P-DOPING OF GROUP-III-NITRIDE BUFFER LAYER STRUCTURE ON A HETEROSUBSTRATE

Abstract

An epitaxial group-ill-nitride buffer-layer structure is provided on a heterosubstrate, wherein the buffer-layer structure has at least one stress-management layer sequence including an interlayer structure arranged between and adjacent to a first and a second group-ill-nitride layer, wherein the interlayer structure comprises a group-ill-nitride interlayer material having a larger band gap than the materials of the first and second group-ill-nitride layers, and wherein a p-type-dopant-concentration profile drops, starting from at least 11018 cm-3, by at least a factor of two in transition from the interlayer structure to the first and second group-ill-nitride layers.

Claims

1. An epitaxial group-Ill-nitride buffer-layer structure comprising: a substrate; a back barrier layer; an active layer; and at least one stress management layer sequence comprising an interlayer structure, a first group-Ill-nitride layer, and a second group-Ill-nitride layer, the interlayer structure arranged between and adjacent to the first and the second group-Ill-nitride layer, wherein the interlayer structure comprises a group-Ill-nitride interlayer material having a larger band gap than the materials of the first and second group-Ill-nitride layers, wherein a p-type-dopant-concentration profile drops, starting from at least 110.sup.18 cm.sup.3, by at least a factor of two in transition from the interlayer structure to the first and second group-Ill-nitride layers, and wherein, the stress management layer sequence is arranged between the substrate and the active layer.

2. The buffer-layer structure according to claim 1, wherein the back barrier layer is made of AlGaN and has a graded carbon concentration.

3. The buffer-layer structure according to claim 2, wherein a carbon concentration of back barrier layer is highest at an interface between back barrier layer 360.

4. The buffer-layer structure according to claim 1, wherein the p-type dopant concentration profile drops in transition from the interlayer structure to the first and second group-Ill-nitride layers by at least one order of magnitude or by at least two orders of magnitude.

5. The buffer layer structure according to claim 1, comprising at least two stress management layer sequences wherein a second stress management layer sequence that has a second interlayer structure is arranged at a larger distance from the substrate than a first stress management layer sequence.

6. The buffer layer structure according to claim 5, wherein the second interlayer structure differs from the first interlayer structure in at least one of the following: a layer thickness of at least one of the interlayers of the interlayer structure, a p-type dopant concentration in at least one of the interlayers of the interlayer structure, a material composition of at least one of the interlayers of the interlayer structure, and/or a number of interlayers in the interlayer structure.

7. The buffer layer structure according to claim 1, wherein an additional group-Ill-nitride layer is deposited on top of the buffer layer structure and wherein the additional layer has a graded p-type-dopant-concentration profile, wherein the p-type dopant concentration is higher in a first section of the additional layer adjacent to the buffer layer than in a second section of the additional layer further away from the buffer layer.

8. The buffer layer structure according to claim 1, further comprising a buffer stack deposited between the substrate and the stress management layer sequence, the buffer stack comprising a compositionally graded AlGaN buffer layer having a Ga fraction increasing with increasing distance from the substrate.

9. The buffer layer structure according to claim 8, wherein the buffer stack has a p-type-dopant concentration of at least 110.sup.17 cm.sup.3.

10. The buffer layer structure according to claim 1, further comprising a buffer stack deposited between the substrate and the stress management layer sequence, the buffer stack comprising a superlattice formed by a stack of alternating group-Ill-nitride layers of two kinds.

11. The buffer layer structure according to claim 10, wherein the buffer stack has a p-type-dopant concentration of at least 110.sup.17 cm.sup.3.

12. The buffer layer structure according to claim 1, wherein the substrate is a silicon substrate or a heterosubstrate.

13. The buffer layer structure according to claim 1, wherein the back barrier layer is arranged between the active layer and the stress management layer.

14. The buffer layer according to claim 1, wherein the buffer layer structure is an epitaxial group-Ill-nitride buffer layer structure formed in a group-Ill-nitride device, a transistor, a FET, a normally-on or a normally-off HEMT or MIS-HEMT, a Schottky diode, a PIN diode, or a LED.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein:

[0046] FIG. 1 shows an embodiment of a buffer layer structure according to the invention;

[0047] FIG. 2 shows an embodiment of a buffer layer structure according to the invention;

[0048] FIG. 3 shows an embodiment of a group-Ill-nitride device according to the invention;

[0049] FIG. 4a shows a concentration diagram for parts of a transistor based on a buffer layer structure according to the invention;

[0050] FIG. 4b shows a current-voltage characteristic of the transistor of FIG. 4a);

[0051] FIG. 5 shows another embodiment of a buffer layer structure according to the first aspect of the invention;

[0052] FIGS. 6a to 6bc show an aluminum content profile (a)), a hole concentration profile (b)) and band gap profiles at 0 V (c)) of variation 1 of the structure of FIG. 5;

[0053] FIGS. 7a to 7c show an aluminum content profile (a)), a hole concentration profile (b)) and band gap profiles at 0 V (c)) of variation 2 of the structure of FIG. 5; and

[0054] FIGS. 8a to 8c show an aluminum content profile (a)), a hole concentration profile (b)) and band gap profiles at 300 V (c)) of variation 3 of the structure of FIG. 5.

DETAILED DESCRIPTION

[0055] FIG. 1 shows an embodiment of a buffer layer structure 100 according to the invention. On a substrate 110, which in the present embodiment is made of silicon or silicon carbide, an intermediate layer may be grown. This intermediate layer is not shown in detail in FIG. 1, but only indicated by a space above the substrate 110. Such intermediate layer, which may contain a sub-layer structure, serves for achieving nucleation and initial lattice adaptation. In some embodiments, however, the intermediate layer may be omitted, and the buffer layer structure grown directly on the substrate. Examples of an intermediate layer in the form of a buffer stack will be described further below.

[0056] The buffer layer structure 100 comprises a stress management layer sequence S. The stress management layer sequence is formed of three group-Ill-nitride layers 120 to 140, of which a single interlayer 130 is sandwiched between a first group-Ill-nitride layer 120 and a second group-Ill-nitride layer 140.

[0057] In this stress management layer sequence S, the first group-Ill-nitride layer 120 is deposited first. It is made of GaN. In other embodiments, it is made of AlGaN. The thickness of the first group-Ill-nitride layer 120 is typically between 300 and 2000 nm, for instance 590 nm.

[0058] Directly on the first group-Ill-nitride layer 120, the single interlayer 130 is deposited. In the present example, it is made of AlGaN. Alternative embodiments use AlN, or AlInN, or AlInGaN.

[0059] The thickness of the single interlayer is typically between 10 and 50 nm. In the present example it is 30 nm.

[0060] Directly on the single interlayer 130, the second group-Ill-nitride layer 140 is grown. In the present example, it is made of the same material as the first group-Ill-nitride layer, that is GaN. In other embodiments, it is made of AlGaN. Its thickness is typically between 300 nm and 1.5 m. In the present example it is 1 m thick.

[0061] The single interlayer 130 has a carbon concentration of 110.sup.19 cm.sup.3. In contrast, the first group-Ill-nitride layer and the second group-Ill-nitride layer have a lower carbon concentration of not more than 110.sup.18 cm.sup.3. In the present example, their carbon concentration is 110.sup.17 cm.sup.3. Due to the different composition of the AlGaN of the single interlayer 130 in comparison with the GaN of the first and second group-Ill-nitride layers 120 and 140, a difference in lattice constants is achieved, which correlates with a difference in band gap. The difference in lattice constants introduces a stress component that at least partially compensates stress created by the growth on the heterosubstrate. The stress-management layer sequence S made of GaNAlGaNGaN is thus able to reduce a stress which occurs in the buffer layer structure and any layers grown on top of it. On the other hand, the stress management layer sequence also introduces a band structure profile that tends to allow the formation of parasitic conductive channels, which may even comprise a 2DEG near the interfaces between the GaN and AlGaN materials. By introducing carbon at the given concentration levels the building of a 2DEG at the interfaces of single interlayer 130 and first and second group-Ill-nitride layers 120 and 140 is suppressed. Due to its higher carbon concentration, especially the single interlayer layer 130 exhibits also a high resistivity which is needed for the subsequent building of high efficient electronic devices based on such a buffer layer structure.

[0062] FIG. 2 shows an embodiment of a buffer layer structure 200 according to the invention. Compared with the buffer layer structure 100 of FIG. 1, the buffer layer structure 200 comprises a nucleation layer 255 and a superlattice 250 between a second group-Ill-nitride layer 220 and the substrate 210. The superlattice 250 and the nucleation layer form an example of an intermediate layer. The nucleation layer 255 is an AlN layer. The superlattice 250 in this embodiment is a high temperature AlGaN/low temperature AlGaN superlattice with a thickness of 100 nm. The nominal composition of the high- and low-temperature AlGaN layers is identical. The nucleation layer 255 and the superlattice 250 further improve the stress management of the buffer layer structure 200. The nucleation layer 255 and the superlattice 250 have a carbon concentration of 510.sup.18 cm.sup.3 in this example, which improves the resistivity of the buffer layer structure. Other favorable embodiments of the superlattice are AlGaN/GaN or AlN/GaN superlattices.

[0063] The use of several first group-Ill-nitride interlayers within the buffer layer structure further improves the stress management, in particular in the case of a silicon substrate, where the lattice mismatch to group-Ill-nitride materials is particularly high. Therefore, on the stress management layer sequence S comprising the first group-Ill-nitride layer 220, the single interlayer 230 and the second group-Ill-nitride layer 240, two repetitions of the stress management layer sequence are deposited. A first repetition contains a further single interlayer 231 and a further group-Ill-nitride layer 241, the group-Ill-nitride layer 240 is the first group-Ill-nitride layer in regard of the second single interlayer 231 and at the same time the second group-Ill-nitride layer in regard of the first single interlayer 230. A second repetition contains a further single interlayer 232 and a further group-Ill-nitride layer 242. Here the group-Ill-nitride layer 241 is the first group-Ill-nitride layer in regard of the third single interlayer 232 and the second group-Ill-nitride layer in regard of the second single interlayer 231. The first single interlayer 230 and its repetitions 231, 232 are thus all arranged between and adjacent to group-Ill-nitride layers, which in comparison have a lower band gap.

[0064] In an embodiment also properties other than the thickness of the single interlayers can vary, preferably the Al content can decrease in the direction pointing away from the substrate. In another embodiment (not shown) more than one layer with lower band gap may follow the first group-Ill-nitride layer (or any of its repetitions).

[0065] The single interlayer 231 in this embodiment has identical properties (composition, thickness, growth temperature) as single interlayer layer 230. The group-Ill-nitride layer 241 corresponds to the third group-Ill-nitride layer 240. However, in the present example, it is somewhat thicker than the third group-Ill-nitride layer 240, namely 2 m thick. It is thus possible, but not a requirement to have the layers of the second stress management layer sequence S1 identical to corresponding layers of the original stress management layer sequence S. It is also possible to vary layer thicknesses between different repetitions of the second stress management layer sequence S2. However, the thickness of the first group-Ill-nitride layer 230 and its corresponding repetitions in the layers 231 and 232 is preferably the same.

[0066] FIG. 3 shows a transistor 300 as an application case of the group-Ill-nitride buffer layer structure according to the invention. In this embodiment, an AlGaN buffer layer 350 having a Ga fraction increasing with increasing distance from a silicon substrate 310 is grown on the silicon substrate 310.

[0067] On the AlGaN buffer layer 350, a stress management layer sequence S is formed of three group-Ill-nitride layers 320 to 340, of which a single interlayer 330 is sandwiched between a first group-Ill-nitride layer 320 and a second group-Ill-nitride layer 340. For the detailed characteristics of these layers, we refer to the above description of the layers 120 to 140 of the embodiment of FIG. 1.

[0068] The stress management layer sequence S is followed by a back barrier layer 360. The back barrier layer 360 is made of AlGaN and has a graded carbon concentration starting with a carbon concentration of 110.sup.18 cm.sup.3 at an interface between back barrier layer 360 and third group-Ill-nitride layer 340, and ending with a carbon concentration of less than 410.sup.18 cm.sup.3 at an interface between back barrier layer 360 and a following active layer 370.

[0069] The active layer 370, which forms a channel layer that contains a 2DEG charge carrier channel active in operation of the transistor, is made of GaN and has a carbon concentration of 410.sup.16 cm.sup.3. In the present example, it is 40 nm thick. A transistor based on the described structure shows the following properties: a charge carrier densitiy of around 110.sup.13 cm.sup.2, a high electron mobility of 1000-2500 cm.sup.2/(V.Math.s) and a low sheet resistance clearly below 400 /sq.

[0070] On the active layer 370, a barrier layer 380 is grown. The barrier layer is an AlGaN (AlGaInN) layer in the present example. In other embodiments, other compositions may be used that are suitable to form a potential barrier for charge carriers that are in the channel layer. The barrier layer has a thickness of 25 nm in the present example. A cap layer 390 is grown on the barrier layer 380 and forms a uppermost layer of the transistor structure (disregarding contacts). In the present case for d mode HEMT devices, the cap layer 390 is a p-doped GaN layer with a thickness of 4nm. In the cap layer 390 carbon is used a dopant at a concentration level of 10.sup.17 cm.sup.3 in this example.

[0071] The buffer layer structure of this embodiment leads to a favorable stress management of the whole structure and at the same time achieves a high resistance below the active layers. The graded back barrier layer 360 additionally prevents carriers from entering the underlying layers of the buffer layer structure and the substrate, which improves the electrical performance of the device. The active layer 370 (GaN or InGaN) grown on the buffer layer structure exhibits a high crystal quality and a low resistance due to its low carbon concentration. With the GaN cap layer 390 a good electrical contact to contact structures and external devices is achieved. To reduce the series resistance of the HEMT device additional recess etching of GaN-cap and part of the AlGaN-layer in lateral sections for ohmic contact formation is favorable.

[0072] FIG. 4a) shows a concentration diagram for parts of a transistor based on a buffer layer structure according to the invention as obtained from a secondary ion mass spectroscope (SIMS) measurement. FIG. 4b shows a current-voltage characteristic of the transistor of FIG. 4a).

[0073] FIG. 4a) shows traces of an aluminum concentration 4000, a carbon concentration 4100 and an oxygen concentration 4200 on a logarithmic scale as a function of depth for a portion of the transistor including a buffer layer structure. Maxima 430, 431, 432 and 433 of the aluminium concentration 4000 are detected at the positions of the single interlayers. This is indicative of an Al contribution to AlGaN stoichiometry of the single interlayers.

[0074] A maximum 480 in the aluminum trace 4000 represents an AlGaN barrier layer. As can be seen from the diagram of FIG. 4a), also the carbon concentration 4100 and the oxygen concentration exhibit respective maxima at the positions of the first group-Ill-nitride layers, i.e., at the Al peaks. The carbon concentration in the single interlayers is 210.sup.20 cm.sup.3, while the carbon concentration in the GaN layers between the single interlayers is slightly below 110.sup.17 cm.sup.3. Thus the carbon concentration in the single interlayers is 3 orders of magnitude higher than in the GaN layers there between. The formation of a 2DEG in the buffer layer structure is thereby suppressed. As shown in the diagram of FIG. 4b) the transistor based on the described buffer layer structure with high carbon content (4300) exhibit high breakdown voltage and low leakage current. The characteristic curve of a transistor according to the invention is herein compared with the characteristic curve (4301) of a transistor based on a buffer layer structure which differs from the claimed structure in the fact that a low p-type concentration of less than 110.sup.18 cm.sup.3 is used in the interlayer structures.

[0075] FIG. 5 shows another embodiment of a buffer layer structure 500 according to the first aspect of the invention. Compared with the buffer layer structures of the FIGS. 1 to 4, the buffer layer structure 500 comprises a buffer layer 550, a substrate 510, and an interlayer structure 530 comprising three different layers instead of a single layer. After the growing of the first group-Ill-nitride layer 520, in this example made of unintentionally doped GaN, a second group-Ill-nitride interlayer 536 is grown also formed of GaN. The second group-Ill-nitride interlayer is followed by a first group-Ill-nitride interlayer 535 and a third group-Ill-nitride interlayer 537, whereby the third group-Ill-nitride interlayer 536 also formed of GaN and is followed by a second group-Ill-nitride layer 540. The second and third group-Ill-nitride interlayers 535 and 537 have thicknesses of 50 nm in this embodiment. Thicknesses between 20 and 200 nm are favorable for these layers. The first group-Ill-nitride interlayer 535 comprises a group-Ill-nitride interlayer material having a larger band gap than the materials of the first and second group-Ill-nitride layers 520, 540, in this case AlGaN, and has a thickness of 30 nm. In the following variations of the buffer layer structure including different modifications of the Al content of the first group-Ill-nitride interlayer 535 and of the p-dopant concentration will be presented and their effects on the prevention of the 2DEG building.

[0076] Variation 1

[0077] Variation 1 of the layer structure shown in FIG. 5 includes the use of a first group-Ill-nitride interlayer 535 with constant aluminum content. The aluminum content profile for the structure is shown in FIG. 6a) in principle. As stated before in this embodiment the second and third group-Ill-nitride interlayers 536 and 537 are made of GaN as well as the first and second group-Ill-nitride layers 520 and 540. The first group-Ill-nitride interlayer 535 includes AlGaN. FIG. 6b) shows the hole concentration profile for the structure, the hole concentration is 510.sup.18 cm.sup.3 within the whole interlayer structure, thus in all three group-Ill-nitride interlayers 535, 536 and 537. The first and second group-Ill-nitride layers 520 and 540 are unintentionally doped. The hole concentration in the interlayer structure is achieved by intentionally doping the interlayers with magnesium or carbon. The shown hole concentration profile for this layer structure leads to the energy profiles of the layer structure shown in FIG. 6c), whereby 601 represents the conduction band energy profile and 602 represents the valence band profile. As can be seen in FIG. 6c) the building of a 2DEG at the interfaces between AlGaN and GaN, thus between the first group-Ill-nitride interlayer 535 and the second and third group-Ill-nitride interlayers 536 and 537 is effectively suppressed.

[0078] Variation 2

[0079] Variation 2 of the layer structure shown in FIG. 5 includes the use of a first group-Ill-nitride interlayer 535 with a gradient in the aluminum content. The aluminum content profile for the structure is shown in FIG. 7a) in principle. In this embodiment the second and third group-Ill-nitride interlayers 536 and 537 are made of GaN as well as the first and second group-Ill-nitride layers 520 and 540. The aluminum content of the first group-Ill-nitride interlayer 535 increases from 10% at the interface to the second group-Ill-nitride interlayer 536 to 70% at the interface to third group-Ill-nitride interlayer 537. The increase can be either continuous as shown in the diagram or stepwise.

[0080] FIG. 7b) shows the hole concentration profile for the structure, the hole concentration for this embodiment is 110.sup.18 cm.sup.3 within the whole interlayer structure, thus in all three group-Ill-nitride interlayers 535, 536 and 537. The first and second group-Ill-nitride layers 520 and 540 are unintentionally doped.

[0081] As can be seen in FIG. 7c) the building of a 2DEG at the interfaces between AlGaN and GaN, thus between the first group-Ill-nitride interlayer 535 and the second and third group-Ill-nitride interlayers 536 and 537 is effectively suppressed with the relatively smaller hole concentration respectively p-dopant-concentration if a graded AlGaN layer is used as the first group-Ill-nitride interlayer.

[0082] Variation 3

[0083] Variation 3 of the layer structure shown in FIG. 5 shows compared to Variation 2 another advantageous hole concentration profile with which the building of parasitic 2DEG can be effectively suppressed. The aluminum content profile for the structure, shown in FIG. 8a) is in principle the same as the one shown in FIG. 7a)

[0084] FIG. 8b) shows the hole concentration profile for the structure, the hole concentration for this embodiment is 510.sup.18 cm.sup.3 only in the second and third group-Ill-nitride interlayers 536 and 537, the first group-Ill-nitride interlayer 535 is unintentionally doped as well as the first and second group-Ill-nitride layers 520 and 540.

[0085] As can be seen in FIG. 8c) where the energy bands fo the structure at 300V are shown, the building of a 2DEG at the interfaces between AlGaN and GaN, thus between the first group-Ill-nitride interlayer 535 and the second and third group-Ill-nitride interlayers 536 and 537 can also effectively be suppressed without intentional doping of the first group-Ill-nitride interlayer 535 if the doping level of the second and third group-Ill-nitride interlayers 536 and 537 is sufficiently high, even in operation.

[0086] As described in FIG. 2 relating to single interlayers also with the described interlayer structures comprising three group-Ill-nitride interlayers repetitions of the stress management layer sequence are advantageous. For example a first stress management layer sequence comprising a single interlayer made of AlN followed by a three stress management layer sequences each comprising a three-layer interlayer structure of GaNAlGaNGaN can be advantageous.

[0087] The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.