Solid-State Electronics 48 (2004) 175–178

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Short Communication

Thermal stability of WSiX Schottky contacts on n-type 4H-SiC

Jihyun Kim a, F. Ren a,*, A.G. Baca b, G.Y. Chung c, S.J. Pearton d

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

c Sterling Semiconductor, Tampa, FL 33619, USAd Department of Material Science and Engineering, University of Florida, Gainesville, FL 32611, USA

Received 11 November 2002; received in revised form 17 January 2003; accepted 2 February 2003

Abstract

Sputter-deposited WSi0:45 rectifying contacts were characterized on n-type 4H-SiC as a function of annealing and

measuremental temperature. The as-deposited contacts show evidence of recombination-dominated carrier transport

and a high series resistance due to ion-induced damage occurring during the Ar plasma-assisted deposition. Annealing

at 500 �C for 1 min produced a maximum barrier height of 1.15 eV and reduced the diode ideality factor. The contacts

were degraded by annealing at >700 �C but showed reduced forward and reverse currents when measured at elevated

temperature (300 �C) compared to the more common Ni rectifying contacts.

� 2003 Elsevier Ltd. All rights reserved.

1. Introduction

SiC Schottky rectifiers are gaining interest for use in

high power, high temperature electronic switching ap-

plications and for hydrogen gas sensing in long-term

space flights. This has stimulated interest in under-

standing the electrical properties and thermal stability of

different metal rectifying contacts in SiC [1–20]. In par-

ticular, 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 rela-

tively 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 �Cshowed excellent long-term stability as an ohmic contact

on n-type 6H-SiC [20].

These results suggest that WSiX may be a promising

candidate as a stable Schottky metallization on n-type

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

392-9513.

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

0038-1101/$ - see front matter � 2003 Elsevier Ltd. All rights reserv

doi:10.1016/S0038-1101(03)00113-8

SiC. The characteristics of a metal/SiC contact strongly

depend on the Schottky barrier height /B, given by

/B ¼ /m � vS

where /m is the metal workfunction and vS the electron

affinity of the SiC [4]. It is important to establish clean

surface conditions prior to metal deposition in order to

avoid a high density of interface states [5]. In this paper

we show that sputtered WSi0:45 shows a barrier height of

1.15 eV after a 500 �C post-deposition annealing treat-

ment and retains a barrier height of P 1 eV to anneal

temperatures above �700 �C. The thermal stability is

better than that of Ni, which is a very common Schottky

metallization for 4H-SiC.

2. Experimental

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

SiC. Approximately 10 lm of undoped (n � 2� 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.

The samples were cleaned sequentially in acetone, iso-

propanol and buffered oxide etchant (BOE) prior to

ed.

Ni(2000Å)

WSiX/Ti/Au

4H-SiC

Epi SiC (10 m)µ

Fig. 1. Schematic of WSiX /SiC Schottky diode.

0.0 0.2 0.4 0.6 0.8 1.0

10-11

1x10-9

1x10-7

1x10-5

1x10-3

1x10-1

WSi/n-SiC

Cur

rent

(A)

Bias(V)

As deposit 500oC, 1min, N2

700oC, 1min, N2

900oC, 1min, N2

1100oC, 1min, N2

1x10-8

1x10-6

1x10-4

1x10-2

WSi/n-SiC

As deposit 500oC, 1min, N

2

700oC, 1min, N2

900oC, 1min, N2

o

Cur

rent

(A)

176 J. Kim et al. / Solid-State Electronics 48 (2004) 175–178

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]. The 120 lm diameter Scho-

ttky diodes were patterned using standard photolithog-

raphy processing (Fig. 1). For comparison similar diodes

with e-beam evaporated Ni Schottky contacts were also

fabricated. The thermal stability of the WSiX diodes were

tested for anneals up to 1100 �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) using an HP4156C parameter analyzer.

-10 -8 -6 -4 -2 0

1x10-10

1100 C, 1min, N2

Bias(V)

Fig. 2. Forward (top) and reverse (bottom) I–V characteristics

from WSiX diodes on 4H-SiC as a function of post-deposition

annealing temperature.

3. Results and discussion

Fig. 2 shows the forward (top) and reverse (bottom)

I–V characteristics for WSiX diodes as a function of

post-deposition annealing temperature. The as-depos-

ited diodes show signatures of sputter-induced damage

causing a high series resistance. The average energy of

incident Arþ ions on the SiC sample is �100 eV, but this

is sufficient to cause displacement damage in the semi-

conductor. The post-deposition annealing appears effec-

tive at recovering this ion-induced damage. For anneals

at P 900 �C, the diodes show high forward and reverse

current which is indicative of a reaction between the WSi

and the SiC, lowering the effective barrier height.

The current density J for a Schottky diode can be

written

J ¼ JS exp eV � JRON

nkT

� ��� 1

�

where the saturation current density JS is given by

JS ¼ A��T 2 exp

�� e/B

kT

�

where A�� is the Richardson’s constant (146 A cm�2 K�2

for SiC), T is the absolute measurement temperature, ethe electronic charge, k is Boltzmann’s constant, V the

applied voltage, n the ideality factor and RON the on-

state resistance. From the linear portions of the forward

I–V characteristics in Fig. 2, we extracted the barrier

height for the WSiX contacts and also the ideality factors

as a function of post-deposition annealing temperature

from the relations

/B ¼ kTe

lnA��T 2

JS

� �

n ¼ e2:3kT

oVo logðJÞ

� �

The resulting values are summarized in Table 1. The

as-deposited contacts show an ideality factor of �2,

which is indicative of the presence of a high density of

recombination centers. The 500 �C anneal treatment

increases the barrier height and decreases the ideality

factor. This appears to be the optimum conditions for

maximizing the barrier height. For comparison, Ni de-

posited on the same wafers showed a slightly lower /B

(1.1 eV) and higher ideality factor, as also shown in

Table 1. By comparison, Ti shows a /B of �0.8 eV in

n-type SiC, Au has �1.7 eV, TiW has 1.2 eV and Pt has

1.4 eV [11]. Annealing at P900 �C severely decreases the

Table 1

Summary of electrical data for annealed contacts

Contact Barrier

height (eV)

Ideality factor

WSi (as-deposited) 0.97 2.1

WSi (500 �C, 1 min, N2) 1.2 1.2

WSi (700 �C, 1 min, N2) 0.99 1.1

WSi (900 �C, 1 min, N2) 0.69 1.2

WSi (1100 �C, 1 min, N2) 0.41 2.2

Ni (as-deposited) 1.1 1.7

Table 2

Summary of electrical data as a function of measurement

temperature

Barrier height (eV) Ideality factor

WSi contacts

25 �C 1.2 1.1

100 �C 1.3 1.1

200 �C 1.4 1.1

300 �C 1.5 1.1

Ni contacts

25 �C 1.0 1.7

100 �C 1.2 1.4

200 �C 1.3 1.3

300 �C 1.4 1.3

10-7

J. Kim et al. / Solid-State Electronics 48 (2004) 175–178 177

barrier height and also degrades the ideality factors of

the WSiX contacts.

Fig. 3 shows the forward I–V characteristics from Ni

and WSiX contacts in SiC, as a function of measurement

temperature. The WSiX sample was annealed at 500 �Cprior to the measurements. As is seen by comparing the

magnitude of the currents at a given forward bias, the

WSiX has slightly lower current, particularly at higher

measuremental temperature. This suggests that this

metallization system has a barrier height that decreases

less with temperature than Ni does, and this would be an

advantage in high temperature applications. This is

0.0 0.5 1.0 1.5

1x10-10

1x10-8

1x10-6

1x10-4Ni/n-SiC

Cur

rent

(A)

Bias(V)

25oC 100oC 200oC 300oC 200oC 100oC 25oC

0.0 0.5 1.0 1.5

1x10-10

1x10-8

1x10-6

1x10-4

1x10-2

WSi/n-SiC

Cur

rent

(A)

Bias(V)

25oC 100oC 200oC 300oC 200oC 100oC 25oC

Fig. 3. Forward I–V characteristics from Ni (top) and WSiX(bottom) diodes on 4H-SiC as a function of measurement

temperature. The WSiX contacts were annealed at 500 �C for

1 min after deposition.

confirmed by the data in Table 2, which shows /B and nfor both the WSiX and Ni contacts as a function of

measurement temperature. Future work will focus on

-20 -10 010

-10

10-9

10-8

Cur

rent

(A)

Bias(V)

Ni(25oC) Ni(100oC) Ni(200oC) Ni(300oC)

10-10

10-9

10-8

10-7

WSi/n-SiC

Cur

rent

(A)

Bias(V)

WSi(25oC) WSi(100oC) WSi(200oC) WSi(300oC)

-20 -10 0

Fig. 4. Reverse I–V characteristics from Ni (top) and WSiX(bottom) diodes on 4H-SiC as a function of measuremental

temperature. The WSiX contacts were annealed at 500 �C for

1 min after deposition.

178 J. Kim et al. / Solid-State Electronics 48 (2004) 175–178

measuring the barrier height of WSiX in p-type 4H-SiC

in order to see if the sum of /Bn and /Bp are in good

agreement with the temperature dependence of the

bandgap, i.e. EgðT Þ ¼ 3:0–3:3� 10�4 (T––300 K) for

4H-SiC [10].

In the case it would prove that the WSiX contacts

obeyed the Schottky–Mott-model in which /Bp would be

given by Eg � ð/m � vSÞ [4].Fig. 4 shows the reverse I–V characteristics from

both Ni (top) and WSi (bottom) contacts on 4H-SiC, as

a function of measuremental temperature in the range of

25–300 �C. The range of reverse current magnitudes is

comparable at high bias, although once again the WSiXcontacts have lower currents at small bias and elevated

temperature (300 �C) which should be an advantage in

high temperature applications.

4. Summary and conclusions

WSi0:45 rectifying contacts have been deposited on

n-type 4H-SiC by rf plasma-assisted sputtering. The

as-deposited contacts show evidence of a high series

resistance related to ion-induced damage, but annealing

at 500 �C for 1 min is sufficient to remove much of this

damage. A maximum barrier height of 1.15 eV was

obtained after this treatment. The contacts are unstable

after annealing above �700 �C and the effective barrier

height decreases. The WSiX contacts appears to have

improved high temperature characteristics compared to

more commonly used Ni rectifying contacts.

Acknowledgements

The work at UF is particularly supported by NASA

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

Space research initiative. The work at Sterling Semi-

conductor is particularly supported by Air Force Con-

tract F33615-01-11-2136 (Dr. James Schofield). Sandia

is a multiprogram laboratory operated by Sandia Cor-

poration for Lockheed–Martin under DOE contract

DE-AC-04-55000.

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