Structure and properties of Ta/Al/Ta and Ti/Al/Ti/Au multilayer metal stacks formed as ohmic contacts on n-GaN

Ievgen Boturchuk; Thomas Walter; Brian Julsgaard; Golta  Khatibi; Sabine Schwarz; Michael  Stöger-Pollach; Kjeld Pedersen; Vladimir N. Popok

doi:10.1007/s10854-019-02167-2

Structure and properties of Ta/Al/Ta and Ti/Al/Ti/Au multilayer metal stacks formed as ohmic contacts on n-GaN

Ievgen Boturchuk, Thomas Walter, Brian Julsgaard, Golta Khatibi, Sabine Schwarz, Michael Stöger-Pollach, Kjeld Pedersen, Vladimir N. Popok

Research output: Contribution to journal/Conference contribution in journal/Contribution to newspaper › Journal article › Research › peer-review

6 Citations (Scopus)

Abstract

Formation of ohmic contacts to GaN is of high practical importance for device fabrication. Due to the wide band gap, formation of multilayer metal structures is required to make electrical connections with low contact resistance. The paper presents a study on structure, composition, adhesion and electrical properties of Ti/Al/Ti/Au and Ta/Al/Ta metal stacks fabricated by e-beam evaporation and thermal annealing in order to provide ohmic contacts to n-type GaN films grown on Si. For the Ti-based case, an interdiffusion of Au and Ga into the stack is found, which is probably caused by a granular structure of the top Ti layer making no proper barrier. Ti of the bottom layer is observed to diffuse into GaN, forming a thin layer of titanium nitride with a low Schottky barrier at GaN interface allowing ohmic contact as shown by electrical measurements. The Ta-based stacks have the expected layered structure with minor interdiffusion of Al and Ta at the interfaces of these two metals. No direct microscopic evidences for Ta diffusion into GaN is observed. However, the formation of a thin tantalum nitride layer at the GaN interface can be deduced from the current–voltage measurements showing ohmic electrical contacts with low contact resistivity of 1.2 × 10 ⁻³ Ω cm ². Four-point bending tests on the both types of samples show that cracks always develop at the interface between Si and buffer layers of GaN. No exfoliation of the metallization layers is observed allowing us to conclude about good adhesion of the metal stacks. Thus, the fabricated Ti- and Ta-based multilayer structures show good electrical performance and reliable adhesion making them promising for the formation of ohmic contacts to n-type GaN.

Original language	English
Journal	Journal of Materials Science: Materials in Electronics
Volume	30
Issue	19
Pages (from-to)	18144-18152
Number of pages	9
ISSN	0957-4522
DOIs	https://doi.org/10.1007/s10854-019-02167-2
Publication status	Published - Oct 2019

Keywords

ALGAN/GAN
GALLIUM NITRIDE

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10.1007/s10854-019-02167-2

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@article{53f651124f834e8eb94a97f950aa8dae,

title = "Structure and properties of Ta/Al/Ta and Ti/Al/Ti/Au multilayer metal stacks formed as ohmic contacts on n-GaN",

abstract = "Formation of ohmic contacts to GaN is of high practical importance for device fabrication. Due to the wide band gap, formation of multilayer metal structures is required to make electrical connections with low contact resistance. The paper presents a study on structure, composition, adhesion and electrical properties of Ti/Al/Ti/Au and Ta/Al/Ta metal stacks fabricated by e-beam evaporation and thermal annealing in order to provide ohmic contacts to n-type GaN films grown on Si. For the Ti-based case, an interdiffusion of Au and Ga into the stack is found, which is probably caused by a granular structure of the top Ti layer making no proper barrier. Ti of the bottom layer is observed to diffuse into GaN, forming a thin layer of titanium nitride with a low Schottky barrier at GaN interface allowing ohmic contact as shown by electrical measurements. The Ta-based stacks have the expected layered structure with minor interdiffusion of Al and Ta at the interfaces of these two metals. No direct microscopic evidences for Ta diffusion into GaN is observed. However, the formation of a thin tantalum nitride layer at the GaN interface can be deduced from the current–voltage measurements showing ohmic electrical contacts with low contact resistivity of 1.2 × 10 −3 Ω cm 2. Four-point bending tests on the both types of samples show that cracks always develop at the interface between Si and buffer layers of GaN. No exfoliation of the metallization layers is observed allowing us to conclude about good adhesion of the metal stacks. Thus, the fabricated Ti- and Ta-based multilayer structures show good electrical performance and reliable adhesion making them promising for the formation of ohmic contacts to n-type GaN. ",

keywords = "ALGAN/GAN, GALLIUM NITRIDE",

author = "Ievgen Boturchuk and Thomas Walter and Brian Julsgaard and Golta Khatibi and Sabine Schwarz and Michael St{\"o}ger-Pollach and Kjeld Pedersen and Popok, {Vladimir N.}",

year = "2019",

month = oct,

doi = "10.1007/s10854-019-02167-2",

language = "English",

volume = "30",

pages = "18144--18152",

journal = "Journal of Materials Science: Materials in Electronics",

issn = "0957-4522",

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}

Structure and properties of Ta/Al/Ta and Ti/Al/Ti/Au multilayer metal stacks formed as ohmic contacts on n-GaN. / Boturchuk, Ievgen; Walter, Thomas; Julsgaard, Brian et al.
In: Journal of Materials Science: Materials in Electronics, Vol. 30, No. 19, 10.2019, p. 18144-18152.

Research output: Contribution to journal/Conference contribution in journal/Contribution to newspaper › Journal article › Research › peer-review

TY - JOUR

T1 - Structure and properties of Ta/Al/Ta and Ti/Al/Ti/Au multilayer metal stacks formed as ohmic contacts on n-GaN

AU - Boturchuk, Ievgen

AU - Walter, Thomas

AU - Julsgaard, Brian

AU - Khatibi, Golta

AU - Schwarz, Sabine

AU - Stöger-Pollach, Michael

AU - Pedersen, Kjeld

AU - Popok, Vladimir N.

PY - 2019/10

Y1 - 2019/10

N2 - Formation of ohmic contacts to GaN is of high practical importance for device fabrication. Due to the wide band gap, formation of multilayer metal structures is required to make electrical connections with low contact resistance. The paper presents a study on structure, composition, adhesion and electrical properties of Ti/Al/Ti/Au and Ta/Al/Ta metal stacks fabricated by e-beam evaporation and thermal annealing in order to provide ohmic contacts to n-type GaN films grown on Si. For the Ti-based case, an interdiffusion of Au and Ga into the stack is found, which is probably caused by a granular structure of the top Ti layer making no proper barrier. Ti of the bottom layer is observed to diffuse into GaN, forming a thin layer of titanium nitride with a low Schottky barrier at GaN interface allowing ohmic contact as shown by electrical measurements. The Ta-based stacks have the expected layered structure with minor interdiffusion of Al and Ta at the interfaces of these two metals. No direct microscopic evidences for Ta diffusion into GaN is observed. However, the formation of a thin tantalum nitride layer at the GaN interface can be deduced from the current–voltage measurements showing ohmic electrical contacts with low contact resistivity of 1.2 × 10 −3 Ω cm 2. Four-point bending tests on the both types of samples show that cracks always develop at the interface between Si and buffer layers of GaN. No exfoliation of the metallization layers is observed allowing us to conclude about good adhesion of the metal stacks. Thus, the fabricated Ti- and Ta-based multilayer structures show good electrical performance and reliable adhesion making them promising for the formation of ohmic contacts to n-type GaN.

AB - Formation of ohmic contacts to GaN is of high practical importance for device fabrication. Due to the wide band gap, formation of multilayer metal structures is required to make electrical connections with low contact resistance. The paper presents a study on structure, composition, adhesion and electrical properties of Ti/Al/Ti/Au and Ta/Al/Ta metal stacks fabricated by e-beam evaporation and thermal annealing in order to provide ohmic contacts to n-type GaN films grown on Si. For the Ti-based case, an interdiffusion of Au and Ga into the stack is found, which is probably caused by a granular structure of the top Ti layer making no proper barrier. Ti of the bottom layer is observed to diffuse into GaN, forming a thin layer of titanium nitride with a low Schottky barrier at GaN interface allowing ohmic contact as shown by electrical measurements. The Ta-based stacks have the expected layered structure with minor interdiffusion of Al and Ta at the interfaces of these two metals. No direct microscopic evidences for Ta diffusion into GaN is observed. However, the formation of a thin tantalum nitride layer at the GaN interface can be deduced from the current–voltage measurements showing ohmic electrical contacts with low contact resistivity of 1.2 × 10 −3 Ω cm 2. Four-point bending tests on the both types of samples show that cracks always develop at the interface between Si and buffer layers of GaN. No exfoliation of the metallization layers is observed allowing us to conclude about good adhesion of the metal stacks. Thus, the fabricated Ti- and Ta-based multilayer structures show good electrical performance and reliable adhesion making them promising for the formation of ohmic contacts to n-type GaN.

KW - ALGAN/GAN

KW - GALLIUM NITRIDE

UR - http://www.scopus.com/inward/record.url?scp=85073417916&partnerID=8YFLogxK

U2 - 10.1007/s10854-019-02167-2

DO - 10.1007/s10854-019-02167-2

M3 - Journal article

SN - 0957-4522

VL - 30

SP - 18144

EP - 18152

JO - Journal of Materials Science: Materials in Electronics

JF - Journal of Materials Science: Materials in Electronics

IS - 19

ER -

Structure and properties of Ta/Al/Ta and Ti/Al/Ti/Au multilayer metal stacks formed as ohmic contacts on n-GaN

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