High temperature thermal stability of Au/Ti/WSixSchottky contacts on n-type 4H-SiC

Jihyun Kim a, F. Ren a,*, A.G. Baca b, R.D. Briggs b, S.J. Pearton c

a Department of Chemical Engineering, University of Florida, P.O. Box 116005, Gainesville, FL 32611, USAb Sandia National Laboratories, Albuquerque, NM 87185, USA

c Department of Material Science and Engineering, University of Florida, Gainesville, FL 32611, USA

Received 2 November 2002; received in revised form 14 January 2003; accepted 17 January 2003

Abstract

The thermal stability of Au/Ti/WSix contacts on 4H-SiC was examined by Auger electron spectroscopy and current–

voltage measurements. The silicide-based contacts on SiC are found to exhibit improved thermal stability compared to

pure W contacts. The Au/Ti/WSix contacts show a maximum Schottky barrier height of �1.15 eV as obtained from

current–voltage (I–V ) measurements. After 500 �C anneals, the Ti diffuses to the surface of the contact structure,

followed by a Au-rich layer and finally the WSix. After 1000 �C anneals, the Ti and Au showed significant mixing.

Particulates formed on the surface in the latter case were Au-rich phases.

� 2003 Elsevier Science Ltd. All rights reserved.

1. Introduction

Due to their excellent set of transport and band gap

properties, SiC Schottky rectifiers are gaining interest

for use in high power, high temperature electronic

switching applications and for sensing of combustion

gases in industry and other situations such as long-term

space flights. The need to understand the long-term

aging characteristics of the contacts, particularly at ele-

vated temperatures has stimulated interest in under-

standing the electrical properties and thermal stability of

different metal rectifying contacts on SiC [1–20]. In

particular, systems such as WC [8], Ti0:58W0:42 [10] and

Pt/Ti/WSi/Ni [20] show very promising stability on SiC

for high temperature applications. The WC showed a

relatively low barrier height of 0.79 eV at 300 K in n-

type 6H-SiC [8], while Ti0:58W0:42 showed a higher value

of 1.22 eV in n-type 6H-SiC over the temperature range

24–300 �C [10]. These are promising for rectifying con-

tacts, while the Pt/Ti/WSi/Ni annealed at 1000 �C

showed excellent long-term stability as an ohmic contact

on n-type 4H-SiC [20].

We have previously found that sputter-deposited

WSix-based contacts show maximum barrier heights of

1.15 eV after a 500 �C post-deposition anneal to remove

ion-induced damage [21]. The contacts showed reduced

forward and reverse currents when measured at elevated

temperatures compared to the more common Ni recti-

fying contacts used in SiC technology. These results

suggest that WSix may be a promising candidate as a

stable Schottky metallization on n-type SiC, but little

work has appeared on this particular aspect. In this

paper we report on the thermal stability of the contact

against intermixing, as measured by Auger electron

spectroscopy.

2. Experimental

The starting substrates were nþ (n � 1019 cm�3) 4H-

SiC. Approximately 10 lm of undoped (n � 5� 1015

cm�3) was grown on these substrates by vapor phase

epitaxy technique. E-beam evaporated Ni (200 nm) was

deposited for full backside area ohmic contacts and was

annealed at 970 �C for 3 min to obtain a low resistance.

*Corresponding author. Tel.: +1-352-392-4757; fax: +1-352-

392-9513.

E-mail address: ren@che.ufl.edu (F. Ren).

0038-1101/03/$ - see front matter � 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0038-1101(03)00069-8

Solid-State Electronics 47 (2003) 1345–1350

www.elsevier.com/locate/sse

The samples were cleaned sequentially in acetone, iso-

propanol and buffered oxide etchant (BOE) prior to

sputter deposition of 700 �AA of WSi0:45 using an Ar

plasma and separate W and Si targets. Detailed char-

acterization of these types of films in GaN have been

reported previously [21–23]. To reduce the sheet resis-

tance of the metallization, a further 200 �AA of Ti and 700�AA of Au were deposited on top of the WSix. The 120 lmdiameter Schottky diodes were patterned using standard

photolithography processing. The thermal stability of

the Au/Ti/WSix diodes were tested for anneals up to

1000 �C for 1 min under flowing N2 in a Heatpulse 610T

furnace. Current–voltage (I–V ) measurements were

performed in the temperature range (25–300 �C) usingan HP4156C parameter analyzer. The Auger electron

spectroscopy (AES) was performed on a Physical Elec-

tronics 660 Scanning Auger Microprobe with a 10 keV,

1 lA beam at 30� for the sample normal. Profiling wasachieved by sputtering with a 3 keV Ar ion beam at a

current of 2 lA rastered over a 3 mm2 area. The sputter

rate for Au was 200 �AA/min, for Ti was 90 �AA/min and for

WSi was 60 �AA/min. Optical and secondary electron im-ages of the analysis areas were also obtained.

3. Results and discussion

Fig. 1 shows reverse leakage current characteristics

from the diodes as a function of post-deposition anneal

temperature (top) and as a function of measurement

temperature for a post-deposition anneal of 500 �C,which was found to produce the maximum Schottky

barrier height of 1.15 eV. More detail on the electrical

characteristics of the contacts is given elsewhere [24].

The ideality factors were typically 1.1 after the optimum

-10 -8 -6 -4 -2 0

2.0x10-3

4.0x10-3

6.0x10-3

8.0x10-3

1.0x10-2

As deposited500oC, 1min, N2

700oC, 1min, N2

900oC, 1min, N2

1000oC, 1min, N2

-30 -20 -10 0

5.0x10-8

1.0x10-7

1.5x10-7

WSi/n-SiC

WSi(25oC)

WSi(100oC)

WSi(200oC)

WSi(300oC)

Fig. 1. Reverse I–V characteristics from Au/Ti/WSi contacts on

6H-SiC as a function of post-deposition anneal temperature

(top) or as a function of measurement temperature after an

optimized post-deposition anneal of 500 �C.

Fig. 2. Secondary electron images of Au/Ti/WSi contacts

as-deposited (top) or after 500 �C (center) or 1000 �C (bottom)

anneals.

1346 J. Kim et al. / Solid-State Electronics 47 (2003) 1345–1350

anneal, whereas the unannealed (as-deposited) contacts

showed ideality factors of �2 and a lower barrier heightof 0.97 eV. The barrier height decreased for anneals

above about 600 �C, reaching values of 0.99 eV after 700�C anneals, 0.69 eV after 900 �C and 0.41 eV after 1000

�C. The reverse current densities were slightly lower atelevated measurement temperatures than for Ni Scho-

ttky contacts on the same wafers.

Fig. 2 shows secondary electron images of the as-

deposited contact (top), after 500 �C anneal (center) andafter 1000 �C anneal (bottom). The first two show

Table 1

Concentration of elements detected on the as-received surfaces

(in at.%)a

Sample C O Au Ti Si

1––as-deposited 54 1 45 nd nd

2––500 �C 48 34 7 7 4

3––1000 �C 36 25 11 14 13

aAES does not detect hydrogen and helium and all concen-

trations are normalized to 100%.

Fig. 3. AES survey spectra from Au/Ti/WSi contacts as-

deposited (top) or after 500 �C (center) or 1000 �C (bottom)

anneals.

Fig. 4. AES elemental depth profiles from Au/Ti/WSi contacts

as-deposited (top) or after 500 �C (center) or 1000 �C (bottom)

anneals.

J. Kim et al. / Solid-State Electronics 47 (2003) 1345–1350 1347

relatively smooth morphology, while the highest tem-

perature anneal produces many small (1–3 lm) particle-like features and some larger particles. These changes in

morphology correspond to changes in the near-surface

composition, as summarized in Table 1.

AES survey spectra from the as-deposited, 500 and

1000 �C annealed samples are shown in Fig. 3 and the

corresponding depth profiles are shown in Fig. 4. The

as-deposited sample shows abrupt and smooth interfaces

between the various layers in the metallization scheme.

After 500 �C annealing, the Ti diffuses out into the

Au, while the WSi/SiC interface remains abrupt. The

movement of Ti towards the surface is accompanied by

its oxidation. After the 1000 �C annealing, the WSi/SiC

interface still remains fairly abrupt, while the Ti and Au

are completely intermixed and there is outdiffusion of C

from the substrate into the reacted surface region.

Under these conditions, the contact shows a more

ohmic-type behavior and the formation of the b-phaseW2N compound is detected by X-ray diffraction exper-

iments.

Elemental maps are shown for the 1000 �C annealed

sample in Figs. 5 and 6. These were obtained after the

sputter-depth profile was complete and show that the

particles present on the surface contain Au. The pres-

ence of the other elements is expected because the profile

was stopped near the WSi/SiC interface.

The AES data basically confirms the electrical results

in that the contact shows significant reaction for anneals

above 600–700 �C and this will define the upper limit forthe stable range of device or sensor operation on which

these contacts are employed. Note that it is the presence

of the Au and Ti that dominates the stability charac-

teristics and future work should focus on overlayers that

provide low sheet resistance without compromising the

thermal and electrical stability.

4. Summary and conclusions

The thermal stability of Au/Ti/WSi contacts on SiC

has been investigated by AES and I–V measurements.

Following the sputter deposition, annealing at 500 �Cremoves ion-induced damage and produces the best

electrical properties. The contact metallurgy is stable

under these conditions, but the use of higher annealing

temperatures leads to significant intermixing of the

contact and a degradation of the barrier height and

ideality factor.

Fig. 5. Elemental maps of W, Au and Ti from the 1000 �C annealed contact after sputter-depth profiling to near the WSi/SiC interface.

1348 J. Kim et al. / Solid-State Electronics 47 (2003) 1345–1350

Acknowledgements

The work at UF is particularly supported by NASA

(NAG10-316, Dr. William Knott) and the UCF-UF

Space research Initiative, and also by NSF DMR-

0101438. Sandia is a multiprogram laboratory operated

by Sandia Corporation for Lockheed–Martin under

DOE contract DE-AC-04-85000.

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