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High temperature thermal stability of plasma-deposited tungsten nitride Schottky contacts to GaAs
Solid-Slate
Pergamon
003%1101(94)00148-0
Electronics Vol. 38, No. 3, pp. 679-682. 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-I I01195 $9.50 + 0.00
HIGH TEMPERATURE THERMAL STABILITY PLASMA-DEPOSITED TUNGSTEN NITRIDE SCHOTTKY CONTACTS TO GaAs CHANG
WOO LEE’ and YONG
OF
TAE KIM’
‘Department of Physics, Korea Advanced Institute of Science and Technology, P.O. Box 305-701, Taejon and ‘Semiconductor Materials Laboratory, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, Korea (Receitled 5 January 1994: in reuised,form 26 April
1994)
Abstract-Electrical and physical properties of plasma enhanced chemical vapor deposited tungsten nitride (W,,N,,) Schottky contacts to GaAs are investigated at the rapid thermal annealing (RTA) temperature of 500-1000°C for 30 s. The cross-sectional transmission electron microscopy and second ion mass spectroscopy reveal that the W,,N3j Schottky contacts to GaAs maintains the integrity of interface during RTA at 1000°C for 30 s without any reaction of W-As or interdiffusion phenomena. Barrier heights and ideality factors for as-deposited W,,N,, contacts to GaAs are 0.86 eV and 1.04. which are changed to 0.72 eV and 1.128 after RTA at 1000°C for 30 s.
As a result, it is found that reliable and higher barrier height can be achieved with PECVD-tungsten nitride Schottky contact to GaAs after the annealing process at the temperature range of 50&1000°C without As overpressure and capping layers. Si doped, (100) oriented n-type GaAs wafers (N, = 1 x 10” cm-‘) are etched in a mixture of HCI and Hz0 (1: IO) and followed by rinse/dry process prior to loading into the plasma enhanced chemical vapor deposition (PECVD) chamber. Native oxide layer is etched off by the remote plasma cleaning process using Ar-Hz gases. 2000 A thick W,,N,, film is deposited with WF,-NH,-Hz gas system (flow ratio of WF,/NH3/Hz = 2/l/25) in a homemade parallel typed PECVD chamber[9] and the deposition temperature is fixed at 350°C and rf power density is fixed at 0.52 W/cm’. The stoichiometry of W,,N,, is determined with Rutherford backscattering spectrometry and the thickness of PECVD-W,,N,, on GaAs is measured with the ,%-ray backscattering method and stylus instrument. The W,,N,, thin films are patterned with conventional lithography process for the Schottky diodes with a diameter of 100 pm. Without capping layers such as SiN, and phosphosilicate glass films rapid thermal annealing (RTA) is carried out in an Ar ambient using a face-to-face proximity capping method at annealing temperatures of 500-1000 C for 30 s. RTA effects on the crystal structure and interfacial reaction of W,,N,, films deposited on GaAs are investigated with XRD. Figure l(a) shows the XRD pattern for as-deposited W,,N,,. In this figure the W,,N,, film consists of (110) oriented cc-W along with (100) oriented WN and (200) oriented W2N phase
Thermal stability and high barrier height of Schottky contact to GaAs are the major requirements for high temperature self aligned gate field effect transistor (SAGFET). Therefore, refractory metal and nitrides such as, tungsten (W)[l], tungsten silicide (WSi,)[2,3], tungsten nitride (WN,)[4,5] and tungsten silicon nitride (WSiN)[6] have been investigated as a thermally stable Schottky contact because these refractory materials have been reported to suppress the diffusion of Ga, As and metal atoms even at 85O’C. Among the above mentioned materials, refractory metal nitrides are considered as promising candidates for gate material because they exhibit more improved thermal stability and higher barrier height than those of refractory metal Schottky contacts[7]. However, sputtered metal and metal nitride thin films have some disadvantages such as tensile film stress, ion bombardment damage and non-uniformity, which are strongly related with the leakage current and delamination of metal film. In this work, we have suggested the plasma enhanced chemical vapor deposited tungsten nitride as a thermally stable Schottky contact to GaAs because PECVD-tungsten nitride experiences near-zero or slightly compressive stress and even though tungsten nitride is not nucleated on GaAs surface by low pressure chemical vapor deposition (LPCVD)[8], PECVD-tungsten nitride can be easily deposited at 35O’C. Thus, the characteristics of plasma enhanced chemical vapor deposited (PECVD) W,,N,, Schottky contacts to GaAs have been investigated using secondary ion mass spectroscopy (SIMS), cross sectional transmission microscopy (XTEM), X-ray diffraction (XRD) and I-V characteristics of Schottky diodes. 679
680
Chang Woo Lee and Yong Tae Kim
grains. Then, annealing at 500°C suggests that (111) oriented W,N peak becomes stronger than asdeposited W,N peak as shown in Fig. l(b) because WN phase is metastable state and this WN peak is exhausted by the transformation of W,N. Furthermore, Fig. l(c) shows that this (111) oriented WIN peak is not extinguished even at the temperature up to 1000°C for 30 s and W-As compound peaks are not observed. However, XRD pattern for PECVD-W contact to GaAs [Fig. l(d)] shows several W-As compound peaks such as (1 lo), (103) oriented WAS,, (113) and (220) oriented W, As,. Therefore, the XRD results mean that N atoms react with W grains and form the polycrystalline W2N grains and this W,N film is less reactive with GaAs. Thermal stability of W,,N3, Schottky contact to GaAs is investigated with SIMS measurement. SIMS profiles for as-deposited and annealed W,,N,, contact to GaAs at 1000°C for 30 s are obtained as shown in Fig. 2(a) and (b). The comparison of both figures indicates that relative intensities of “Ga+ and “As+ secondary ions in the W,,N,, films are less than the background level and not changed before and after RTA. This means that W,,N,, film blocks the out-diffusion of Ga and As during RTA even at 1000°C for 30 s. In addition,
40
Diffraction
60
106
104
102
1 0
3
6
9
0
3
6
9
12
Sputter Time (min) Fig. 2. SIMS depth profiles for PECVD-W,,N,, on GaAs (a) as-deposited and (b) after RTA at 1000°C for 30 s.
SIMS profiles imply that out-diffusion of N atoms does not take place seriously and F impurities originated from the reduction process of WF, are relatively reduced after annealing, but still accumulated at the interface of W-N and GaAs after RTA at 1000°C for 30 s. From the XRD and SIMS results, it is concluded that although the roles of N and F atoms are not exactly explained, W,,N,, Schottky contact to GaAs has thermal stability of high temperature annealing process. The interfacial reaction of 1000 A W,,N,, film and GaAs is investigated with XTEM studies after RTA process at 1000°C for 30 s. Figure 3 shows that there is no reaction between W,,N,, and GaAs so that integrity of W,,N,,/GaAs interface can be maintained at the RTA temperature up to 1000°C. According to the suggested model explaining the role of tungsten nitride diffusion barrier[l 11,the role of We7Nj3 diffusion barrier preventing interdiffusion of Ga, As and metal atoms can be ascribed to both f.c.c.-W,N and N atoms: i.e. f.c.c.-W,N grains are
80
Angle 10 (deg.)
Fig. 1. X-ray diffraction patterns for PECVD-W,,N,, on GaAs as a function of RTA temperature (a) as-deposited state, (b) 500°C for 30 s and (c) 1000°C for 30 s. (d) PECVDW on GaAs after RTA at 1000°C for 30s.
Fig.
3. XTEM W,,N,,/GaAs
micrograph for the interface after RTA at 1000°C for 30 s.
of
Tungsten nitride Schottky contacts to GaAs
681
120
0’
‘=I
asderh
’
600
’
’
800
’
’
1000
RTA Temperature (“C) Fig. 5. The resistivities of PECVD-W,,N,, on GaAs as a function of RTA temperature.
IO-8 -
0
L
0.2
0.4
0.6
0.8
1.0
Forward and Reverse Bias (V) Fig. 4. Typical I-V characteristics of PECVD-W,,N,, Schottky contacts to GaAs (a) as-deposited and (b) after RTA at 1000°C for 30s.
thermodynamically stable than b.c.c.-W and part of excessive nitrogen atoms located among equiaxial structured W,N grain boundaries block the grain boundary diffusion paths of W, Ga and As atoms. The I-V characteristics of Schottky diode with forward bias are analyzed in terms of the thermionic emission model[ 121; more
J=A*T?exp(-q&/kT)[exp(qV,/nkT)-
The forward and reverse leakage current densities of Fig. 4(a) are lower than those of Fig. 4(b) by one order of magnitude because forward and reverse current densities of as-deposited diode may be reduced due to higher resistivity of PECVD-W,,N,, contact metal or pinning by the native oxide at the interface. Figure 5 shows the variation of the resistivities of W,,N,, thin films as a function of RTA temperature. The resistivity of as-deposited W6,NJ1 films on GaAs decreases from 100 to 20 p&cm after RTA at 1000°C for 30 s. The reason for lower resistivity of annealed W,,N,, film is attributed to grain growth of W,N and out-diffusion of nitrogen incorporated with W films. Figure 6 summarizes the variation of barrier heights and ideality factors of W,, N,, /GaAs Schottky diodes as-deposited and annealed in the RTA temperature range of SO&1000°C for 30s. Ideality factors maintain nearly steady state value of 1.04-1.10 and barrier heights are also slightly degraded from 0.86 to 0.79eV even after RTA at 900°C. Thus, it suggests that all these
11,
where J is the current density, A * is the effective Richardson constant equal to 8.64 A/cm2 K2 for GaAs, T is the measurement temperature, k is the Boltzmann constant, V, is the applied voltage, & is the Schottky barrier height of the diode and n is the ideality factor. As an example, Fig. 4(a) and (b) show the forward and reverse Z-V characteristics of PECVD-W,,N,, Schottky contacts to GaAs asdeposited and after RTA at 1000°C for 30 s without As overpressure. The barrier height &, and ideality factor n of the as-deposited PECVD-W,,N,,/GaAs diode are 0.86 eV and 1.04, respectively. After capless RTA at 1OOO”C,&, of W,, N,, /GaAs diode decreases to 0.72 eV and n increases from 1.04 to 1.128, which indicates that I-V characteristics of annealed diode are not excessively deviated from ideal characteristics.
I *g
0.6 -
X k
‘i:
0.4 -
%
---=+=-C-
a
0.2 -
0
‘2’ us-dep.
’ 600
’
’ 800
’
’ IOOn
Annealing Temperature (“C) Fig. 6. Variations of barrier heights and ideality factors of PECVD-W,,N,, Schottky contacts to GaAs according to annealing temperatures (O-barrier height and m-ideality factor of PECVD-W,,N,, Schottky contacts to GaAs).
682
Chang Woo Lee and Yong Tae Kim
PECVD-W,,N,, contacts to GaAs show excellent Schottky diode characteristics for temperature as high as 900°C. Furthermore, these values of 4, and n are not significantly degraded even at 1000°C. This indicates that the current mechanism is not changed from the ideal thermionic model because nitrogen atoms in tungsten nitride grain boundaries block the interdiffusion or the chemical interaction between W,, N,, layer and GaAs in the annealing temperature ranges of SO&1000°C. In conclusion, we have proposed a thermally stable PECVD-W,,N,, Schottky contacts to GaAs for high temperature self aligned gate. The resistivity of PECVD-W,,N,, films deposited on GaAs is varied from 100 to 20 p&cm and I-V characteristics of the GaAs diodes exhibit close to idea1 diode: the barrier heights decrease from 0.86 to 0.72 eV and ideality factors increase from 1.04 to I.128 before and after annealing at 1000°C for 30 s. In addition, it is also found that the thermal stability of PECVD-W,,N,, Schottky contacts to GaAs is maintained during the post-annealing temperature up to 1000°C without As overpressure. The improved thermal stability of PECVD-W,,NX3 Schottky contacts to GaAs is attributable to the nitrogen atoms incorporated into W films because f.c.c.-W,N, which is more stable than b.c.c.-W grains, and N atoms in W,N grain boundaries prevent the interdiffusion and the chemical
reaction of W,,N,, annealing process.
layer and surface of GaAs during
REFERENCES I.
K. M. Yu, S. K. Cheung, T. Sanda. J. M. Jaklevic, W. Cheung and E. E. Haller. .I. appl. Phvs. 60, 3235
N.
(1986). 2. A. G. Lahav. C. S. Wu and F. A. Baiocchi, J. Var. Sci. Technol. B6, 1785 (1988). 3. Z. Zhongde, N. W. Cheung, Z. J. Lemnios, M. D. Strathman and J. B. Stimmell. J. Vat. Sci. Technol. B4, 1398 (1988). 4. N. Uchitomi, M. Nagaoka. K. Shimada, T. Mozoguchi and N. Toyoda, J. Var. Sci. Technol. EB4, 1392 (1986). 5. H. Yamagishi, Japan. J. appl. Phys. 23, L895 (1984). 6. H. Sugahara and J. Nagano, Appl. Surf. Sri. 41142,207 (1989). 7. L. C. Zhang, S. K. Cheung, C. L. Liang and N. W. Cheung, Appl. Phys. L&f. SO, 445 (1987). 8. T. Nakajima, K. Watanabe and N. Watanabe, J. Electrochem. Sot. 134, 3175 (1987). 9. Y. T. Kim, S.-K. Min. J. S. Hong and C. K. Kim, Appl.
Phys. L,ett. 58, 837 (1991). IO. C. W. Lee, Y. T. Kim and S-K. Min, Appl. Phys. Leff. 63, 3312 (1993). I I. C. W. Lee, Y. T. Kim, S.-K. Min, C. Lee, J. Y. Lee and Y. W. Park, Mat. Res. Sot. Symp. Proc. (Edited by S. P. Murarka ef al.) 318, 335 (1994). 12. E. H. Rhoderick and R. H. Williams, Metalsemiconductor Contacts, 2nd edn, Chap. 3. p. 94. Clarendon Press, Oxford (1988).
Pergamon
003%1101(94)00148-0
Electronics Vol. 38, No. 3, pp. 679-682. 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-I I01195 $9.50 + 0.00
HIGH TEMPERATURE THERMAL STABILITY PLASMA-DEPOSITED TUNGSTEN NITRIDE SCHOTTKY CONTACTS TO GaAs CHANG
WOO LEE’ and YONG
OF
TAE KIM’
‘Department of Physics, Korea Advanced Institute of Science and Technology, P.O. Box 305-701, Taejon and ‘Semiconductor Materials Laboratory, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, Korea (Receitled 5 January 1994: in reuised,form 26 April
1994)
Abstract-Electrical and physical properties of plasma enhanced chemical vapor deposited tungsten nitride (W,,N,,) Schottky contacts to GaAs are investigated at the rapid thermal annealing (RTA) temperature of 500-1000°C for 30 s. The cross-sectional transmission electron microscopy and second ion mass spectroscopy reveal that the W,,N3j Schottky contacts to GaAs maintains the integrity of interface during RTA at 1000°C for 30 s without any reaction of W-As or interdiffusion phenomena. Barrier heights and ideality factors for as-deposited W,,N,, contacts to GaAs are 0.86 eV and 1.04. which are changed to 0.72 eV and 1.128 after RTA at 1000°C for 30 s.
As a result, it is found that reliable and higher barrier height can be achieved with PECVD-tungsten nitride Schottky contact to GaAs after the annealing process at the temperature range of 50&1000°C without As overpressure and capping layers. Si doped, (100) oriented n-type GaAs wafers (N, = 1 x 10” cm-‘) are etched in a mixture of HCI and Hz0 (1: IO) and followed by rinse/dry process prior to loading into the plasma enhanced chemical vapor deposition (PECVD) chamber. Native oxide layer is etched off by the remote plasma cleaning process using Ar-Hz gases. 2000 A thick W,,N,, film is deposited with WF,-NH,-Hz gas system (flow ratio of WF,/NH3/Hz = 2/l/25) in a homemade parallel typed PECVD chamber[9] and the deposition temperature is fixed at 350°C and rf power density is fixed at 0.52 W/cm’. The stoichiometry of W,,N,, is determined with Rutherford backscattering spectrometry and the thickness of PECVD-W,,N,, on GaAs is measured with the ,%-ray backscattering method and stylus instrument. The W,,N,, thin films are patterned with conventional lithography process for the Schottky diodes with a diameter of 100 pm. Without capping layers such as SiN, and phosphosilicate glass films rapid thermal annealing (RTA) is carried out in an Ar ambient using a face-to-face proximity capping method at annealing temperatures of 500-1000 C for 30 s. RTA effects on the crystal structure and interfacial reaction of W,,N,, films deposited on GaAs are investigated with XRD. Figure l(a) shows the XRD pattern for as-deposited W,,N,,. In this figure the W,,N,, film consists of (110) oriented cc-W along with (100) oriented WN and (200) oriented W2N phase
Thermal stability and high barrier height of Schottky contact to GaAs are the major requirements for high temperature self aligned gate field effect transistor (SAGFET). Therefore, refractory metal and nitrides such as, tungsten (W)[l], tungsten silicide (WSi,)[2,3], tungsten nitride (WN,)[4,5] and tungsten silicon nitride (WSiN)[6] have been investigated as a thermally stable Schottky contact because these refractory materials have been reported to suppress the diffusion of Ga, As and metal atoms even at 85O’C. Among the above mentioned materials, refractory metal nitrides are considered as promising candidates for gate material because they exhibit more improved thermal stability and higher barrier height than those of refractory metal Schottky contacts[7]. However, sputtered metal and metal nitride thin films have some disadvantages such as tensile film stress, ion bombardment damage and non-uniformity, which are strongly related with the leakage current and delamination of metal film. In this work, we have suggested the plasma enhanced chemical vapor deposited tungsten nitride as a thermally stable Schottky contact to GaAs because PECVD-tungsten nitride experiences near-zero or slightly compressive stress and even though tungsten nitride is not nucleated on GaAs surface by low pressure chemical vapor deposition (LPCVD)[8], PECVD-tungsten nitride can be easily deposited at 35O’C. Thus, the characteristics of plasma enhanced chemical vapor deposited (PECVD) W,,N,, Schottky contacts to GaAs have been investigated using secondary ion mass spectroscopy (SIMS), cross sectional transmission microscopy (XTEM), X-ray diffraction (XRD) and I-V characteristics of Schottky diodes. 679
680
Chang Woo Lee and Yong Tae Kim
grains. Then, annealing at 500°C suggests that (111) oriented W,N peak becomes stronger than asdeposited W,N peak as shown in Fig. l(b) because WN phase is metastable state and this WN peak is exhausted by the transformation of W,N. Furthermore, Fig. l(c) shows that this (111) oriented WIN peak is not extinguished even at the temperature up to 1000°C for 30 s and W-As compound peaks are not observed. However, XRD pattern for PECVD-W contact to GaAs [Fig. l(d)] shows several W-As compound peaks such as (1 lo), (103) oriented WAS,, (113) and (220) oriented W, As,. Therefore, the XRD results mean that N atoms react with W grains and form the polycrystalline W2N grains and this W,N film is less reactive with GaAs. Thermal stability of W,,N3, Schottky contact to GaAs is investigated with SIMS measurement. SIMS profiles for as-deposited and annealed W,,N,, contact to GaAs at 1000°C for 30 s are obtained as shown in Fig. 2(a) and (b). The comparison of both figures indicates that relative intensities of “Ga+ and “As+ secondary ions in the W,,N,, films are less than the background level and not changed before and after RTA. This means that W,,N,, film blocks the out-diffusion of Ga and As during RTA even at 1000°C for 30 s. In addition,
40
Diffraction
60
106
104
102
1 0
3
6
9
0
3
6
9
12
Sputter Time (min) Fig. 2. SIMS depth profiles for PECVD-W,,N,, on GaAs (a) as-deposited and (b) after RTA at 1000°C for 30 s.
SIMS profiles imply that out-diffusion of N atoms does not take place seriously and F impurities originated from the reduction process of WF, are relatively reduced after annealing, but still accumulated at the interface of W-N and GaAs after RTA at 1000°C for 30 s. From the XRD and SIMS results, it is concluded that although the roles of N and F atoms are not exactly explained, W,,N,, Schottky contact to GaAs has thermal stability of high temperature annealing process. The interfacial reaction of 1000 A W,,N,, film and GaAs is investigated with XTEM studies after RTA process at 1000°C for 30 s. Figure 3 shows that there is no reaction between W,,N,, and GaAs so that integrity of W,,N,,/GaAs interface can be maintained at the RTA temperature up to 1000°C. According to the suggested model explaining the role of tungsten nitride diffusion barrier[l 11,the role of We7Nj3 diffusion barrier preventing interdiffusion of Ga, As and metal atoms can be ascribed to both f.c.c.-W,N and N atoms: i.e. f.c.c.-W,N grains are
80
Angle 10 (deg.)
Fig. 1. X-ray diffraction patterns for PECVD-W,,N,, on GaAs as a function of RTA temperature (a) as-deposited state, (b) 500°C for 30 s and (c) 1000°C for 30 s. (d) PECVDW on GaAs after RTA at 1000°C for 30s.
Fig.
3. XTEM W,,N,,/GaAs
micrograph for the interface after RTA at 1000°C for 30 s.
of
Tungsten nitride Schottky contacts to GaAs
681
120
0’
‘=I
asderh
’
600
’
’
800
’
’
1000
RTA Temperature (“C) Fig. 5. The resistivities of PECVD-W,,N,, on GaAs as a function of RTA temperature.
IO-8 -
0
L
0.2
0.4
0.6
0.8
1.0
Forward and Reverse Bias (V) Fig. 4. Typical I-V characteristics of PECVD-W,,N,, Schottky contacts to GaAs (a) as-deposited and (b) after RTA at 1000°C for 30s.
thermodynamically stable than b.c.c.-W and part of excessive nitrogen atoms located among equiaxial structured W,N grain boundaries block the grain boundary diffusion paths of W, Ga and As atoms. The I-V characteristics of Schottky diode with forward bias are analyzed in terms of the thermionic emission model[ 121; more
J=A*T?exp(-q&/kT)[exp(qV,/nkT)-
The forward and reverse leakage current densities of Fig. 4(a) are lower than those of Fig. 4(b) by one order of magnitude because forward and reverse current densities of as-deposited diode may be reduced due to higher resistivity of PECVD-W,,N,, contact metal or pinning by the native oxide at the interface. Figure 5 shows the variation of the resistivities of W,,N,, thin films as a function of RTA temperature. The resistivity of as-deposited W6,NJ1 films on GaAs decreases from 100 to 20 p&cm after RTA at 1000°C for 30 s. The reason for lower resistivity of annealed W,,N,, film is attributed to grain growth of W,N and out-diffusion of nitrogen incorporated with W films. Figure 6 summarizes the variation of barrier heights and ideality factors of W,, N,, /GaAs Schottky diodes as-deposited and annealed in the RTA temperature range of SO&1000°C for 30s. Ideality factors maintain nearly steady state value of 1.04-1.10 and barrier heights are also slightly degraded from 0.86 to 0.79eV even after RTA at 900°C. Thus, it suggests that all these
11,
where J is the current density, A * is the effective Richardson constant equal to 8.64 A/cm2 K2 for GaAs, T is the measurement temperature, k is the Boltzmann constant, V, is the applied voltage, & is the Schottky barrier height of the diode and n is the ideality factor. As an example, Fig. 4(a) and (b) show the forward and reverse Z-V characteristics of PECVD-W,,N,, Schottky contacts to GaAs asdeposited and after RTA at 1000°C for 30 s without As overpressure. The barrier height &, and ideality factor n of the as-deposited PECVD-W,,N,,/GaAs diode are 0.86 eV and 1.04, respectively. After capless RTA at 1OOO”C,&, of W,, N,, /GaAs diode decreases to 0.72 eV and n increases from 1.04 to 1.128, which indicates that I-V characteristics of annealed diode are not excessively deviated from ideal characteristics.
I *g
0.6 -
X k
‘i:
0.4 -
%
---=+=-C-
a
0.2 -
0
‘2’ us-dep.
’ 600
’
’ 800
’
’ IOOn
Annealing Temperature (“C) Fig. 6. Variations of barrier heights and ideality factors of PECVD-W,,N,, Schottky contacts to GaAs according to annealing temperatures (O-barrier height and m-ideality factor of PECVD-W,,N,, Schottky contacts to GaAs).
682
Chang Woo Lee and Yong Tae Kim
PECVD-W,,N,, contacts to GaAs show excellent Schottky diode characteristics for temperature as high as 900°C. Furthermore, these values of 4, and n are not significantly degraded even at 1000°C. This indicates that the current mechanism is not changed from the ideal thermionic model because nitrogen atoms in tungsten nitride grain boundaries block the interdiffusion or the chemical interaction between W,, N,, layer and GaAs in the annealing temperature ranges of SO&1000°C. In conclusion, we have proposed a thermally stable PECVD-W,,N,, Schottky contacts to GaAs for high temperature self aligned gate. The resistivity of PECVD-W,,N,, films deposited on GaAs is varied from 100 to 20 p&cm and I-V characteristics of the GaAs diodes exhibit close to idea1 diode: the barrier heights decrease from 0.86 to 0.72 eV and ideality factors increase from 1.04 to I.128 before and after annealing at 1000°C for 30 s. In addition, it is also found that the thermal stability of PECVD-W,,N,, Schottky contacts to GaAs is maintained during the post-annealing temperature up to 1000°C without As overpressure. The improved thermal stability of PECVD-W,,NX3 Schottky contacts to GaAs is attributable to the nitrogen atoms incorporated into W films because f.c.c.-W,N, which is more stable than b.c.c.-W grains, and N atoms in W,N grain boundaries prevent the interdiffusion and the chemical
reaction of W,,N,, annealing process.
layer and surface of GaAs during
REFERENCES I.
K. M. Yu, S. K. Cheung, T. Sanda. J. M. Jaklevic, W. Cheung and E. E. Haller. .I. appl. Phvs. 60, 3235
N.
(1986). 2. A. G. Lahav. C. S. Wu and F. A. Baiocchi, J. Var. Sci. Technol. B6, 1785 (1988). 3. Z. Zhongde, N. W. Cheung, Z. J. Lemnios, M. D. Strathman and J. B. Stimmell. J. Vat. Sci. Technol. B4, 1398 (1988). 4. N. Uchitomi, M. Nagaoka. K. Shimada, T. Mozoguchi and N. Toyoda, J. Var. Sci. Technol. EB4, 1392 (1986). 5. H. Yamagishi, Japan. J. appl. Phys. 23, L895 (1984). 6. H. Sugahara and J. Nagano, Appl. Surf. Sri. 41142,207 (1989). 7. L. C. Zhang, S. K. Cheung, C. L. Liang and N. W. Cheung, Appl. Phys. L&f. SO, 445 (1987). 8. T. Nakajima, K. Watanabe and N. Watanabe, J. Electrochem. Sot. 134, 3175 (1987). 9. Y. T. Kim, S.-K. Min. J. S. Hong and C. K. Kim, Appl.
Phys. L,ett. 58, 837 (1991). IO. C. W. Lee, Y. T. Kim and S-K. Min, Appl. Phys. Leff. 63, 3312 (1993). I I. C. W. Lee, Y. T. Kim, S.-K. Min, C. Lee, J. Y. Lee and Y. W. Park, Mat. Res. Sot. Symp. Proc. (Edited by S. P. Murarka ef al.) 318, 335 (1994). 12. E. H. Rhoderick and R. H. Williams, Metalsemiconductor Contacts, 2nd edn, Chap. 3. p. 94. Clarendon Press, Oxford (1988).