Ohmic contact and interfacial reaction of Ti/Al/Pt/Aumetallic multi-layers on n-AlxGa1xN/GaN heterostructures

B. Shen a,*, H.M. Zhou a, J. Liu a, Y.G. Zhou a, R. Zhang a,Y. Shi a, Y.D. Zheng a, T. Someya b, Y. Arakawa b

a National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, Chinab Research Center for Advanced Science and Technology and Institute of Industrial Science, University of Tokyo, Komaba 4-6-1,

Meguro-ku, Tokyo 153-8904, Japan

Abstract

The specific contact resistivity (qC) and interfacial reaction between Au/Pt/Al/Ti metallic multi-layers and Si-dopedn-type AlxGa1xN (n-AlGaN) layers in modulation-doped Al0:22Ga0:78N/GaN heterostructures have been investigated.

By means of the measurements based on the transmission line model, the qC as low as 1.6 104 X cm2 is obtained.Based on the X-ray diffraction analysis, it is found that N atoms in n-AlGaN layer diffuse out and a much amounts of

N-vacancies are formed in n-AlGaN layer near the interface after the sample is annealed at temperatures higher than

500 C. It induces the heavy n-type doped region in n-AlGaN near the interface, and thus leads to the decrease of theqC. With increasing the annealing temperature, more N atoms in n-AlGaN layer diffuse out and react with Ti atoms.Ti2N phase is formed at the interface after the sample is annealed at 800 C. In this case, the qC further decreases. 2003 Elsevier Science B.V. All rights reserved.

Keywords: AlxGa1xN/GaN heterostructures; Ohmic contact; Interfacial reactions

1. Introduction

AlxGa1xN/GaN heterostructure field-effect

transistors (HFETs) are the most promising elec-

tronic devices for high power, high temperature,and microwave applications [1,2]. To achieve a

high performance of the devices, the ohmic contact

of high quality on AlxGa1xN/GaN heterostruc-

tures is crucial [3,4]. The parasitic ohmic contact

resistance will decrease both the transconductance

and the saturation current and seriously limit the

performance of the devices [5,6]. In the past several

years, the low-resistance ohmic contact on n-type

GaN (n-GaN) films with the specific contact re-

sistivity (qC) as low as 1 108 X cm2 has beenrealized [7]. However, the ohmic contact on n-typeAlxGa1xN (n-AlGaN) layer in AlxGa1xN/GaN

heterostructures is not satisfied until now [8,9].

The best specific contact resistivity on n-AlGaN is

several orders of magnitude lower than that on

n-GaN. The mechanism of the metal/n-AlGaN

ohmic contact and the interfacial reaction between

metallic layers and n-AlGaN layers are also nec-

essary to be further investigated.In this study, the specific contact resistivity bet-

ween Au/Pt/Al/Ti metallic multilayers and Si-doped

*Corresponding author. Fax: +86-25-3328130.

E-mail address: boshen@jlonline.com (B. Shen).

0925-3467/03/$ - see front matter 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0925-3467(03)00083-1

Optical Materials 23 (2003) 197–201

www.elsevier.com/locate/optmat

n-AlGaN layers in modulation-doped Al0:22Ga0:78-

N/GaN heterostructures as the function of the

annealing temperature and time was investigated

by means of the measurements based on the

transmission line model. The interfacial reaction

between the metallic multi-layers and the n-Al-GaN layers was also discussed based on the X-ray

diffraction analysis.

2. Experimental

Modulation-doped Al0:22Ga0:78N/GaN hetero-

structures were grown on (0 0 0 1) surface of sap-phire (a-Al2O3) substrates using metal-organicchemical vapor deposition (MOCVD). First, a

nucleation GaN buffer layer was grown at 488 C,follow by a 2.0-lm-thick unintentionally dopedGaN layer deposited at 1071 C. Then, an unin-tentionally doped Al0:22Ga0:78N (i-AlGaN) layer,

followed by a 65-nm-thick Si-doped Al0:22Ga0:78N

layer were deposited, both at 1080 C. Fig. 1 showsthe x=2h X-ray diffraction rocking curve of thesample. The scanning is along the GaN [0 0 0 1]

direction. Besides the peak from the GaN [0 0 0 2]

reflection, the peak with perfect symmetry from

the Al0:22Ga0:78N layer is observed. The fringe

peaks distributed on the two sides of the

Al0:22Ga0:78N peak indicate that the Al0:22Ga0:78N/

GaN interface is very uniform and sharp. The

detailed growth of the heterostructures was de-

scribed in Ref. [10]. The carrier concentration in

n-AlGaN layer is 1.26 1018 cm3 at room tem-

perature determined by means of Van der Pauw

Hall measurement.

Ti/Al/Pt/Au (15 nm/100 nm/40 nm/200 nm)metallic multi-layer for the ohmic contact was

deposited on the n-AlGaN layer by means of

e-beam evaporation. The method of the trans-

mission line measurements (TLMs) was used to

determine the specific contact resistivity of the

samples [11]. The metal film for the TLMs was

patterned by standard photolithographic lift-off

technique. Fig. 2 shows the schematic circularohmic contact pattern and the equivalent circuit

for the TLMs. In Fig. 2, the radius of the central

metallic spot r0 is 100 lm. The inner radius r1 andouter radius r01 of the inside metal ring are 110 and400 lm, respectively. The inner radius r2 and outerradius r02 of the outside metal ring are 420 and 700lm, respectively. Between metal rings and spot,

16.0 16.4 16.8 17.2 17.610-1

101

103

105

107

AlGaN

GaN

Inte

nsity

(a.u

.)

angle (degree)

Fig. 1. x=2h X-ray diffraction rocking curve of a modulation-doped Al0:22Ga0:78N heterostructure. The scanning is along the

GaN [0 0 0 1] direction.

Fig. 2. (a) Schematic metal/n-AlGaN circular contact pattern,

and (b) the equivalent circuit for the circular contact.

198 B. Shen et al. / Optical Materials 23 (2003) 197–201

there are the SiON dielectric layers. The samples

were annealed in high-purity N2 at temperatures

between 300 and 900 C for 5 min before thecurrent–voltage I-V checking and the ohmic con-tact measurements.

The X-ray diffraction analysis was performedusing the D/max/A X-ray diffraction spectrometer

operated at 30 kV, 60 mA. The X-ray wavelength

was 0.1542 nm.

3. Results and discussion

If the annealing temperature is higher than 500C, the I–V curve of the samples will show thelinear relation, indicating the good ohmic contact

between metal layer and n-AlGaN.

The specific contact resistivity qC of the samplesas the function of the annealing temperature is

shown in Fig. 3. The qC decreases rapidly withincreasing temperature. At 500 C, the qC is 1.5103 X cm2. At 850 C, it decreases to 1.6 104X cm2.For understanding the mechanism of the

change of qC, the interfacial reaction of the Ti/Al/Pt/Au/n-AlGaN contact was investigated by means

of XRD analysis. Fig. 4 shows the XRD spectra of

the sample with the as-deposited metal film on

n-AlGaN (Fig. 4(a)), and the samples annealed at

700 C (Fig. 4(b)), 800 C (Fig. 4(c)) and 900 C(Fig. 4(d)), respectively.

In Fig. 4(a), the diffraction peaks from GaN,

AlGaN, a-Al2O3 and each metal thin film areobserved. If the annealing temperature is higher

than 500 C, the alloy phases between Ti and Alare formed. As shown in Fig. 4(b), the diffraction

peaks from AlTi, AlTi and AlTi3 phases are ob-served. Meanwhile, the peaks from Al and Ti films

disappear. As shown in Fig. 4(c), the diffraction

peak of Ti2N phase, which is located at 37.0, isobserved when the annealing temperature is 800

C. This indicates that a much amount of N atomsdiffuse out from n-AlGaN and react with Ti atoms

to form the new Ti–N phase. As shown in Fig.

4(d), besides the Ti2N diffraction peak, the peakfrom new Au3Pt phase is also observed when the

annealing temperature is 900 C, indicating theinterfacial reaction between Au and Pt films occurs

at this temperature. Meanwhile, the peaks from

Au and Pt films disappear.

From the above results, we can find that: for Ti/

Al/Pt/Au metallic multi-layer, if the annealing

temperature is higher than 500 C, the inter-dif-fusion at the Ti/Al interface occurs firstly, and the

new Ti–Al alloy phases are formed. The Pt film

and Au film are stable between 500 and 800 C. Ifthe annealing temperature is higher than 900 C,Pt/Au interfacial reaction occurs. No evidences

indicate the Al/Pt interfacial reaction even if the

annealing temperature is 900 C.Due to the limitation of the detecting resolution

of the XRD technique, no XRD peak of Ti–N

alloy phase is observed when the annealing tem-

perature is lower than 800 C. However, we stillthink the our-diffusion of the N atoms from

n-AlGaN layer into Ti layer or Ti–Al alloy layer

occurs at 500 C or further lower annealing tem-peratures. But the amount of the out-diffused N

atoms at such temperature is not enough to formnew Ti–N alloy phase. With increasing the an-

nealing temperature, the our-diffusion of the N

atoms becomes more violent, and thus the Ti2N

phase is observed at 800 C.The our-diffusion of the N atoms leaves the

N-vacancies in n-AlGaN layer near the metal/

n-AlGaN interface. Because a N-vacancy is deter-

mined to serve as the shallow donor in AlxGa1xNor GaN, the high n-doped region in n-AlGaN

forms near the metal/n-AlGaN interface, and thus

450 500 550 600 650 700 750 800 850 9000

1

2

3

4

5

6

7

8

9

ρ C (

2x10

-4Ω

cm2 )

Temperature(˚C)

Fig. 3. The specific contact resistivity qC of the samples as thefunction of the annealing temperature in N2 ambient for 5 min.

B. Shen et al. / Optical Materials 23 (2003) 197–201 199

the specific contact resistivity qC decreases. With in-creasing the annealing temperature, the N-vacancy

concentration in n-AlGaN increases, and thus the

qC further decreases.From the above results and discussion, we can

find that it is difficult to obtain the high N-vacancy

concentration in n-AlGaN and the very low qC at500 C or further lower annealing temperatures.This is much different from the situation in the Ti/Al/Pt/Au layer on n-GaN. However, the high

temperature annealing is suspected to influence

the electrical properties of AlxGa1xN/GaN het-

erostructures, and thus cause the degradation

of the device performance [6]. Therefore, for metal/

n-AlGaN ohmic contact, it is necessary to look for

a more suitable metal element to replace Ti if

we do not want to increase the annealing temper-ature.

4. Conclusion

The specific contact resistivity and interfacial

reaction between Au/Pt/Al/Ti metallic multi-layers

and n-AlGaN layers in modulation-doped

Al0:22Ga0:78N/GaN heterostructures have been in-

vestigated by means of the TLMs and XRD

analysis. The qC decreases with increasing an-nealing temperature, and the qC of 1.6 104X cm2 is obtained at 850 C. It is found that Natoms in n-AlGaN layer diffuse out and a much

amounts of N-vacancies are formed in n-AlGaN

layer near the interface after the sample is an-

nealed at temperatures higher than 500 C. It in-duces the heavy n-type doped region in n-AlGaN

near the interface, and thus leads to the decrease ofthe qC. With increasing the annealing temperature,more N atoms in n-AlGaN layer diffuse out and

Fig. 4. The XRD spectra of (a) the sample with the as-deposited metal film on n-AlGaN, and the samples annealed at (b) 700 C,(c) 800 C, and (d) 900 C, respectively, in N2 ambient for 5 min.

200 B. Shen et al. / Optical Materials 23 (2003) 197–201

react with Ti atoms. Ti2N phase is formed at the

interface after the sample is annealed at 800 C. Inthis case, the qC further decreases.

Acknowledgements

This work was supported by Special Funds for

Major State Basic Research Projects of China

#G20000683, the National Natural Science

Foundation of China (nos. 60136020, 69976014,

and 69987001), and the National High Technology

Research and Developments Project of China.

References

[1] Q. Chen, J.W. Wang, R. Gaska, M.A. Khan, M.S. Shur,

G.J. Sullivan, A.L. Sailor, J.A. Higgings, A.T. Ping, I.

Asesida, IEEE Electron Dev. Lett. 19 (1998) 44.

[2] U.K. Mishra, Y.F. Uw, B.P. Keller, S. Keller, S.P.

Denbaars, IEEE Trans. Microwave Theor. Tech. 46

(1998) 756.

[3] J. Wurfi, V. Abrosimova, J. Helsenbeck, E. Nebauer, W.

Tiegetr, G. Trankle, Microelectron. Reliab. 39 (1999)

1737.

[4] M.S. Shur, R. Gaska, A. Bykhovski, Solid-State Electron.

43 (1999) 1451.

[5] S. Heikanan, S. Keller, S.P. Denbaars, U.K. Mishira,

Appl. Phys. Lett. 78 (2001) 2876.

[6] K.O. Schweitz, P.K. Wang, S.E. Mohney, D. Gottold,

Appl. Phys. Lett. 80 (2002) 1954.

[7] Q.Z. Liu, S.S. Lau, Solid-State Electron. 42 (1998)

677.

[8] S.J. Pearton, J.C. Zolper, R.J. Shul, F. Ren, J. Appl. Phys.

86 (1999) 1.

[9] J.W. Zhang, T.P. Zhang, W. Wang, B.J. Ning, G.Y. Wu,

Chinese J. Semicond. 22 (2001) 737 (in Chinese).

[10] B. Shen, T. Someya, Y. Arakawa, Appl. Phys. Lett. 76

(2000) 2746.

[11] G.K. Reeves, Solid-State Electron. 23 (1980) 487.

B. Shen et al. / Optical Materials 23 (2003) 197–201 201