel layer 316. In this configuration, the buffer and transition layer of Figure 5 are avoided. Nevertheless, the device 300 shown in Figure 6 still provides the desirable effects of utilizing the ScAlN barrier layer disclosed herein.

[0045] Figure 7 illustrates a more basic and less restrictive configuration than that shown in Figure 6. In this configuration, the device 400 has a barrier layer 412 directly deposited on the substrate 410, and a channel layer 414 directly deposited on the barrier layer 412.

[0046] Finally, Figure 8 illustrates yet another non-limiting configuration. In this configuration, the device 500 has a substrate 510, which comprises a nucleation layer 512, a GaN buffer layer 514, a charge relaxation transition layer 516, a ScAlN barrier layer 518, a UID GaN channel layer 520, and optionally one or more capping layers 522.

[0047] In this configuration, the capping layer can be an etching stop layer, an intermediate layer, an upper barrier layer, or a combination of one or more of these layers. These capping layers can also be used in any other configuration as illustrated in Figures 6 and 7, as non-limiting examples.

[0048] The substrates of the various configurations disclosed herein can be any substrate suitable for the purposes described herein, such as SiC, sapphire, GaN, AlN, and Si. Since the various heterostructures disclosed herein are N-polarized, the orientation of the substrate must match the subsequent growth approach used to form the nucleation layer and / or buffer layer (if any), so that the charge generation layer and UID channel layer are formed with an upper surface having an N-polarized orientation.

[0049] Naturally, only the barrier layer and channel layer must be N-polar. Other layers can have different configurations. While the entire heterostructure is most typically N-polarized, alternative strategies for controlling or reversing the polarization of group III nitride crystals are known in the art. In the most typical approach, the first growth layer (such as the nucleation layer) is grown such that its surface is reliably N-polarized, and that polarity is maintained relative to the rest of the heterostructure. However, the buffer and / or nucleation layer below the barrier layer may have any combination of crystal orientations, as long as, firstly, the buffer and / or nucleation layer does not form unintended leak pathways, and secondly, the buffer and / or nucleation layer can be used to successfully grow the subsequent N-polarized barrier and channel layers.

[0050] It is particularly effective to grow individual layers so that each layer serves as a base layer for the appropriate epitaxial growth of the next layer. In some cases, nucleation layers may be useful for transitions from one material to the next, for example, from a substrate to a buffer or barrier layer. In addition, charge relaxation transition layers and / or charge relaxation buffer layers may also be useful when placed between certain layers. Furthermore, as is known in the art, a single thin N-polar intermediate layer (such as AlN) or multiple thin N-polar intermediate layers (such as GaN and AlN) may be incorporated between the barrier layer and the channel layer.

[0051] It should be understood that using ScAlN as a barrier material results in a 2-3 times increase in charge density compared to standard AlGaN HEMTs, thus enhancing charge confinement and providing a much greater advantage. Furthermore, reversing the material polarization to N polarity as disclosed herein provides considerably more freedom to design the material structure, which yields the enhanced charge confinement described above, into many different configurations that will be apparent to those skilled in the art upon consideration of this disclosure.

[0052] Furthermore, while this disclosure relates to heterostructures, it should be understood that these heterostructures themselves may be components of devices that have well-known components such as gates and / or ohmic contacts in order to function as transistors.

[0053] Various features illustrated and described in one figure or embodiment can be combined, without limitation, with other features illustrated and described in other figures and embodiments, and all such combinations are deemed to be disclosed herein.

[0054] One or more embodiments of the present disclosure have been described. However, it is understood that various modifications may be made without departing from the spirit and scope of the invention. For example, different materials and configurations may be used, and transistor structures having different shapes or configurations may benefit from the present disclosure. Accordingly, other embodiments are within the scope of the following claims.

Claims

1. A high electron mobility transistor heterostructure, circuit board and N-polarity channel layer, An N-polar barrier layer disposed between the substrate and the N-polar channel layer, comprising a rare earth group III nitride material, It has, Furthermore, the high electron mobility transistor heterostructure further comprises a charge relaxation transition layer between the substrate and the N polarity barrier layer, wherein the charge relaxation transition layer contains a rare earth group III nitride alloy, and the composition of the rare earth group III nitride alloy is graded relative to the rare earth group III nitride material of the N polarity barrier layer.

2. The heterostructure according to claim 1, wherein the rare earth group III nitride material is selected from the group consisting of ScAlN, YAlN, LaAlN, PrAlN, GdAlN, ErAlN, and combinations thereof.

3. The heterostructure according to claim 1, wherein the rare earth group III nitride material is selected from the group consisting of ScAlN, YAlN, and combinations thereof.

4. Furthermore, the heterostructure according to claim 1, further comprising at least one of a nucleation layer and a buffer layer between the substrate and the charge relaxation transition layer.

5. The heterostructure according to claim 4, wherein the nucleation layer includes an AlN nucleation layer.

6. Furthermore, the heterostructure according to claim 4, further comprising a buffer layer between the nucleation layer and the charge relaxation transition layer.

7. The heterostructure according to claim 6, wherein the buffer layer includes a GaN buffer layer.

8. Furthermore, the heterostructure according to claim 1, further comprising at least one capping layer disposed on the opposite side of the N-polar channel layer from the N-polar barrier layer.

9. The heterostructure according to claim 8, wherein the at least one capping layer is selected from the group consisting of an etching stop layer, an intermediate layer, an upper barrier, and combinations thereof.

10. The heterostructure according to claim 1, wherein the N polar channel layer includes a GaN channel layer.

11. The heterostructure according to claim 1, wherein the rare earth group III nitride material of the N polarity barrier layer is N polarity oriented.

12. The heterostructure according to claim 6, wherein the buffer layer is N polarity oriented.

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