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TEPZZ 7676 ZA_T(11) EP 2 767 620 A1

(12) EUROPEAN PATENT APPLICATION

(43) Date of publication: 20.08.2014 Bulletin 2014/34

(21) Application number: 13155540.1

(22) Date of filing: 15.02.2013

(51) Int Cl.:C30B 25/18 (2006.01) C30B 29/40 (2006.01)

H01L 29/267 (2006.01) H01L 21/02 (2006.01)

(84) Designated Contracting States: AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TRDesignated Extension States: BA ME

(71) Applicant: Azzurro Semiconductors AG01237 Dresden (DE)

(72) Inventors: • Lutgen, Stephan

01309 Dresden (DE)

• Murad, Saad01705 Freital (DE)

• Chitnis, Ashay01309 Dresden (DE)

(74) Representative: Eisenführ SpeiserPatentanwälte Rechtsanwälte PartGmbB Anna-Louisa-Karsch-Strasse 210178 Berlin (DE)

(54) P-doping of group-III-nitride buffer layer structure on a heterosubstrate

(57) The invention relates to an epitaxial group-III-nitride buffer-layer structure (100) on a heterosubstrate,wherein the buffer-layer structure (100) comprises atleast one stress-management layer sequence S includ-ing an interlayer structure (530) arranged between andadjacent to a first and a second group-III-nitride layer(120, 140), wherein the interlayer structure (130) com-

prises a group-III-nitride interlayer material having a larg-er band gap than the materials of the first and secondgroup-III-nitride layers (120, 140), and wherein a p-type-dopant-concentration profile drops, starting from at least1x1018 cm-3, by at least a factor of two in transition fromthe interlayer structure (130) to the first and secondgroup-III-nitride layers (120, 140).

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Description

[0001] The present invention relates to an epitaxialgroup-III-nitride buffer layer structure on a heterosub-strate. It also relates to a device structure, in particularto a layer structure of a transistor, a field-effect transistor(FET), a high-electron-mobility transistor (HEMT), in par-ticular a normally-on or normally-off HEMT or a metal-insulator-semiconductor (MIS)HEMT, a Schottky diode,or a P-I-N structure.[0002] Most group-III-nitride based device structures,in particular transistor structures used nowadays for radiofrequency (RF) or high-voltage (HV) power-conversiondevices, are fabricated on a heterosubstrate, this meanson a substrate of a material other than a group-III-nitridematerial, such as a Si, SiC, or Al2O3 (Sapphire) substrate.The ability to use Si as a heterosubstrate is particularlyadvantageous because it allows using relatively cheapwafers having large industry-standard diameters, andbecause it forms the basis for monolithically integratinggroup-III-nitride devices into silicon-based integrated cir-cuits made by a CMOS or related technology.[0003] Such epitaxial group-III-nitride layer structuresgrown on a heterosubstrate, however, require a sophis-ticated buffer layer structure between the substrate andthe active layer(s) in order to manage stress and defectswithin the crystalline structure.[0004] In order to control the resistive properties of thebuffer layer structure in a group-III-nitride layer structure,Fe doping has been widely used. However, the use ofFe has some disadvantages. More specifically, Fe dop-ing results in an undesired tilt and twist of the crystallitestructure, as can be revealed by x-ray diffractometry(XRD). Furthermore, the inventors found that Fe, whenprovided as a dopant during the growth of the epitaxiallayer structure, segregates towards a channel layer,which in operation carries a two-dimensional electrongas, hereinafter also referred to in short as 2DEG. Thepresence of Fe in the channel layer is detrimental forachieving a desired high electron concentration in the2DEG. Finally, Fe doping causes an undesired Fe con-tamination of the reactor used for deposition of the group-III-nitride layer structure. This causes an undesired Febackground doping, typically in concentrations up toabout 1017cm-3 in nominally undoped upper HEMT-de-vice layers and on the wafer surface. Since the presenceof Fe induces traps for charge carriers, an unintentionalFe doping reduces a dynamical behavior of the on-re-sistance Ron of group-III-nitride based HEMT devices.Due to the contamination risk, Fe doping is not consid-ered compatible with wafer processing in CMOS proc-esses. This forms an obstacle for an integration of thefabrication of group-III-nitride devices into existing, well-established CMOS process lines on silicon wafers.[0005] The document US 7,884,393 discloses the useof a homosubstrate in the form of a GaN substrate toachieve an extremely low dislocation density in an epi-taxial group-III-nitride layer structure grown on the ho-

mosubstrate. The low dislocation density achievable bygrowth on a homosubstrate makes a carbon concentra-tion in the different layers of the layer structure variableto some degree. By growing on a homosubstrate andcontrolling a carbon concentration, according to US7,884,393, the quality of a buffer layer and a channellayer of group-III-nitride field-effect transistors andHEMTs is improved. As an application, US 7,884,393describes a HEMT structure grown on a GaN substrate,which has single high-resistance buffer layer depositedimmediately on the GaN substrate, a single GaN channellayer immediately on the buffer layer, and a single barrierlayer immediately on the channel layer.. The buffer layeris in different embodiments made of GaN or AlGaN andhas a carbon concentration of 4x1017cm-3 or higher. Thehighest carbon concentration disclosed for the buffer lay-er in US 7,884,393 is 2x1018cm-3. The adjacent channellayer does not form a part of the buffer layer, but formsan active layer of the HEMT. It is made of either GaN orInGAN and has a carbon concentration of not more than4x1016cm-3. A lower concentration of carbon in the chan-nel layer is described in US 7,884,393 as desirable forobtaining a high purity and thus a high electron mobility.Group-III-nitride buffer layer structures grown on heter-osubstrates, however, and in particular silicon, have amuch higher dislocation density compared to thosegrown on a homosubtrate. This high dislocation densitycan currently not be avoided. Typical dislocation densi-ties achieved in state-of-the-art buffer layer structures onheterosubstrates are in the range of 5x107-5x109cm-2.[0006] A first aspect of the invention disclosed in thepresent specification is an epitaxial group-III-nitride buff-er layer structure on a heterosubstrate. The buffer layerstructure comprises at least one stress-management lay-er sequence including an interlayer structure arrangedbetween and adjacent to a first and a second group-III-nitride layer, wherein the interlayer structure comprisesa group-III-nitride interlayer material having a larger bandgap than the materials of the first and second group-III-nitride layers, and wherein a p-type-dopant-concentra-tion profile drops, starting from at least 1x1018 cm-3, byat least a factor of two in transition from the interlayerstructure to the first and second group-III-nitride layers.[0007] The p-type-dopant concentration profile match-es the hole concentration profile if no compensation ef-fects occur. If compensation effects have to be consid-ered, the p-type-dopant concentration is higher than thedesired hole concentration. For instance, the p-type-do-pant concentration may be higher by a factor of 5, 10,20, 50 or 100 than the hole concentration achieved. Thus,in different embodiments, the p-type-dopant concentra-tion profile considering compensation effects drops,starting for instance from at least 1x1019 cm-3 (factor 10),or from 1x1020 cm-3 (factor 100), by at least a factor oftwo in transition from the interlayer structure to the firstand second group-III-nitride layers.[0008] Suitable p-type dopants are for instance carbonand magnesium.

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[0009] The buffer layer structure of the invention isbased on the recognition, that group-III nitrides grown ona heterosubstrate are subjected to substantial stress dur-ing the manufacturing process. In comparison with a dep-osition on a homosubstrate, this requires a special stressmanagement in the buffer-layer structure. One stressmanagement measure is including in the buffer layerstructure a stress-management layer sequence of an in-terlayer structure that is arranged between and adjacentto a first group-III-nitride layer and a second group-III-nitride layer, which have a smaller band gap than a group-III-nitride interlayer material. However, the provision ofthis stress-management layer sequence implies the riskof a formation of undesired parasitic conductive channelsin the buffer layer structure. The invention is based onthe further recognition that an incorporation of carbon ormagnesium as p-type dopant in the stress-managementlayer sequence with a suitable concentration profileacross the stress-management layer sequence is effec-tive in avoiding the formation of such parasitic conductivechannels in the buffer layer structure. This way, it be-comes even possible in some embodiments to avoid anyintentional incorporation of iron atoms, which, as de-scribed, have been used in the prior art to achieve a highlyresistive buffer layer structure.[0010] The p-type concentration profile of the presentinvention has a concentration contrast between the in-terlayer structure on one hand and the first and secondgroup-III-nitride layers on the other hand. Namely, the p-type-dopant-concentration profile drops, starting from atleast 1x1018 cm-3, by at least a factor of two in transitionfrom the interlayer structure to the first and second group-III-nitride layers. Herein the drop in the p-type-dopantconcentration can either be sudden or continous. Theinventors have recognized in their experiments that alower contrast or lower concentration values will not beefficient in avoiding parasitic conductive channels in thestress-management layer sequence. The inventors arethe first to manufacture a group-III-nitride based bufferlayer structure with a stress management layer structurethat achieves the mentioned high p-type concentrationcontrast between the interlayer structure and the first andsecond layers even at dislocation densities in the rangeof 5x107-5x109cm-2, which are for instance observedwhen using a silicon substrate as the heterosubstrate.Thus, it is only the combination of the mentioned p-typedopant concentrations and their minimum difference asdefined above, which can provide a highly-resistive bufferlayer structure for group-III-nitrides on heterosubstrateswithout having to use transition metal atoms such as iron,which have negative side effects described earlier.[0011] The high resistivity of the buffer layer structureenables a subsequent manufacture of highly efficientelectronic group-III-nitride semiconductor devices basedon the buffer stress-management layer sequence, suchas group-III-nitride-based high-electron mobility transis-tors or Schottky diodes. Consequently the buffer layerstructure according to the first aspect of the invention

allows providing a strain-managed buffer layer structureon a heterosubstrate, which due to its high resistivity isparticularly suited as a base for highly efficient electronicdevices requiring a high resistance of the buffer layerstructure. With the buffer layer structures according tothe invention high resistivities, which means high break-down electric field strength of more than 1 MV/cm or even2 or 3 MV/cm can be reached. By increasing an Al contentin the interlayer structure the breakdown electric fieldstrength can be additionally increased.[0012] In the following, embodiments of the buffer layerstructure of the first aspect of the invention will be de-scribed.[0013] In a preferred embodiment of the stress-man-agement layer sequence the interlayer structure has ap-type-dopant concentration profile which drops in tran-sition from the interlayer structure to the first and secondgroup-III-nitride layer by at least one order of magnitudeor even by at least two orders of magnitude. The buildingof a 2DEG in the buffer stress-management layer se-quence can be suppressed with higher effectivity thehigher the drop in p-type-dopant concentration is.[0014] Preferably the p-type-dopant concentration isconstant throughout the interlayer structure.[0015] In a preferable embodiment the first or the sec-ond, or the first and the second group-III-nitride layer areonly unintentionally doped, this means a measured p-type dopant concentration of not more than 1x1017 /cm3.Higher ranges of p-type-dopant concentration of the firstor the second, or the first and the second group-III-nitridelayer in the preferable range of 1x1018 /cm3 or even1x1019 /cm3 can be tolerated with corresponding higherp-type-dopant concentration in the interlayer structure.[0016] In one embodiment of the invention the inter-layer structure is a single layer and the group-III-nitrideinterlayer material is of homogeneous composition,which has a larger band gap than the first and secondgroup-III-nitride layers. The concentration of the p-typedopant in the interlayer structure is at least 6x1018 cm-3,a p-type dopant concentration of at least 1x1020cm-3 ispreferred. P-type-dopant concentrations of at least6x1018 cm-3 in a single interlayer have the additional ben-efit of being able to compensate undesired parasitic ox-ygen concentrations in the layer. Thus, higher oxygencontent in the layer is tolerable, which reduces the fab-rication costs of the buffer-layer structure, because com-plex and expensive measures to reduce or avoid an un-wanted oxygen incorporation into the layers duringgrowth can be omitted. But even values of the p-type-dopant concentration of the single interlayer of up to1x1021 cm-3 are beneficial in this regard.[0017] In another preferred embodiment the interlayerstructure comprises a first group-III-nitride interlayer thatis arranged between and adjacent to a second group-III-nitride interlayer and a third group-III-nitride interlayer.The first, second and third group-III-nitride interlayerhave a respective homogeneous composition. A bandgap of the first group-III-nitride interlayer is larger than

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band gaps of the second group-III-nitride interlayer andof the third group-III-nitride interlayer. The p-type-dopantconcentration throughout the interlayer structure is atleast 5x1018cm-3. Such an interlayer structure allows theuse of a p-type-dopant concentration at a level between5x1018cm-3and 1x1019cm-3. The doping level that canbe electrically activated is limited. Moderate p-type do-pant concentrations in the interlayer structure which areable to eliminate the parasitic 2DEG channels are pre-ferred because a higher p-type doping level can causeundesired process fluctuations. Nevertheless if processfluctuations can be compensated in another way, alsohigher p-type dopant concentrations are preferred.[0018] In a variation of this embodiment the materialof the first and second group-III-nitride layer is the sameas the material of second and third group-III-nitride inter-layers and the group-III-nitride interlayer material havinga larger band gap than the materials of the first and sec-ond group-III-nitride layers is the material of the firstgroup-III-nitride interlayer.[0019] In another preferred embodiment the interlayerstructure comprises a first group-III-nitride interlayer thatis arranged between and adjacent to a second group-III-nitride interlayer and a third group-III-nitride interlayerand the first group-III-nitride interlayer is compositionallygraded. In this embodiment a p-type doping concentra-tion of 1x1018 cm-3 throughout the interlayer structure isalready sufficient to avoid the building of a parasitic 2DEG channel.[0020] By using the described three-layer structure ofthe interlayer structure and especially a compositionallygraded first group-III-nitride interlayer, p-type dopingconcentrations of 5x1018 cm-3, respectively 1x1018 cm-3

in the first group-III-nitride interlayer are sufficient to com-pensate undesired parasitic oxygen concentrations in therespective layer.[0021] In another embodiment, the group-III-nitride in-terlayer material having a larger band gap additionallyhas a larger oxygen concentration than either the first orthe second group-III-nitride layer, or than both the firstand the second group-III-nitride layer.[0022] The first group-III-nitride interlayer is preferablymade of AlGaN having an Al mole fraction that increaseswith increasing distance from the substrate. Such an in-crease of the Al mole fraction with increasing distanceimproves the stress management of the buffer layerstructure as well as it allows the use of lower p-type-dopant concentrations in the interlayer structure.[0023] A compositional grading of the second group-III-nitride interlayer or the third group-III-nitride interlayeror the second and the third group-III-nitride interlayer ispart of another embodiment of the invention. The gradingof these layers is especially advantageous in regard ofavoiding additional stress due to lattice mismatches be-tween the first group-III-nitride interlayer on one side andthe first and second group-III-nitride layer on the otherside.[0024] In another embodiment of the invention the con-

centration of the p-type dopant is higher in the secondand third group-III-nitride interlayers than in the firstgroup-III-nitride interlayer.[0025] As p-type dopant carbon or magnesium, or acombination of carbon and magnesium are preferred.[0026] In preferred embodiments, there is no intention-al Fe doping throughout the buffer layer structure. Thisis for instance expressed in an Fe concentration of lessthan 4x1016cm-3. However, in some embodiments it maystill be useful to also include iron in the buffer layer struc-ture. Due to the presence of p-type dopant according tothe present invention, iron can be incorporated in thebuffer-layer structure in much lower concentrations, com-pared to prior-art buffer-layer structures.[0027] In a favorable embodiment of the invention thebuffer layer structure comprises at least two stress-man-agement layer sequences, wherein a second stress-management layer sequence is arranged at a larger dis-tance from the substrate than a first stress-managementlayer sequence and has a second interlayer structure .With additional interlayer structures the stress manage-ment of the buffer layer structure on the heterosubstrateand the quality of subsequent active layers can be im-proved.[0028] This can be further improved by the use of asecond interlayer structure, which differs from the firstinterlayer structure in at least one of the following: a layerthickness of at least one of the interlayers of the interlayerstructure, a p-type-dopant-concentration in at least oneof the interlayers of the interlayer structure, a materialcomposition of at least one of the interlayers of the inter-layer structure, and a number of interlayers in the inter-layer structure.[0029] Another preferable embodiment comprises anadditional group-III-nitride layer which is deposited ontop of the buffer layer structure and wherein the additionallayer has a graded p-type-dopant concentration, whereinthe p-type dopant concentration is higher in a first sectionof the additional layer adjacent to the buffer layer than ina second section of the additional layer further away fromthe buffer layer. Favorably the buffer layer structure isgrown on a heterosubstrate such as a silicon, a silicon-on-insulator, a silicon carbide substrate, a Diamond or aGa2O3-substrate.[0030] In preferred embodiments, the stress-manage-ment layer sequence is configured to introduce a com-pressive stress in buffer layer structure during growth,which fully or at least partially compensates a tensilestress developing after growth during a cooling down ofthe buffer layer structure from growth temperature toroom temperature.[0031] Preferably, the group-III-nitride interlayer mate-rial having a larger band gap than the materials of thefirst and second group-III-nitride layers is a binary, ter-nary or quaternary first group-III-nitride material that hashigher aluminum content than a second group-III-nitridematerial forming the first and second group-III-nitride lay-er. AlN, AlGaN, AlInGaN or AlInN are favorable materials

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forming the group-III-nitride interlayer material. In someembodiments, the layer made of the group-III-nitride in-terlayer material is a low-temperature layer, that is, it isgrown at a lower temperature than the adjacent first andsecond group-III-nitride layers. Also the use of similargrowth temperatures in the interlayer structure and thefirst and second group-III-nitride layers can be useful incertain embodiments.[0032] In order to further improve the stress manage-ment and the resistivity, an embodiment of the inventionfurther comprises a buffer stack deposited between theheterosubstrate and the stress-management layer se-quence, the buffer stack comprising

a) either a compositionally graded AlGaN buffer layerhaving a Ga fraction increasing with increasing dis-tance from the heterosubstrate,

b) or a superlattice formed by a stack of alternatinggroup-III-nitride layers of two kinds,

wherein the buffer stack has a p-type-dopant concentra-tion of at least 1x1018 cm-3.[0033] In regard of a further improved stress-manage-ment in combination with high resistivity in the whole buff-er layer structure an embodiment of the invention is pre-ferred, which comprises a nucleation layer between thebuffer stack and the substrate, wherein the nucleationlayer has a p-type dopant concentration of at least 1x1018

cm-3.[0034] In a further embodiment, the invention relatesto a group-III-nitride device on a heterosubstrate, in par-ticular a transistor, FET, a normally-on or a normally-off(MIS)-HEMT, Schottky diode, PIN structure, comprisingthe epitaxial group-III-nitride buffer layer structures ac-cording to the first aspect of the invention or one of itsembodiments. The buffer layer structures according tothe invention are also advantageous for the use in opto-electronic devices such as LEDs or laser diodes. Thedevice performance is substantially improved by includ-ing the buffer layer structure of the present invention.[0035] Preferably the group-III-nitride device compris-es an only unintentionally doped channel active group-III-nitride layer, which means a p-type-dopant concentra-tion of not more than 1x1017cm-3, preferably lower than4x1016cm-3. Such a low p-type-dopant concentration inthe active layer enables highly efficient performance ofthe group-III-nitride device, where high conductivity is re-quired in the active group-III-nitride layer.[0036] In another embodiment the group-III-nitride de-vice is a HEMT. Here, the active layer forms a channellayer. The HEMT further comprises a barrier layer on theactive layer. A 2DEG can thus form at an interface be-tween the channel layer and the barrier layer. Thin inter-mediate layers between the channel layer and the activelayers may be used, as is known as such in the prior art.Preferably, the HEMT further comprises a cap layer onthe barrier layer. The cap layer may comprise one or

more p-type dopants. Carbon or magnesium may beused as the p-dopant or as one of the p-type dopantsherein as well.[0037] In a second aspect, the invention relates to amethod for fabricating a p-type-doped group-III-nitridebuffer layer structure on a heterosubstrate, the methodcomprising

- fabricating an epitaxial group-III-nitride buffer layerstructure on a heterosubstrate, wherein the bufferlayer structure is fabricated as comprising a stress-management layer sequence including an interlayerstructure arranged between and adjacent to a firstand a second group-III-nitride layer, wherein the in-terlayer structure comprises a group-III-nitride inter-layer material having a larger band gap than the ma-terials of the first and second group-III-nitride layers,and wherein the stress management layer sequenceis fabricated to have a p-type-dopant-concentrationprofile, which drops, starting from at least 1x1018

cm-3, by at least a factor of two in transition from theinterlayer structure to the first and second group-III-nitride layers.

[0038] Favorably, metal-organic vapor epitaxy(MOVPE) or metal-organic chemical vapor deposition(MOCVD) is used as a deposition method.[0039] In a preferred embodiment of the method thelayer made of the group-III-nitride interlayer material hav-ing a larger band gap than the materials of the first andsecond group-III-nitride layers is deposited at a temper-ature lower than the temperature at which the first group-III-nitride layer and the second group-III-nitride layer aregrown. With this temperature management, growth rateand V/III-ratio a high difference between the p-type do-pant concentration in the group-III-nitride interlayer ma-terial and in the first or second group-III-nitride layer canbe achieved. At the same time, this is advantageous forthe stress-management function of the layer sequence.[0040] In a preferred embodiment of the method beforethe deposition of the stress-management layer sequencea AlN nucleation layer is deposited on the substrate bya vapor phase deposition technique using a hydrogen-free carrier gas, preferably N2, at a temperature between800 and 1030 °C, followed by a deposition of a bufferstack before depositing the stress-management layer se-quence by using Tetraethylgallium and Trimethylalumi-num as precursors at a temperature between 800 and1030 °C, wherein the buffer stack comprises

a) either a compositionally graded AlGaN buffer layerhaving a Ga fraction increasing with increasing dis-tance from the heterosubstrate,

b) or a superlattice formed by a stack of alternatinggroup-III-nitride layers of two kinds. The low temper-atures during the growth of the AlN nucleation layerand the buffer stack are advantageous for the incor-

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poration of carbon concentrations of more than1x1017 cm-3 and lead to a better crystal quality.

[0041] In order to control the p-type dopant concentra-tion in the different layers of the buffer layer structureseveral growth parameters other than the growth tem-perature can be controlled and adjusted. Under other-wise constant growth conditions, increasing a flow rateof a nitrogen source used in the preparation of a layerleads to a decreased p-type dopant concentration in thelayer. An opposite effect is achieved by a higher flow rateof a gallium precursor. A higher V/III ratio during thegrowth of a layer also leads to a lower p-type dopantconcentration in the layer. A similar effect is achieved bya higher ambient pressure in the reactor and by a lower-ing the growth rate of a layer. An additional means tocontrol and adjust the p-type dopant concentration is theuse of special C-precursors like for example CBr4, CCI4.[0042] Further embodiments of the invention will bedescribed hereinafter with reference to the drawings, inwhich:

Fig. 1 shows an embodiment of a buffer layer struc-ture according to the first aspect of the invention;

Fig. 2 shows another embodiment of a buffer layerstructure according to the first aspect of the inven-tion;

Fig. 3 shows an embodiment of a group-III-nitridedevice according to the invention,

Fig. 4 a) shows a concentration diagram for parts ofa transistor based on a buffer layer structure accord-ing to the invention,

Fig. 4 b) shows a current-voltage characteristic ofthe transistor of Fig. 4 a),

Fig. 5 shows another embodiment of a buffer layerstructure according to the first aspect of the inven-tion,

Fig. 6 shows an aluminum content profile (a)), a holeconcentration profile (b)) and band gap profiles at 0V (c)) of variation 1 of the structure of Fig. 5.

Fig. 7 shows an aluminum content profile (a)), a holeconcentration profile (b)) and band gap profiles at 0V (c)) of variation 2 of the structure of Fig. 5.

Fig. 8 shows an aluminum content profile (a)), a holeconcentration profile (b)) and band gap profiles at300 V (c)) of variation 3 of the structure of Fig. 5.

[0043] Fig. 1 shows an embodiment of a buffer layerstructure 100 according to the first aspect of the invention.On a substrate 110, which in the present embodiment is

made of silicon or silicon carbide, an intermediate layermay be grown. This intermediate layer is not shown indetail in Fig. 1, but only indicated by a space above thesubstrate 110. Such intermediate layer, which may con-tain a sub-layer structure, serves for achieving nucleationand initial lattice adaptation. In some embodiments, how-ever, the intermediate layer may be omitted, and the buff-er layer structure grown directly on the substrate. Exam-ples of an intermediate layer in the form of a buffer stackwill be described further below.[0044] The buffer layer structure 100 comprises astress-management layer sequence S. The stress-man-agement layer sequence is formed of three group-III-ni-tride layers 120 to 140, of which an single interlayer 130is sandwiched between a first group-III-nitride layer 120and a second group-III-nitride layer 140.[0045] In this stress-management layer sequence S,the first group-III-nitride layer 120 is deposited first. It ismade of GaN. In other embodiments, it is made of AlGaN.The thickness of the first group-III-nitride layer 120 is typ-ically between 300 and 2000 nm, for instance 590 nm.[0046] Directly on the first group-III-nitride layer 120,the single interlayer 130 is deposited. In the present ex-ample, it is made of AlGaN. Alternative embodiments useAlN, or AlInN, or AlInGaN.[0047] The thickness of the single interlayer is typicallybetween 10 and 50 nm. In the present example it is 30 nm.[0048] Directly on the single interlayer 130, the secondgroup-III-nitride layer 140 is grown. In the present exam-ple, it is made of the same material as the first group-III-nitride layer, that is GaN. In other embodiments, it is madeof AlGaN. Its thickness is typically between 300 nm and1.5 mm. In the present example it is 1 mm thick.[0049] The single interlayer 130 has a carbon concen-tration of 1x1019cm-3. In contrast, the first group-III-nitridelayer and the second group-III-nitride layer have a lowercarbon concentration of not more than 1x1018cm-3. Inthe present example, their carbon concentration is1x1017cm-3. Due to the different composition of the Al-GaN of the single interlayer 130 in comparison with theGaN of the first and second group-III-nitride layers 120and 140, a difference in lattice constants is achieved,which correlates with a difference in band gap. The dif-ference in lattice constants introduces a stress compo-nent that at least partially compensates stress createdby the growth on the heterosubstrate. The stress-man-agement layer sequence S made of GaN-AlGaN-GaN isthus able to reduce a stress which occurs in the bufferlayer structure and any layers grown on top of it. On theother hand, the stress-management layer sequence alsointroduces a band structure profile that tends to allow theformation of parasitic conductive channels, which mayeven comprise a 2DEG near the interfaces between theGaN and AlGaN materials. By introducing carbon at thegiven concentration levels the building of a 2DEG at theinterfaces of single interlayer 130 and first and secondgroup-III-nitride layers 120 and 140 is suppressed. Dueto its higher carbon concentration, especially the single

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interlayer layer 130 exhibits also a high resistivity whichis needed for the subsequent building of high efficientelectronic devices based on such a buffer layer structure.[0050] Fig. 2 shows another embodiment of a bufferlayer structure 200 according to the first aspect of theinvention. Compared with the buffer layer structure 100of Fig. 1, the buffer layer structure 200 comprises a nu-cleation layer 255 and a superlattice 250 between a sec-ond group-III-nitride layer 220 and the substrate 210. Thesuperlattice 250 and the nucleation layer form an exam-ple of an intermediate layer. The nucleation layer 255 isan AlN layer. The superlattice 250 in this embodiment isa high temperature AlGaN / low temperature AlGaN su-perlattice with a thickness of 100 nm. The nominal com-position of the high- and low-temperature AlGaN layersis identical. The nucleation layer 255 and the superlattice250 further improve the stress management of the bufferlayer structure 200. The nucleation layer 255 and thesuperlattice 250 have a carbon concentration of 5x1018

cm-3 in this example, which improves the resistivity of thebuffer layer structure. Other favorable embodiments ofthe superlattice are AlGaN/GaN or AlN/GaN superlattic-es.[0051] The use of several first group-III-nitride interlay-ers within the buffer layer structure further improves thestress management, in particular in the case of a siliconsubstrate, where the lattice mismatch to group-III-nitridematerials is particularly high. Therefore, on the stress-management layer sequence S comprising the firstgroup-III-nitride layer 220, the single interlayer 230 andthe second group-III-nitride layer 240, two repetitions ofthe stress management layer sequence are deposited.A first repetition contains a further single interlayer 231and a further group-III-nitride layer 241, the group-III-ni-tride layer 240 is the first group-III-nitride layer in regardof the second single interlayer 231 and at the same timethe second group-III-nitride layer in regard of the first sin-gle interlayer 230. A second repetition contains a furthersingle interlayer 232 and a further group-III-nitride layer242. Here the group-III-nitride layer 241 is the first group-III-nitride layer in regard of the third single interlayer 232and the second group-III-nitride layer in regard of thesecond single interlayer 231. The first single interlayer230 and its repetitions 231, 232 are thus all arrangedbetween and adjacent to group-III-nitride layers, whichin comparison have a lower band gap.[0052] In a further embodiment also properties otherthan the thickness of the single interlayers can vary, pref-erably the Al content can decrease in the direction point-ing away from the substrate. In another embodiment (notshown) more than one layer with lower band gap mayfollow the first group-III-nitride layer (or any of its repeti-tions).[0053] The single interlayer 231 in this embodimenthas identical properties (composition, thickness, growthtemperature) as single interlayer layer 230. The group-III-nitride layer 241 corresponds to the third group-III-ni-tride layer 240. However, in the present example, it is

somewhat thicker than the third group-III-nitride layer240, namely 2 mm thick. It is thus possible, but not arequirement to have the layers of the second stress man-agement layer sequence S1 identical to correspondinglayers of the original stress management layer sequenceS. It is also possible to vary layer thicknesses betweendifferent repetitions of the second stress managementlayer sequence S2. However, the thickness of the firstgroup-III-nitride layer 230 and its corresponding repeti-tions in the layers 231 and 232 is preferably the same.[0054] Fig. 3 shows a transistor 300 as an applicationcase of the group-III-nitride buffer layer structure accord-ing to the invention. In this embodiment, an AlGaN bufferlayer 350 having a Ga fraction increasing with increasingdistance from a silicon substrate 310 is grown on thesilicon substrate 310.[0055] On the AlGaN buffer layer 350, a stress-man-agement layer sequence S is formed of three group-III-nitride layers 320 to 340, of which a single interlayer 330is sandwiched between a first group-III-nitride layer 320and a second group-III-nitride layer 340. For the detailedcharacteristics of these layers, we refer to the above de-scription of the layers 120 to 140 of the embodiment ofFig. 1.[0056] The stress-management layer sequence S isfollowed by a back barrier layer 360. The back barrierlayer 360 is made of AlGaN and has a graded carbonconcentration starting with a carbon concentration of1x1018cm-3 at an interface between back barrier layer360 and third group-III-nitride layer 340, and ending witha carbon concentration of less than 4x1016 cm-3 at aninterface between back barrier layer 360 and a followingactive layer 370.[0057] The active layer 370, which forms a channellayer that contains a 2DEG charge carrier channel activein operation of the transistor, is made of GaN and has acarbon concentration of 4x1016 cm-3. In the present ex-ample, it is 40 nm thick. A transistor based on the de-scribed structure shows the following properties: acharge carrier densitiy of around 1x1013cm-2, a high elec-tron mobility of 1000-2500 cm2/(V·s) and a low sheet re-sistance clearly below 400 Ω/sq.[0058] On the active layer 370, a barrier layer 380 isgrown. The barrier layer is an AlGaN (AlGaInN) layer inthe present example. In other embodiments, other com-positions may be used that are suitable to form a potentialbarrier for charge carriers that are in the channel layer.The barrier layer has a thickness of 25 nm in the presentexample. A cap layer 390 is grown on the barrier layer380 and forms a uppermost layer of the transistor struc-ture (disregarding contacts). In the present case for dmode HEMT devices, the cap layer 390 is a p-dopedGaN layer with a thickness of 4nm. In the cap layer 390carbon is used a dopant at a concentration level of 1017

cm-3 in this example.[0059] The buffer layer structure of this embodimentleads to a favorable stress management of the wholestructure and at the same time achieves a high resistance

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below the active layers. The graded back barrier layer360 additionally prevents carriers from entering the un-derlying layers of the buffer layer structure and the sub-strate, which improves the electrical performance of thedevice. The active layer 370 (GaN or InGaN) grown onthe buffer layer structure exhibits a high crystal qualityand a low resistance due to its low carbon concentration.With the GaN cap layer 390 a good electrical contact tocontact structures and external devices is achieved. Toreduce the series resistance of the HEMT device addi-tional recess etching of GaN-cap and part of the AlGaN-layer in lateral sections for ohmic contact formation isfavorable.[0060] Fig. 4 a) shows a concentration diagram forparts of a transistor based on a buffer layer structureaccording to the invention as obtained from a secondaryion mass spectroscop (SIMS) measurement. Fig. 4bshows a current-voltage characteristic of the transistorof Fig. 4 a).[0061] Fig. 4 a) shows traces of an aluminum concen-tration 4000, a carbon concentration 4100 and an oxygenconcentration 4200 on a logarithmic scale as a functionof depth for a portion of the transistor including a bufferlayer structure. Maxima 430, 431, 432 and 433 of thealuminium concentration 4000 are detected at the posi-tions of the single interlayers. This is indicative of an Alcontribution to AlGaN stoichiometry of the single inter-layers[0062] A maximum 480 in the aluminum trace 4000represents an AlGaN barrier layer. As can be seen fromthe diagram of Fig. 4 a), also the carbon concentration4100 and the oxygen concentration exhibit respectivemaxima at the positions of the first group-III-nitride layers,i.e., at the Al peaks. The carbon concentration in the sin-gle interlayers is 2x1020cm-3, while the carbon concen-tration in the GaN layers between the single interlayersis slightly below 1x1017cm-3. Thus the carbon concen-tration in the single interlayers is 3 orders of magnitudehigher than in the GaN layers therebetween. The forma-tion of a 2DEG in the buffer layer structure is therebysuppressed. As shown in the diagram of Fig. 4 b) thetransistor based on the described buffer layer structurewith high carbon content (4300) exhibit high breakdownvoltage and low leakage current. The characteristic curveof a transistor according to the invention is herein com-pared with the characteristic curve (4301) of a transistorbased on a buffer layer structure which differs from theclaimed structure in the fact that a low p-type concentra-tion of less than 1x1018cm-3 is used in the interlayer struc-tures.[0063] Fig. 5 shows another embodiment of a bufferlayer structure 500 according to the first aspect of theinvention. Compared with the buffer layer structures ofthe Fig. 1 to 4, the buffer layer structure 500 comprisesan interlayer structure 530 comprising three different lay-ers instead of a single layer. After the growing of the firstgroup-III-nitride layer 520, in this example made of unin-tentionally doped GaN, a second group-III-nitride inter-

layer 536 is grown also consisting of GaN. The secondgroup-III-nitride interlayer is followed by a first group-III-nitride interlayer 535 and a third group-III-nitride interlay-er 537, whereby the third group-III-nitride interlayer 536also consist of GaN and is followed by a second group-III-nitride layer 540. The second and third group-III-nitrideinterlayers 535 and 537 have thicknesses of 50 nm inthis embodiment. Thicknesses between 20 and 200 nmare favorable for these layers. The first group-III-nitrideinterlayer 535 comprises a group-III-nitride interlayer ma-terial having a larger band gap than the materials of thefirst and second group-III-nitride layers 520, 540, in thiscase AlGaN, and has a thickness of 30 nm. In the follow-ing variations of the buffer layer structure including dif-ferent modifications of the Al content of the first group-III-nitride interlayer 535 and of the p-dopant concentra-tion will be presented and their effects on the preventionof the 2DEG building.

Variation 1

[0064] Variation 1 of the layer structure shown in Fig.5 includes the use of a first group-III-nitride interlayer 535with constant aluminum content. The aluminum contentprofile for the structure is shown in Fig. 6 a) in principle.As stated before in this embodiment the second and thirdgroup-III-nitride interlayers 536 and 537 are made of GaNas well as the first and second group-III-nitride layers 520and 540. The first group-III-nitride interlayer 535 consistof AlGaN. Fig. 6 b) shows the hole concentration profilefor the structure, the hole concentration is 5x1018 cm-3

within the whole interlayer structure, thus in all threegroup-III-nitride interlayers 535, 536 and 537. The firstand second group-III-nitride layers 520 and 540 are un-intentionally doped. The hole concentration in the inter-layer structure is achieved by intentionally doping the in-terlayers with magnesium or carbon. The shown holeconcentration profile for this layer structure leads to theenergy profiles of the layer structure shown in Fig. 6 c),whereby 601 represents the conduction band energy pro-file and 602 represents the valence band profile. As canbe seen in Fig. 6 c) the building of a 2DEG at the interfacesbetween AlGaN and GaN, thus between the first group-III-nitride interlayer 535 and the second and third group-III-nitride interlayers 536 and 537 is effectively sup-pressed.

Variation 2

[0065] Variation 2 of the layer structure shown in Fig.5 includes the use of a first group-III-nitride interlayer 535with a gradient in the aluminum content. The aluminumcontent profile for the structure is shown in Fig. 7 a) inprinciple. In this embodiment the second and third group-III-nitride interlayers 536 and 537 are made of GaN aswell as the first and second group-III-nitride layers 520and 540. The aluminum content of the first group-III-ni-tride interlayer 535 increases from 10 % at the interface

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to the second group-III-nitride interlayer 536 to 70 % atthe interface to third group-III-nitride interlayer 537. Theincrease can be either continous as shown in the diagramor stepwise.[0066] Fig. 7 b) shows the hole concentration profilefor the structure, the hole concentration for this embod-iment is 1x1018 cm-3 within the whole interlayer structure,thus in all three group-III-nitride interlayers 535, 536 and537. The first and second group-III-nitride layers 520 and540 are unintentionally doped.[0067] As can be seen in Fig. 7 c) the building of a2DEG at the interfaces between AlGaN and GaN, thusbetween the first group-III-nitride interlayer 535 and thesecond and third group-III-nitride interlayers 536 and 537is effectively suppressed with the relatively smaller holeconcentration respectively p-dopant-concentration if agraded AlGaN layer is used as the first group-III-nitrideinterlayer.

Variation 3

[0068] Variation 3 of the layer structure shown in Fig.5 shows compared to Variation 2 another advantageoushole concentration profile with which the building of par-asitic 2DEG can be effectively suppressed. The alumi-num content profile for the structure, shown in Fig. 8 a)is in principle the same as the one shown in Fig. 7 a).[0069] Fig. 8 b) shows the hole concentration profilefor the structure, the hole concentration for this embod-iment is 5x1018 cm-3 only in the second and third group-III-nitride interlayers 536 and 537, the first group-III-ni-tride interlayer 535 is unintentionally doped as well asthe first and second group-III-nitride layers 520 and 540.[0070] As can be seen in Fig. 8 c) where the energybands fo the structure at 300V are shown, the buildingof a 2DEG at the interfaces between AlGaN and GaN,thus between the first group-III-nitride interlayer 535 andthe second and third group-III-nitride interlayers 536 and537 can also effectively be suppressed without intention-al doping of the first group-III-nitride interlayer 535 if thedoping level of the second and third group-III-nitride in-terlayers 536 and 537 is sufficiently high, even in oper-ation.[0071] As described in Fig. 2 relating to single interlay-ers also with the described interlayer structures compris-ing three group-III-nitride interlayers repetitions of thestress-management layer sequence are advantageous.For example a first stress-management layer sequencecomprising a single interlayer made of AlN followed by athree stress-management layer sequences each com-prising a three-layer interlayer structure of GaN-AlGaN-GaN can be advantageous.

Claims

1. An epitaxial group-III-nitride buffer-layer structure ona heterosubstrate, wherein the buffer-layer structure

comprises at least one stress-management layer se-quence including an interlayer structure arrangedbetween and adjacent to a first and a second group-III-nitride layer, wherein the interlayer structure com-prises a group-III-nitride interlayer material having alarger band gap than the materials of the first andsecond group-III-nitride layers, and wherein a p-type-dopant-concentration profile drops, startingfrom at least 1x1018 cm-3, by at least a factor of twoin transition from the interlayer structure to the firstand second group-III-nitride layers.

2. The buffer-layer structure according to claim 1,wherein the p-type-dopant-concentration profiledrops in transition from the interlayer structure to thefirst and second group-III-nitride layers by at leastone order of magnitude.

3. The buffer-layer structure according to claim 1,wherein the p-type-dopant-concentration profiledrops in transition from the interlayer structure to thefirst and second group-III-nitride layers by at leasttwo orders of magnitude.

4. The buffer-layer structure according to one of theclaims 1 to 3, wherein the p-type-dopant-concentra-tion is constant throughout the interlayer structure.

5. The buffer-layer structure of one of the precedingclaims, wherein the interlayer structure is a singlelayer and the group-III-nitride interlayer material isof homogeneous composition, which has a largerband gap than the first and second group-III-nitridelayers and wherein the concentration of the p-typedopant in the interlayer structure is at least6x1018cm-3.

6. The buffer-layer structure of one of the claims 1 to4, wherein

- the interlayer structure comprises a first group-III-nitride interlayer that is arranged betweenand adjacent to a second group-III-nitride inter-layer and a third group-III-nitride interlayer,- the first, second and third group-III-nitride in-terlayer have a respective homogeneous com-position,- a band gap of the first group-III-nitride interlayeris larger than band gaps of the second group-III-nitride interlayer and the third group-III-nitrideinterlayer, and wherein- the p-type-dopant concentration throughoutthe interlayer structure is at least 5x1018cm-3.

7. The buffer-layer structure of one of the claims 1 to4, wherein the interlayer structure comprises a firstgroup-III-nitride interlayer that is arranged betweenand adjacent to a second group-III-nitride interlayer

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and a third group-III-nitride interlayer, and whereinthe first group-III-nitride interlayer is compositionallygraded.

8. The buffer-layer structure of one of the claims 1 to4, wherein

- the interlayer structure comprises a first group-III-nitride interlayer that is arranged betweenand adjacent to a second group-III-nitride inter-layer and a third group-III-nitride interlayer,- wherein the second group-III-nitride interlayeror the third group-III-nitride interlayer or the sec-ond and the third group-III-nitride interlayer arecompositionally graded.

9. The buffer-layer structure of claim 7, wherein the firstgroup-III-nitride interlayer is made of AlGaN havingan Al mole fraction that increases with increasingdistance from the substrate.

10. The buffer layer structure according to one of thepreceding claims, wherein the group-III-nitride inter-layer material having a larger band gap additionallyhas a larger oxygen concentration than either thefirst or the second group-III-nitride layer, or than boththe first and the second group-III-nitride layer.

11. The buffer-layer structure of claim 7 or 9, whereinthe concentration of the p-type dopant is higher inthe second and third group-III-nitride interlayers thanin the first group-III-nitride interlayer.

12. The buffer-layer structure of one of the precedingclaims, wherein the p-type dopant is carbon or mag-nesium, or a combination of carbon and magnesium.

13. The buffer-layer structure of one of the precedingclaims, wherein the first or the second, or the firstand the second group-III-nitride layer has a p-type-dopant concentration between 1x1016 /cm3 and1x1018 /cm3.

14. The buffer layer structure according to one of thepreceding claims, comprising at least two stress-management layer sequences wherein a secondstress-management layer sequence arranged at alarger distance from the substrate than a first stress-management layer sequence has a second interlay-er structure.

15. The buffer layer structure according to claim 14,wherein the second interlayer structure differs fromthe first interlayer structure in at least one of the fol-lowing: a layer thickness of at least one of the inter-layers of the interlayer structure, a p-type-dopant-concentration in at least one of the interlayers of theinterlayer structure, a material composition of at least

one of the interlayers of the interlayer structure, anda number of interlayers in the interlayer structure.

16. The buffer layer structure according to one of thepreceding claims, wherein an additional group-III-ni-tride layer is deposited on top of the buffer layer struc-ture and wherein the additional layer has a gradedp-type-dopant-concentration profile, wherein the p-type-dopant concentration is higher in a first sectionof the additional layer adjacent to the buffer layerthan in a second section of the additional layer furtheraway from the buffer layer.

17. The buffer layer structure according to one of thepreceding claims, wherein the heterosubstrate is ei-ther a silicon substrate, a silicon-on-insulator sub-strate, or a silicon carbide substrate.

18. The buffer layer structure according to one of thepreceding claims, further comprising a buffer stackdeposited between the heterosubstrate and thestress-management layer sequence, the bufferstack comprising

a) either a compositionally graded AlGaN bufferlayer having a Ga fraction increasing with in-creasing distance from the heterosubstrate,b) or a superlattice formed by a stack of alter-nating group-III-nitride layers of two kinds,

wherein the buffer stack has a p-type-dopant con-centration of at least 1x1017 cm-3.

19. A group-III-nitride device, in particular a transistor, aFET, a normally-on or a normally-off HEMT or MIS-HEMT, a Schottky diode, a PIN diode, a LED com-prising an epitaxial group-III-nitride buffer layerstructure on a heterosubstrate according to one ofthe preceding claims.

20. A method for fabricating a p-type-doped group-III-nitride buffer layer structure on a heterosubstrate,the method comprising

- fabricating an epitaxial group-III-nitride bufferlayer structure on a heterosubstrate, the bufferlayer structure comprising a stress-manage-ment layer sequence including an interlayerstructure arranged between and adjacent to afirst and a second group-III-nitride layer, and theinterlayer structure comprising a group-III-ni-tride interlayer material having a larger band gapthan the materials of the first and second group-III-nitride layers, wherein- the stress management layer sequence is fab-ricated with a p-type-dopant-concentration pro-file, which drops, starting from at least 1x1018

cm-3, by at least a factor of two in transition from

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the interlayer structure to the first and secondgroup-III-nitride layers.

21. The method according claim 20 further comprising,before fabricating the stress-management layer se-quence,

- depositing a AlN nucleation layer on the sub-strate at a temperature between 800 and 1030°C by a vapor phase deposition technique usinga hydrogen-free carrier gas- depositing a buffer stack before depositing thestress-management layer sequence by usingTetraethylgallium and Trimethylaluminum asprecursors at a temperature between 800 and1030°C,- wherein the buffer stack comprises

a) either a compositionally graded AlGaNbuffer layer having a Ga fraction increasingwith increasing distance from the hetero-substrate,b) or a superlattice formed by a stack of al-ternating group-III-nitride layers of twokinds, and wherein

the nucleation layer and the buffer stack are fab-ricated with a p-type-dopant concentration of atleast 1x1018 cm-3.

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REFERENCES CITED IN THE DESCRIPTION

This list of references cited by the applicant is for the reader’s convenience only. It does not form part of the Europeanpatent document. Even though great care has been taken in compiling the references, errors or omissions cannot beexcluded and the EPO disclaims all liability in this regard.

Patent documents cited in the description

• US 7884393 B [0005]