- Email: [email protected]
Excellent thermal stability of cobaltaluminum alloy Schottky contacts on GaAs substrates
Solid-State ElectronicsVol. 33, No. 7, pp. 863-867, 1990
Printed in Great
0038-I101/90 s3.00 + 0.00 Copyright 0
Britain. All rights reserved
1990 Pergamon Press plc
EXCELLENT THERMAL STABILITY OF COBALT-ALUMINUM ALLOY SCHOTTKY CONTACTS GaAs SUBSTRATES H. C. CHENG,C. Y. WV and J. J.
ON
SHY
Department of Electronics Engineering, National Chiao-Tung University, Hsinchu, Taiwan, Republic of China (Received 31 August 1989; in revised form 21 December
1989)
Abstract-Schottky contact properties of ftlms with codeposited Co-Al mixture or tri-layered Co/Al/Co structure on GaAs substrate after different rapid thermal annealings (RTAs) have been investigated. Interfacial thermal stability between co-deposited Co-Al mixture and GaAs was attributed to the formation of CoAl compound. However, cobalt at the Co/GaAs interface would strongly react with GaAs to produce Co,GaAs phase for tri-layered Co/Al/Co structure on GaAs. Subsequently, Co,GaAs compound would decompose into CoGa and CoAs phases. Capped with a 3500~A-thick SiO, layer, co-deposited Co-Al films could withstand high-temperature annealing and exhibited a barrier height of 0.88 eV and an ideality factor of 1.08 even after 1050°C rapid thermal annealing for 10 s. As far as the authors are aware, this rapid thermal annealing temperature is the highest in the literature for such excellent Schottky characteristics. The contact was found to achieve a barrier of 09OeV for the co-deposited Co-Al alloy with Co: Al = 1: 1.05 after 850°C rapid thermal annealing for 50 s. Tri-layered Co/Al/Co film was also found to be thermally stable on GaAs after short time annealing (10 s), but the contact property rapidly degraded with annealing time.
1. INTRODUCTION
contacts on GaAs have been widely used in metal-semiconductor field-effect transistors (MESFETs), Schottky diodes, and other microwave devices[ 11.To improve the performance of GaAs ICs, the self-aligned-gate (SAG) process was developed to eliminate the source parasitic resistance[2]. However, SAG processing would require the gate material to sustain a high-temperature (> SSOC) implant activation annealing without degrading the Schottky characteristics of the gate[3-41. Most elemental metals react with GaAs at a lower temperature so that the barrier characteristics deteriorated. In contrast, several refractory metal silicides have been demonstrated to possess superior barrier characteristics than these elemental metals on GaAs. A stable Schottky contact with the barrier height of 0.83 eV has been reported for a TaSi,/GaAs structure after 850°C annealing[5]. Refractory metal nitrides were also candidate gate materials for GaAs. A stable diode of barrier height 0.95 eV and ideality factor 1.09 for a IO-s anneal at 800°C was obtained for tungsten nitride on GaAs. However, the barrier decreased to 0.90 eV and the ideality factor increased to 1.15 after 10 s annealing at 9Oo”C[6]. Sands et a/.[71 found that a NiAl bimetallic metallization on GaAs could achieve a stable and uniform Schottky contact with a barrier height of 0.99 eV and an ideality factor of 1.10 after a 20 s anneal at 650°C. The barrier and ideality factor of NiAl/GaAs annealed at 950°C for 20s degraded to 0.82eV and 2.07, respectively. Schottky
CoAl compound has the same crystal structure as NiAl phase (cubic CsCl structure). In addition, the chemical properties of CoAl alloy are very similar to that of NiAl. The melting point of CoAl (1645°C) is slightly higher than that of NiAl (1638°C). In the present study, the Schottky characteristics of CoAl contacts on GaAs were investigated by current-voltage (I-V) and capacitance-voltage (C-V) measurements. A transmission electron microscope was used to identify the phase transitions of Co-Al alloys. 2. EXPERIMENTAL
DETAILS
The GaAs wafers used were (100) oriented and n type with Si dopant concentration of 1 x 10’6cm-3. The samples were degreased consecutively in boiling trichloroethylene (TCE), acetone, and deionized water for 5 min each. To remove surface damage layer, wafers were etched with H2S04: H,O,: H,O = 5 : 1: 1 solution. The samples were divided into two groups. Some of the samples were used for transmission electron microscopy (TEM) observations and the other were utilized for electrical measurements. Making use of the lift-off process to form diodes, samples for electrical measurements were patterned by photolithography after wafer cleaning. The diameter of the circular diodes was 500pm. Before loading into the dual electron gun evaporation system, samples for TEM examinations were dipped with HCl:H,O = 1: 1 solution for 1 min and specimens for electrical measure-
863
864
H. C. CHENG et al.
ments were dipped with NH,OH : H,O = 1: 1 solution for 1 min to remove the surface native oxide. Cobalt and aluminum were codeposited to obtain the Co-Al alloys with 200 A-thick cobalt or layer-by-layerdeposited to obtain the films of Co(lO0 A)/ Al/Co( 100 A) structure. The thickness of Al layer was selected to make the atomic ratio of Co/Al to be 1.05/l or l/1.05. The film sequence of tri-layered structure was chosen based on the penetration into the native oxide on GaAs and the stabilization of aluminum metal. The chamber was evacuated to 3 x 10v6 Torr. The deposition rate of cobalt was 1 A/s. Following the metal deposition, some of the samples were capped with a 3500 8, thick SiOz layer by a single-electron-gun evaporation system. This cap served to prevent the arsenic outdiffusion from GaAs substrate during high temperature annealing. Subsequently, rapid thermal annealing (RTA) was carried out in a flowing nitrogen ambient using a halogen lamp system. The efficiency of dopant activation in RTA system was reported to be higher than that of conventional furnace[8]. Annealings were performed at temperatures from 650 to 1050°C for 10 s or from 500 to 850°C for 50 s. After annealing, AuGe sintering at 450°C for 1.5 min was used as backside ohmic contact. Finally, the capped Si02 film on the front-side surface of the sample was removed by BOE solution. Samples for TEM observations were cut into 3 mm diameter discs by an ultrasonic cutter. An etching solution with Br, : methonal = 2 : 98 was used until a small hole on the specimen appeared. The electron microscopy was performed with a JEOL2000FX scanning transmission electron microscope (STEM) operating at 160 kV. Current-voltage (I-V) and capacitance-voltage (C-I’) measurements were utilized to appraise the contact properties. 1-V characteristics were measured by an HP4145B semiconductor parameter analyzer. From the thermionic emission theory, the barrier height @+,and ideality factor n can be derived from:
where I is the current, V is the voltage, q is the elementary charge, k is the Boltzmann constant, T is the absolute temperature, a is the diode area, A* is the effective Richardson constant of 8.16 A cmm2 Km2 for GaAs, and Z, is the reverse saturation current. C-V characteristics were determined by using an HP4194A Impedance/Gain-Phase analyzer. The C-V barrier heights @c-” were extracted by plotting Cm2 vs the reverse-bias applied voltage for the bulk doped samples.
3. RESULTS
AND DISCUSSION
The samples which will be mentioned below were marked by a letter and a number. The letter represents the deposition type of metal films, e.g. letter “C” for the co-deposited Co-Al mixture and “T” for n-i-layered Co/Al/Co structure. The number classifies the atomic ratio of Co:Al, e.g. number “1” for Co:Al= 1:1.05 and “2” for Co:Al= 1.05:l. According to the results of TEM observations, interfacial properties of t&layered Co/Al/Co films on GaAs were not stable. After 500°C annealing, Co,GaAs phase with a similar lattice constant as GaAs substrate was formed to have a preferential orientation to GaAs for type T specimens. Co,GaAs would decompose into CoGa and CoAs compounds as the specimens were annealed at the temperatures above 550°C. The results are consistent with those of a previous report[9]. These reactions resulted in a failure of the epitaxial growth of CoAl phase[lO]. TEM analysis of Cl and C2 specimens indicated that only the CoAl phase and a small amount of Co,Al were present for all annealing conditions. An example is shown in Fig. 1. CoGa and CoAs compounds were not found in Cl and C2 samples annealed at different temperatures indicating the interfacial stability of these samples. In addition, no epitaxy of the CoAl phase was found in any type C specimens. Lau et a/.[41 reported that the stability of electrical characteristics of a gate contact after high-temperature annealing was directly correlated to the chemical stability of the gate metallurgy. Samples of type C were therefore thought to possess a better thermal stability of electrical properties than those of type T. In the Z-V measurement, the barrier height and ideality factor were extracted to appraise the Schottky characteristics of various types of metal films on GaAs. For the Cl specimens without a SiO, capping layer, the curves of barrier height and ideality factor vs annealing temperature with 10 s annealing time are shown in Fig. 2. The diode properties of Cl samples were maintained up to 950°C. Barrier height and ideality factor of Cl specimens annealed at 950°C for 10 s were 0.86 eV and 1.13, respectively. However, the ideality factor increased to 1.76 although the contact remained rectifying for the Cl samples annealed at 1050°C for 10 s. Similar results were also found in C2 samples. The barrier height and ideality factor of C2 specimens after 950°C rapid thermal annealing for 10 s were 0.86 eV and 1.10, accordingly. After heat treatment at 1050°C for 10 s, ideality factor of C2 samples rose to 1.60. Moreover, the diode characteristics would degrade faster as the annealing time was raised to 50 s. For example, the Schottky characteristics of C2 specimens deteriorated at temperatures above 650°C and the contact properties became nearly ohmic after 850°C annealing for 50 s. To avoid the outdiffusion of a substantial amount of Ga and As from the metal/GaAs interface, a capping layer of dielectric film on the surface of metal
Co-Al
contacts on GaAs
865
(4
(b)
Fig. 1. Diffraction pattern (a) and indexed pattern (b) of CoAl phase, Cl samples annealed at 550°C
for 50s. film was deposited to improve the thermal stability of the metal/GaAs contact. The plots of barrier height and ideality factor against annealing temperature for Cl and C2 specimens capped with a 35OOA thick SiOr film are shown in Fig. 3. For the short time annealing (IO s), the diode characteristics were well preserved even after 1050°C annealing. The ideality factors for Cl and C2 samples after 1050°C annealing for 10 s were 1.04 and 1.08, respectively. In addition, the barrier heights for Cl and C2 specimens annealed at this condition were 0.84 and 0.88 eV, accordingly. As far as the authors are aware, this rapid thermal annealing temperature is the highest reported to date
for such good Schottky characteristics. Even if the annealing time increased, the contact properties were still excellent at a high-temperature annealing. For example, the barrier height and ideality factor for Cl samples annealed at 850°C for 50 s were 0.90 eV and 1.09, correspondingly. Hence, a layer of dielectric film capped on the metal was very effective to obtain a CoAl/GaAs diode with a good thermal stability. The curves of barrier height and ideality factor of the samples of type T are shown in Fig. 4. The contact properties of type T specimens were also stable until the annealing temperature reached 1050°C for short time annealing. However, the contact characteristics 2.0
0.95
o.g512.0
+--.Cz,$b
,@b
-cl
k-4
*Cl,n
_
C2.n
r’ .$ x
0.75-
.-b t g 0.65 -
1 1.2
_----A_--
650 Annealing
Temperature
(‘Cl
Fig. 2. Schottky barriers Gb and ideality factors n of Cl specimens without oxide cap annealed at different temperatures for 10 s. SSE 33,7--F
750 Annealing
:
650
----!
_---
950
Temperature
A 1
1.0
1050
( ‘C)
Fig. 3. Barrier heights Gb and ideality factors n of SiO,capped Cl and C2 specimens as functions of annealing temperature with a 10 s treatment.
H. C. CHENG et al.
866
2.0
0.95~ -
T1 .@b
O---O
Tz ,#b
b--d
Tlsn
A---A
T2.n
.
-I 1.8
0.8+
I
m
!5 0.65 1.2
0.55’1 650
I
1
b
750
850
Annealing
I
I
I
950
Temperature
1
11.0
1050
( ‘Cl
and ideality factors n against annealing temperature for SiO,-capped Tl and T2 samples annealed for 10s. Fig. 4. Curves of barriers ab
such as Al,, and GaAa that were generated to move the surface Fermi level toward valence band maximum[ 131. Because the Co, GaAs compound occurred for tri-layered Co/Al/Co film on GaAs, the formation of Al,Ga,_.As layer would rely on the reaction of Co,GaAs phase with aluminum layer[lO]. On the other hand, co-deposited Co-Al film did not react with GaAs so that the Al-Ga exchange at the interface could proceed directly. Since all of the Co,GaAs, CoGa and CoAs compounds at the interface would reduce the probability of Al-Ga exchange and enhance the arsenic outdiffusion, the contact barriers of type T diodes were lower than those of type C specimens. The barrier height was also determined from the C-Vmeasurements[l4]. The barrier heights extracted from the C-*-V intercepts were 0.95, 0.95 and 0.98 eV for the C2 specimens annealed at 650, 850 and 105O”C, respectively. The higher values of barrier heights measured from C-V method than those from Z-I’ curve could be ascribed to traps near the surface or deeper in the depletion region[ 151.
4. CONCLUSIONS
degraded at annealing temperatures above 550°C if the annealing time was 50 s. The thermal stability of the electrical properties is consistent with the TEM examinations. Since the co-deposited Co-Al film did not react with GaAs and only a small amount of AlLGa exchange was surmised to occur at the interface, the smoothness and quality of the metal/GaAs interface were not deteriorated during annealing and the diode characteristics could thus be maintained up to a very high temperature. However, the decomposition of the Co,GaAs phase would destroy the interface smoothness, and CoGa as well as CoAs products between CoAl and GaAs would also obstruct the AlGa exchange for type T specimens. Hence, samples of type T exhibited a worse stability of electrical properties than those of type C. As for the thermal stability of type T diodes annealed at 1050°C for 10 s, it could be interpreted in a similar way as that of TaSi,/GaAs[ll]. In Ref.[ll], the authors demonstrate that the diode characteristics of metal/GaAs contact could be maintained if the decomposition of the GaAs was less than 15-20 monolayers. In this study, barriers ranged from 0.74 eV for Tl specimens annealed at 1050°C for 10 s to 0.90 eV for Cl samples annealed at 850°C for 50 sf This phenomenon is attributed to the effect of Al-Ga exchange. As the aluminum exchanged with the gallium at the interface of metal/GaAs, arsenic outdiffusion could be depressed due to the dipole change of the interfacial layer[l2]. In addition, Schottky barrier height increased through two mechanisms: (1) a layer of Al,Ga, _,As that was formed to enlarge the band gap of the GaAs surface. (2) The induced anti-site defects
Chemical properties of co-deposited C-Al films on GaAs were very stable. Whatever the annealing conditions were, reaction products were always CoAl phase and a trace of Co,Al. On the other hand, cobalt at the interface of Co/GaAs strongly reacted with GaAs and formed Co,GaAs phase for trilayered Co/Al/Co structure on GaAs. After annealing at the temperatures above 550°C Co2GaAs compound decomposed into CoGa and CoAs phases which would obstruct the AlGa exchange and enhance the arsenic outdiffusion. Co-deposited Co-Al film exhibited an excellent thermal stability on GaAs substrate. Good Schottky characteristics of co-deposited Co-Al alloys capped with a layer of 3500 A thick SiO, could be maintained until the annealing temperature rose to 1050°C. The C2 specimens achieved a barrier height of 0.88 eV and an ideality factor of 1.08 even after an annealing at 1050°C for 10 s. Moreover, the contact could obtain a barrier of 0.90eV for Cl specimens annealed at 850°C for 50 s. Tri-layered Co/Al/Co structure on GaAs also showed a good thermal stability after short time annealing. However, the diode characteristics of type T specimens degraded at annealing temperatures above 550°C as the annealing time increased to 50 s. Owing to the limited Al&a exchange and the arsenic outdiffusion, the Schottky characteristics for triple layer of Co/Al/Co structure on GaAs were worse than those of co-deposited Co-Al mixture. Acknowledgement-The the Republic of China
research was supported in part by National
Science Council.
Co-Al contacts on GaAs
861
1. X. W. Wu, L. C. Zhang, P. Bradley, D. K. Chin and T. Van Duzer, Appl. Phys. Lett. SO, 287 (1987). 2. J. Y. Josefowicz and D. B. Rensch. J. Vuc. Sci. Technol.
8. S. S. Gill, J. Electrochem. Sot. 135, 1027 (1988). 9. M. Genut and M. Eizenberg, Appl. Phys. La. !#),I358 (1987). 10. T. Sands, J. P. Harbison, W. K. Chan, S. A. Schwarz, C. C. Chang, C. J. Palmstom and V. G. Keramidas,
BS, 1707 (1987). 3. L. C. Zhang, C. L. Liang, S. K. Cheung and N. W. Cheung, J. vat. Sci. Te&nol. BS, 1716 (i987).
11. T. E. Hayens, W. K. Chu, C. C. Han, S. S. Lau and S. T. Picraux, Appl. Phys. Lett. 53, 2200
REFERENCES
4. S. S. Lau. W. X. Chen. E. D. Marshall. C. S. Pai. W. F. Tseng and T. F. Kdech, Appl. Phys. L&. 47,1298 (1985). 5. C. P. Lee, T. H. Liu and T. F. Lei, J. appl. Phys. 65, 642 (1989). 6. H. Yamagishi, Jup. J. appl. Phys. 23, L895 (1984).
7. T. Sands, W. K. Chan, C. C. Chang, E. W. Chase and V. G. Keramidas, Appl. Phys. Lat. 52, 1338 (1988).
Appl. Phys. L&t. 52, 1216 (1988).
(1988).
12. L. J. Brillson, J. Vat. Sci. Technol. 16, 1137 (1979). 13. N. Newman, W. E. Spicer and E. R. Weber, J. Vat. Sci. Technol. BS, 1020 (1987). 14. H. C. Cheng, C. Y. Wu and J. J. Shy, to be published. 1.5. A. Callegari, G. D. Spiers, J. H. Magerlein and H. C. Guthrie, J. uppl. Phys. 61, 2054 (1987).
Printed in Great
0038-I101/90 s3.00 + 0.00 Copyright 0
Britain. All rights reserved
1990 Pergamon Press plc
EXCELLENT THERMAL STABILITY OF COBALT-ALUMINUM ALLOY SCHOTTKY CONTACTS GaAs SUBSTRATES H. C. CHENG,C. Y. WV and J. J.
ON
SHY
Department of Electronics Engineering, National Chiao-Tung University, Hsinchu, Taiwan, Republic of China (Received 31 August 1989; in revised form 21 December
1989)
Abstract-Schottky contact properties of ftlms with codeposited Co-Al mixture or tri-layered Co/Al/Co structure on GaAs substrate after different rapid thermal annealings (RTAs) have been investigated. Interfacial thermal stability between co-deposited Co-Al mixture and GaAs was attributed to the formation of CoAl compound. However, cobalt at the Co/GaAs interface would strongly react with GaAs to produce Co,GaAs phase for tri-layered Co/Al/Co structure on GaAs. Subsequently, Co,GaAs compound would decompose into CoGa and CoAs phases. Capped with a 3500~A-thick SiO, layer, co-deposited Co-Al films could withstand high-temperature annealing and exhibited a barrier height of 0.88 eV and an ideality factor of 1.08 even after 1050°C rapid thermal annealing for 10 s. As far as the authors are aware, this rapid thermal annealing temperature is the highest in the literature for such excellent Schottky characteristics. The contact was found to achieve a barrier of 09OeV for the co-deposited Co-Al alloy with Co: Al = 1: 1.05 after 850°C rapid thermal annealing for 50 s. Tri-layered Co/Al/Co film was also found to be thermally stable on GaAs after short time annealing (10 s), but the contact property rapidly degraded with annealing time.
1. INTRODUCTION
contacts on GaAs have been widely used in metal-semiconductor field-effect transistors (MESFETs), Schottky diodes, and other microwave devices[ 11.To improve the performance of GaAs ICs, the self-aligned-gate (SAG) process was developed to eliminate the source parasitic resistance[2]. However, SAG processing would require the gate material to sustain a high-temperature (> SSOC) implant activation annealing without degrading the Schottky characteristics of the gate[3-41. Most elemental metals react with GaAs at a lower temperature so that the barrier characteristics deteriorated. In contrast, several refractory metal silicides have been demonstrated to possess superior barrier characteristics than these elemental metals on GaAs. A stable Schottky contact with the barrier height of 0.83 eV has been reported for a TaSi,/GaAs structure after 850°C annealing[5]. Refractory metal nitrides were also candidate gate materials for GaAs. A stable diode of barrier height 0.95 eV and ideality factor 1.09 for a IO-s anneal at 800°C was obtained for tungsten nitride on GaAs. However, the barrier decreased to 0.90 eV and the ideality factor increased to 1.15 after 10 s annealing at 9Oo”C[6]. Sands et a/.[71 found that a NiAl bimetallic metallization on GaAs could achieve a stable and uniform Schottky contact with a barrier height of 0.99 eV and an ideality factor of 1.10 after a 20 s anneal at 650°C. The barrier and ideality factor of NiAl/GaAs annealed at 950°C for 20s degraded to 0.82eV and 2.07, respectively. Schottky
CoAl compound has the same crystal structure as NiAl phase (cubic CsCl structure). In addition, the chemical properties of CoAl alloy are very similar to that of NiAl. The melting point of CoAl (1645°C) is slightly higher than that of NiAl (1638°C). In the present study, the Schottky characteristics of CoAl contacts on GaAs were investigated by current-voltage (I-V) and capacitance-voltage (C-V) measurements. A transmission electron microscope was used to identify the phase transitions of Co-Al alloys. 2. EXPERIMENTAL
DETAILS
The GaAs wafers used were (100) oriented and n type with Si dopant concentration of 1 x 10’6cm-3. The samples were degreased consecutively in boiling trichloroethylene (TCE), acetone, and deionized water for 5 min each. To remove surface damage layer, wafers were etched with H2S04: H,O,: H,O = 5 : 1: 1 solution. The samples were divided into two groups. Some of the samples were used for transmission electron microscopy (TEM) observations and the other were utilized for electrical measurements. Making use of the lift-off process to form diodes, samples for electrical measurements were patterned by photolithography after wafer cleaning. The diameter of the circular diodes was 500pm. Before loading into the dual electron gun evaporation system, samples for TEM examinations were dipped with HCl:H,O = 1: 1 solution for 1 min and specimens for electrical measure-
863
864
H. C. CHENG et al.
ments were dipped with NH,OH : H,O = 1: 1 solution for 1 min to remove the surface native oxide. Cobalt and aluminum were codeposited to obtain the Co-Al alloys with 200 A-thick cobalt or layer-by-layerdeposited to obtain the films of Co(lO0 A)/ Al/Co( 100 A) structure. The thickness of Al layer was selected to make the atomic ratio of Co/Al to be 1.05/l or l/1.05. The film sequence of tri-layered structure was chosen based on the penetration into the native oxide on GaAs and the stabilization of aluminum metal. The chamber was evacuated to 3 x 10v6 Torr. The deposition rate of cobalt was 1 A/s. Following the metal deposition, some of the samples were capped with a 3500 8, thick SiOz layer by a single-electron-gun evaporation system. This cap served to prevent the arsenic outdiffusion from GaAs substrate during high temperature annealing. Subsequently, rapid thermal annealing (RTA) was carried out in a flowing nitrogen ambient using a halogen lamp system. The efficiency of dopant activation in RTA system was reported to be higher than that of conventional furnace[8]. Annealings were performed at temperatures from 650 to 1050°C for 10 s or from 500 to 850°C for 50 s. After annealing, AuGe sintering at 450°C for 1.5 min was used as backside ohmic contact. Finally, the capped Si02 film on the front-side surface of the sample was removed by BOE solution. Samples for TEM observations were cut into 3 mm diameter discs by an ultrasonic cutter. An etching solution with Br, : methonal = 2 : 98 was used until a small hole on the specimen appeared. The electron microscopy was performed with a JEOL2000FX scanning transmission electron microscope (STEM) operating at 160 kV. Current-voltage (I-V) and capacitance-voltage (C-I’) measurements were utilized to appraise the contact properties. 1-V characteristics were measured by an HP4145B semiconductor parameter analyzer. From the thermionic emission theory, the barrier height @+,and ideality factor n can be derived from:
where I is the current, V is the voltage, q is the elementary charge, k is the Boltzmann constant, T is the absolute temperature, a is the diode area, A* is the effective Richardson constant of 8.16 A cmm2 Km2 for GaAs, and Z, is the reverse saturation current. C-V characteristics were determined by using an HP4194A Impedance/Gain-Phase analyzer. The C-V barrier heights @c-” were extracted by plotting Cm2 vs the reverse-bias applied voltage for the bulk doped samples.
3. RESULTS
AND DISCUSSION
The samples which will be mentioned below were marked by a letter and a number. The letter represents the deposition type of metal films, e.g. letter “C” for the co-deposited Co-Al mixture and “T” for n-i-layered Co/Al/Co structure. The number classifies the atomic ratio of Co:Al, e.g. number “1” for Co:Al= 1:1.05 and “2” for Co:Al= 1.05:l. According to the results of TEM observations, interfacial properties of t&layered Co/Al/Co films on GaAs were not stable. After 500°C annealing, Co,GaAs phase with a similar lattice constant as GaAs substrate was formed to have a preferential orientation to GaAs for type T specimens. Co,GaAs would decompose into CoGa and CoAs compounds as the specimens were annealed at the temperatures above 550°C. The results are consistent with those of a previous report[9]. These reactions resulted in a failure of the epitaxial growth of CoAl phase[lO]. TEM analysis of Cl and C2 specimens indicated that only the CoAl phase and a small amount of Co,Al were present for all annealing conditions. An example is shown in Fig. 1. CoGa and CoAs compounds were not found in Cl and C2 samples annealed at different temperatures indicating the interfacial stability of these samples. In addition, no epitaxy of the CoAl phase was found in any type C specimens. Lau et a/.[41 reported that the stability of electrical characteristics of a gate contact after high-temperature annealing was directly correlated to the chemical stability of the gate metallurgy. Samples of type C were therefore thought to possess a better thermal stability of electrical properties than those of type T. In the Z-V measurement, the barrier height and ideality factor were extracted to appraise the Schottky characteristics of various types of metal films on GaAs. For the Cl specimens without a SiO, capping layer, the curves of barrier height and ideality factor vs annealing temperature with 10 s annealing time are shown in Fig. 2. The diode properties of Cl samples were maintained up to 950°C. Barrier height and ideality factor of Cl specimens annealed at 950°C for 10 s were 0.86 eV and 1.13, respectively. However, the ideality factor increased to 1.76 although the contact remained rectifying for the Cl samples annealed at 1050°C for 10 s. Similar results were also found in C2 samples. The barrier height and ideality factor of C2 specimens after 950°C rapid thermal annealing for 10 s were 0.86 eV and 1.10, accordingly. After heat treatment at 1050°C for 10 s, ideality factor of C2 samples rose to 1.60. Moreover, the diode characteristics would degrade faster as the annealing time was raised to 50 s. For example, the Schottky characteristics of C2 specimens deteriorated at temperatures above 650°C and the contact properties became nearly ohmic after 850°C annealing for 50 s. To avoid the outdiffusion of a substantial amount of Ga and As from the metal/GaAs interface, a capping layer of dielectric film on the surface of metal
Co-Al
contacts on GaAs
865
(4
(b)
Fig. 1. Diffraction pattern (a) and indexed pattern (b) of CoAl phase, Cl samples annealed at 550°C
for 50s. film was deposited to improve the thermal stability of the metal/GaAs contact. The plots of barrier height and ideality factor against annealing temperature for Cl and C2 specimens capped with a 35OOA thick SiOr film are shown in Fig. 3. For the short time annealing (IO s), the diode characteristics were well preserved even after 1050°C annealing. The ideality factors for Cl and C2 samples after 1050°C annealing for 10 s were 1.04 and 1.08, respectively. In addition, the barrier heights for Cl and C2 specimens annealed at this condition were 0.84 and 0.88 eV, accordingly. As far as the authors are aware, this rapid thermal annealing temperature is the highest reported to date
for such good Schottky characteristics. Even if the annealing time increased, the contact properties were still excellent at a high-temperature annealing. For example, the barrier height and ideality factor for Cl samples annealed at 850°C for 50 s were 0.90 eV and 1.09, correspondingly. Hence, a layer of dielectric film capped on the metal was very effective to obtain a CoAl/GaAs diode with a good thermal stability. The curves of barrier height and ideality factor of the samples of type T are shown in Fig. 4. The contact properties of type T specimens were also stable until the annealing temperature reached 1050°C for short time annealing. However, the contact characteristics 2.0
0.95
o.g512.0
+--.Cz,$b
,@b
-cl
k-4
*Cl,n
_
C2.n
r’ .$ x
0.75-
.-b t g 0.65 -
1 1.2
_----A_--
650 Annealing
Temperature
(‘Cl
Fig. 2. Schottky barriers Gb and ideality factors n of Cl specimens without oxide cap annealed at different temperatures for 10 s. SSE 33,7--F
750 Annealing
:
650
----!
_---
950
Temperature
A 1
1.0
1050
( ‘C)
Fig. 3. Barrier heights Gb and ideality factors n of SiO,capped Cl and C2 specimens as functions of annealing temperature with a 10 s treatment.
H. C. CHENG et al.
866
2.0
0.95~ -
T1 .@b
O---O
Tz ,#b
b--d
Tlsn
A---A
T2.n
.
-I 1.8
0.8+
I
m
!5 0.65 1.2
0.55’1 650
I
1
b
750
850
Annealing
I
I
I
950
Temperature
1
11.0
1050
( ‘Cl
and ideality factors n against annealing temperature for SiO,-capped Tl and T2 samples annealed for 10s. Fig. 4. Curves of barriers ab
such as Al,, and GaAa that were generated to move the surface Fermi level toward valence band maximum[ 131. Because the Co, GaAs compound occurred for tri-layered Co/Al/Co film on GaAs, the formation of Al,Ga,_.As layer would rely on the reaction of Co,GaAs phase with aluminum layer[lO]. On the other hand, co-deposited Co-Al film did not react with GaAs so that the Al-Ga exchange at the interface could proceed directly. Since all of the Co,GaAs, CoGa and CoAs compounds at the interface would reduce the probability of Al-Ga exchange and enhance the arsenic outdiffusion, the contact barriers of type T diodes were lower than those of type C specimens. The barrier height was also determined from the C-Vmeasurements[l4]. The barrier heights extracted from the C-*-V intercepts were 0.95, 0.95 and 0.98 eV for the C2 specimens annealed at 650, 850 and 105O”C, respectively. The higher values of barrier heights measured from C-V method than those from Z-I’ curve could be ascribed to traps near the surface or deeper in the depletion region[ 151.
4. CONCLUSIONS
degraded at annealing temperatures above 550°C if the annealing time was 50 s. The thermal stability of the electrical properties is consistent with the TEM examinations. Since the co-deposited Co-Al film did not react with GaAs and only a small amount of AlLGa exchange was surmised to occur at the interface, the smoothness and quality of the metal/GaAs interface were not deteriorated during annealing and the diode characteristics could thus be maintained up to a very high temperature. However, the decomposition of the Co,GaAs phase would destroy the interface smoothness, and CoGa as well as CoAs products between CoAl and GaAs would also obstruct the AlGa exchange for type T specimens. Hence, samples of type T exhibited a worse stability of electrical properties than those of type C. As for the thermal stability of type T diodes annealed at 1050°C for 10 s, it could be interpreted in a similar way as that of TaSi,/GaAs[ll]. In Ref.[ll], the authors demonstrate that the diode characteristics of metal/GaAs contact could be maintained if the decomposition of the GaAs was less than 15-20 monolayers. In this study, barriers ranged from 0.74 eV for Tl specimens annealed at 1050°C for 10 s to 0.90 eV for Cl samples annealed at 850°C for 50 sf This phenomenon is attributed to the effect of Al-Ga exchange. As the aluminum exchanged with the gallium at the interface of metal/GaAs, arsenic outdiffusion could be depressed due to the dipole change of the interfacial layer[l2]. In addition, Schottky barrier height increased through two mechanisms: (1) a layer of Al,Ga, _,As that was formed to enlarge the band gap of the GaAs surface. (2) The induced anti-site defects
Chemical properties of co-deposited C-Al films on GaAs were very stable. Whatever the annealing conditions were, reaction products were always CoAl phase and a trace of Co,Al. On the other hand, cobalt at the interface of Co/GaAs strongly reacted with GaAs and formed Co,GaAs phase for trilayered Co/Al/Co structure on GaAs. After annealing at the temperatures above 550°C Co2GaAs compound decomposed into CoGa and CoAs phases which would obstruct the AlGa exchange and enhance the arsenic outdiffusion. Co-deposited Co-Al film exhibited an excellent thermal stability on GaAs substrate. Good Schottky characteristics of co-deposited Co-Al alloys capped with a layer of 3500 A thick SiO, could be maintained until the annealing temperature rose to 1050°C. The C2 specimens achieved a barrier height of 0.88 eV and an ideality factor of 1.08 even after an annealing at 1050°C for 10 s. Moreover, the contact could obtain a barrier of 0.90eV for Cl specimens annealed at 850°C for 50 s. Tri-layered Co/Al/Co structure on GaAs also showed a good thermal stability after short time annealing. However, the diode characteristics of type T specimens degraded at annealing temperatures above 550°C as the annealing time increased to 50 s. Owing to the limited Al&a exchange and the arsenic outdiffusion, the Schottky characteristics for triple layer of Co/Al/Co structure on GaAs were worse than those of co-deposited Co-Al mixture. Acknowledgement-The the Republic of China
research was supported in part by National
Science Council.
Co-Al contacts on GaAs
861
1. X. W. Wu, L. C. Zhang, P. Bradley, D. K. Chin and T. Van Duzer, Appl. Phys. Lett. SO, 287 (1987). 2. J. Y. Josefowicz and D. B. Rensch. J. Vuc. Sci. Technol.
8. S. S. Gill, J. Electrochem. Sot. 135, 1027 (1988). 9. M. Genut and M. Eizenberg, Appl. Phys. La. !#),I358 (1987). 10. T. Sands, J. P. Harbison, W. K. Chan, S. A. Schwarz, C. C. Chang, C. J. Palmstom and V. G. Keramidas,
BS, 1707 (1987). 3. L. C. Zhang, C. L. Liang, S. K. Cheung and N. W. Cheung, J. vat. Sci. Te&nol. BS, 1716 (i987).
11. T. E. Hayens, W. K. Chu, C. C. Han, S. S. Lau and S. T. Picraux, Appl. Phys. Lett. 53, 2200
REFERENCES
4. S. S. Lau. W. X. Chen. E. D. Marshall. C. S. Pai. W. F. Tseng and T. F. Kdech, Appl. Phys. L&. 47,1298 (1985). 5. C. P. Lee, T. H. Liu and T. F. Lei, J. appl. Phys. 65, 642 (1989). 6. H. Yamagishi, Jup. J. appl. Phys. 23, L895 (1984).
7. T. Sands, W. K. Chan, C. C. Chang, E. W. Chase and V. G. Keramidas, Appl. Phys. Lat. 52, 1338 (1988).
Appl. Phys. L&t. 52, 1216 (1988).
(1988).
12. L. J. Brillson, J. Vat. Sci. Technol. 16, 1137 (1979). 13. N. Newman, W. E. Spicer and E. R. Weber, J. Vat. Sci. Technol. BS, 1020 (1987). 14. H. C. Cheng, C. Y. Wu and J. J. Shy, to be published. 1.5. A. Callegari, G. D. Spiers, J. H. Magerlein and H. C. Guthrie, J. uppl. Phys. 61, 2054 (1987).