- Email: [email protected]
Thermal stability of the Pd-Al alloy Schottky contacts to n-GaAs
MATERIALS SClENCE & ENCINEERRW
B
Materials Scienceand EngineeringB49 (1997) 144- 151
Thermal stability of the Pd-Al TX Department
of Materials
Science
alloy Schottky contacts to II-GaAs
Huang *, J.G. Pang
mm Engineering,
National
Tsing Hera lkicersity,
Hsinchu
30 033, Taiwan.
ROC
Received 4 April 1997;accepted 11 June 1997
Abstract Thermal stability of the Pd-Al alloy Schottky contacts to n-GaAs have been investigated by X-ray diffraction (XRD), cross-sectional transmission electron microscopy (XTEM) and Auger depth profiling. Electrical characteristics of the Schottky diodes were assessed by current-voltage measurement. Four different alloy compositions, i.e. PddOAl,“, Pd48A152,Pd52A138and Pd,8Al,,, were codeposited by dual electron-gun evaporation and then annealed by rapid thermal processing at temperatures 500-1000°C for 20 s. The Al-rich films were chemically stable on GaAs up to lOOO”C, however, the contacts degraded and exhibited poor diode characteristics when interfacial diffusion was prominent and the interface became rough. The Pd-rich films were chemically less stable and the interfacial reaction occurred at a lower temperature with increasing Pd content. After high-temperature annealing, more As outdiffused into the Al-rich films. On the other hand, more Ga outdiffused into the Pd-rich Nms resulting in the chemical reaction and the formation of PdGa compound. The Pd,,AI,, contact exhibited the best stability among all Pd-Al alloy metailizations on GaAs. A barrier height of 0.97 V and an ideality factor of 1.09 were obtained for Pd,,Al,,/n-GaAs Schottky diode annealed at 900°C for 20 s. 0 1997 Elsevier Science SA. Ke~~*urds:
Contact stability; P&Al alloy; GaAs
1, Introduction
Refractory metal silicides and nitrides, e.g. WSi, [2], [3] and TaSi, [4], with appropriate compositions have been reported to be thermally stable after high temperature annealing and have been demonstrated to yield good results. On the other hand, Al is the most common gate metal used in microwave FETs. However, because of the low melting point of Al (66O”Q Al/GaAs interface is only metallurgically stable below 500°C [5,6]. The thermal stability and lateral uniformity of Al/GaAs interface do not satisfy the requirements of SAG process. In principle, chemical stability of metal/GaAs interface can be well accounted for by using thermodynamic data associated with bulk compounds [7,8]. From a thermodynamic point of view, in order to assure a low chemical reactivity between metal and GaAs, one could select metallic compounds with strong metal-metal bond strength. Hence transition metal aluminides, which have higher melting temperatures than pure Al and favorably large heat of formations, are considered as new candidates for SAG process. The W-Al alloy has been successfully applied as self-aligned gate for GaAs MESFETs [9-l 11. Sands et al. [12] studied the NiAl Schottky metallization on GaAs and found that WN,
The high speed integrated circuits based on GaAs metal semiconductor field effect transistors (MESFETs) have been considered as one of the most promising devices for the next generation electronic devices. Many fabrication processes have been proposed for GaAs MESFETs. Among them, the self-aligned gate (SAG) process, which can reduce the unnecessary source, drain resistances and improve device performance, has been extensively employed for enhancement mode digital logic MESFETs. In the SAG process, the gate metal serves as the implantation mask for source and drain regions. The implanted GaAs wafer is then subjected to a high temperature ion activation annealing, either 800-850°C for lo-20 min for furnace annealing or 800-400°C for 5-20 s for rapid thermal annealing. The gate metal must maintain good Schottky characteristics after high temperature annealing. In searching for high temperature stable metallizations for GaAs SAG process, almost all pure metals were found to be not thermally stable on GaAs [l]. * Corresponding author. 0921-5107/97/$17.00Q 1997Elsevier ScienceS.A. All rights reserved. P11s0921-5107(97)00113-x
T.S. Humg,
J.G. Pang / Murerials
Stience
NiAl is more stable than pure Ni and Al, although it is still not stable enough to withstand the post-implantation annealing treatment. Blanpain et al. [13] have investigated the thermal stability of Pt-Al alloy films on GaAs and their results indicate that for the composition region between 50 and 67 at.“? Al, the alloy films fuhill the thermal stability requirements imposed by GaAs SAG technology. Our previous investigations on MO-AI [14,15] and Ta-Al [16] alloy Schottky contacts to fz-GaAs indicated that the MO-AI alloy metallizations with Al compositions between 25 and 73 at.% and the Ta-Al alloys with Al content between 63 and 70 at.O/o were stable after rapid thermal annealing up to 900°C. For these thermally stable Mo-Al/GaAs and Ta-Al/GaAs Schottky contacts, the barrier heights increased with annealing temperature. The barrier height enhancement is beneficial to the GaAs digital logic devices application based on the enhancement mode FETs. Ni, Pd and Pt belong to the same group of metals in the periodic table of elements. The PdAl compound has a large heat of formation (AH= - 92 kJ/g-atom), which is more negative than that of NiAl (AH= - 59 kJ/g-atom) and is similar to that of PtAl (AH= - 100 kJ/g-atom) [17]. Therefore, the Pd-Al alloy films may also possess good stability on GaAs as Pt-Al alloy metallizations. In this paper we report both the metallurgical and the electrical characteristics of electronbeam evaporated Pd-Al alloy contacts to n-GaAs after isochronous rapid thermal annealing at temperatures 500-1000°C. According to the Pd-Al binary phase diagram [18], there exist three intermediate phases with a wide composition range and melting temperature higher than 900°C. Therefore, we have investigated four alloy contacts, namely Pd,,Al,O, Pd48A152, Pd,,Al,, and Pd,*Al,,. The PddOAl,, alloy lies in the 6-Pd,Al, phase regime. The compositions of both Pd,,Al,, and Pd,,Al,, alloys are in the phase regime of /3-PdAl, one is Al-rich while the other is Pd-rich. The PdBBAl,, alloy lies in the p-Pd,Al phase regime.
2. Experimental
procedures
Si-doped (100) GaAs wafers (17= 2.2 x lOI cmA3) were degreased in boiling trichloroethylene, acetone and then etched with H2S04:H,0,:H,0 (5:l:l by volume) to remove surface damage layer and undesirable impurities. A short dip in NH,OH:H,O (1:l) was performed immediately before loading the wafers into the vacuum chamber to further remove native oxide. The thin Pd-Al alloy films were codeposited by a dual electron-gun evaporation system equipped with a cryopump under a base vacuum of 3 x 10 - 7 Torr. The deposition rate of Al was fixed at 0.2 nm s- * and the Pd deposition rate was varied according to the desired
and Engineering
B49 (1997)
144-151
145
composition. The film thickness was about 100 nm. A small piece of oxidized Si wafer was also placed next to the GaAs substrate as a control sample to facilitate the Rutherford back scattering analysis of the film compositions. The compositions of the analyzed alloy films were Pd,,Al,O, Pd,,Al,,, Pd,,Al,, and Pd,,A132. To prepare Schottky diodes, a layer of SiO, about 100 nm was deposited by electron-beam evaporation onto PdAl films as an annealing cap. The samples were annealed in a Heatpulse 410 incoherent lamp rapid thermal annealing furnace at temperatures 500- 1000°C for 20 s under flowing nitrogen atmosphere. After heat treatment, the samples for making Schottky diodes were etched with HF:H,O (1:l) to remove SiO, layer and then further processed by standard photolithography and aqua regia solution etch to make circular contact pads of 500 urn diameter. Backside ohmic contacts of Schottky diodes were formed by evaporation In followed by a 1 min annealing at 400°C. The thermal stability and interface structures of PdAl/GaAs contacts were investigated by X-ray diffraction (XRD), cross-sectional transmission electron microscopy (XTEM) and Auger depth profiling analysis. The electrical resistivity of the film was measured by collinear four-point probe in conjunction with the film thickness measurement using a surface profiler. The electrical properties of the Schottky diodes were characterized by dark current-voltage (I-V) measurement. All I-V measurements were performed by a HP4145B semiconductor parameter analyzer.
3. Results
3.1.1. P&AII,,lGcrAs
contact
Fig. 1 shows the XRD patterns for Pd,,Al,,/GaAs annealed at different temperatures. The as-deposited sample showed the characteristic diffraction pattern of amorphous phase. After 500°C anneal, the amorphous film crystallized into Pd,Al, phase. Pd,Al, was the dominant phase in the film up to 800°C. Additional peaks of /3-PdAl emerged after 900°C anneal. After annealing at lOOO”C, the XRD intensities of Pd,Al, peaks significantly decreased and ,L?-PdAl became the major phase. Although the XRD analysis did not show phases associated with the reaction between the film and GaAs substrate, the Auger depth profiles.- Fig. 2(a), revealed apparent interdiffusion near the interface at 900°C. The &lm also contained some oxygen and a layer of aluminum oxide covered the surface. At lOOO”C, a lot of As outdiffused into the film as shown in Fig. 2(b) and simultaneously more oxygen indiffused to the film. The interfacial diffusion at 900°C seems not to alter the original phases within the film. The XTEM
146
T.S. Hrrang, J.G. Pang/Materials
Science and Engitleering 849 (1997) 144- 151
analysis, Fig. 3, indicated that the grains of Pd,Al, and ,B-PdAl adjoined directly to the substrate and no other new phase was observed. However, the interface was not flat any more. 3.12. Pd~8Alj2/G~As contact The XRD analysis for Pd,,Al,,/GaAs annealed at different temperatures indicated that the P-PdAl phase formed in the as-deposited film and after annealing in the temperature range of 500-lOOO”C, only /?-PdAl existed and its diffraction intensity increased. Other phases associated with interfacial reactions were not detected by XRD analysis. The interfacial stability was further confirmed by Auger depth profiling and XTEM analysis, The Auger depth profiles of Pd,,Al,,/GaAs annealed at 900°C Fig. 4(a), indicated that the interface was still quite sharp. The bright field XTEM image, Fig. 5, shows a flat and smooth interface in 900°C annealed sample. In contrast to Pd,,Al,,/GaAs, the outdiffused As in 1000°C annealed Pd,,Al,,/GaAs accumulated in the interfacial region and the oxygen content in the film was negligible, as shown in Fig. 4(b). The Pd,,Al,,/GaAs contact exhibited the best stability among all samples prepared in this study. 3.1.3. Pd,,Al,,/GaAs contact The results of XRD analysis for Pd,,Al,,/GaAs annealed at various temperatures are shown in Fig. 6. The ,L?-PdAl was detected in the as-deposited sample and the contact interface was stable up to 700°C anneal. The diffraction intensity of (110) P-PdAl peak decreased after annealing above 800°C. Concurrently, a small
Al,o/GaA~, Pd
9OO OC, 20 S
{a’
Ga ,_ / /
I
0 I
5
IO
Pd40Also/GaAs,
15
20
25
1000 ‘C, OC, 20 s
b)
Pd
Ga /
.1=: s t; . ;.p Y .
\
I
1 ‘l’,’
E. 8
.
0
. . . . . .. .*....; . . . . . .. A.1 . . . . . . . . . . . . ._.-.-.-........ 1 I I I I 5 10 15 20 25 30
Sputter
time (min)
Fig. 2. Auger depth profiles for Pd,,Al,O/GaAs 900°C: and (b) 1000°C.
annealed at: (a)
amount of the interfacial reaction product PdGa was observed in 800°C annealed sample and the (210) PdGa diffraction intensity increased with annealing temperature above 800°C. Auger depth profiles, Fig. 7, indicated that Ga outdiffused into the PdAl film at 800°C whereas the amount of As in the film was insignificant.
20
30
40
50
60
70
80
28 (degree) Fig. 1. XRD patterns of Pd,,Al,,/GaAs: (a) as-deposited and annealed at: (b) 800°C; (c) 900°C; and (d) 1000°C.
3.1.4. Pd6,Ai,JGnAs contact Fig. 8 shows the XRD patterns for Pd,,Al,,/GaAs annealed at different temperatures. The as-deposited sample showed the (111) peak of Pd phase only. After 500°C anneal, the Pd,Al and a small amount of PdGa were detected. At 6OO”C, more PdGa was formed and some PdAs, also emerged. The Pd,Al disappeared at 700°C and the dominant phases in the film became
147
PdGa and PdAs,. At 800°C and higher temperatures, PdAs, decomposed and only PdGa was retained after 1000°C anneal.
Pd,,AI,,/GaAs,
900 OC
Pd,,AI,,/GaAs,
1000 OC
3.2. Film resisfivit]
Fig. 9 shows the variation of film resistivities with annealing temperatures. The resistivity of Pd,,Al,,/ GaAs was the lowest among all samples. It decreased from 5.3 to 1.3 1tR cm below 700°C and then increased with temperature. The increase of resistivity above 700°C may be due to the oxidation of the film as revealed in Auger depth profiling analysis. The resistivity of Pd4*A1,, film decreased monotonously with increasing temperature. After 900°C anneal, the film resistivity reduced to 23 I.IR cm. Because the interface of the sample was stable up to 900°C the decrease of resistivity with temperature would be a result of grain growth of the major ,8-PdAl phase in the film as detected in the XRD analysis. After annealing below 800°C the Mm resistivity of Pd,,Al,,/ GaAs exhibited a similar temperature dependence to that of Pd,,Al,,/GaAs. The film resistivity of the as-deposited Pd,,Al,,/GaAs was the highest among all samples. It decreased after annealing below 700°C and varied irregularly above 800°C. The interfacial reactions occurred extensively above 600°C resulting in the formation of several different phases in the reacted films, therefore, the variation of resistivity became irregular.
0
10 20 30 Sputter Time (min)
Fig. 4. Auger depth profiles for Pd,,Al,jGaAs 900°C; and (b) 1000°C.
40 annealed at: (a)
It is worth mentioning that the thermally stable Pd-Al alloy films on GaAs substrate have resistivities smaller than 60 ltQ cm, which is also much smaller than the resistivities of other refractory alloy films on GaAs, e.g. WSi, ( > 100 @2 cm) [2], WN, ( > 70 @2 cm) [3], WAl,Y (50- 100 Q cm) [9] and TaAl, ( b 200 uQ cm) [16]. The low electrical resistivity is one of the important requirements for the contact metals in the semiconductor devices applications. 3.3. Electrical clzaracteristics of Schottlcy diocles
PdAl
Pd2Ah
AlOx
Epoxy
GaAs
Fig. 3. XTEM bright field image and corresponding schematic drawing for Pd,,Al,,/GaAs annealed at 900°C for 20 s.
The typical log I-V curves of Pd,,Al,,/GaAs Schottky diodes are shown in Fig. 10. The forward log I-V curves of all stable diodes can be well analyzed with thermionic emission model [19]. The ideality factor, II
148
T.S. Humg,
J.G. Pung,‘M~rerials
S&we
and Engitzeeritlg
B49 (1997)
144- 151
Pd&I,,/GaAs,
800 “C
Ga
Fig. 5. XTEM brightfield image for Pd,,Al,,/GaAs 900°C for 20 s.
annealed at
and the barrier heights, q&,, extracted from the forward log I-V curves of the stable Pd-Al/GaAs diodes are plotted as a function of annealing temperature in Fig. 11. The Pd,,,Al,,/GaAs diodes annealed below 600°C exhibited good Schottky characteristics with n values smaller than 1.04. It started to degrade after 700°C anneal and the ideality factor increased abruptly. After 900°C anneal, the forward log I-V curve did not show linear region to be analyzed with thermionic emission model for obtaining reliable ideality factor and barrier height. The n values of Pd,,Al,,/GaAs diodes annealed below 500°C were also lower than 1.04. It increased slightly to 1.17 at 700°C and then decreased to 1.09
40
50
60
20 (degree) Fig. 6. XRD patterns of Pd,,Al,,/GaAs tures.
annealed at various tempera-
0
5
10
15
20
25
30
35
40
Sputter Time (min) Fig. 7. Auger depth profiles for Pd,,Al,,/GaAs
annealed at 800°C.
after 900°C anneal. A barrier height of 0.97 V with an ideality factor of 1.09 was obtained for 900°C annealed diode. The excellent thermal stability of Pd,,Al,JGaAs diodes can be correlated to the metallurgical stability of the contacts as revealed in the structure arnlysis.
20
30
40
50
60
70
80
28 (degree) Fig. 8. XRD patterns of Pd,Al,dGaAs: (a) as-deposited and annealed at: (b) 500°C; (c) 600°C; (d) 7OO“C; and Ie) 800°C.
T.S.
Huang,
J.G.
Pang
/Materials
Science
and
Engirrreriag
120rS-
~100 I I F180 I c 2 6 z I$
B49
(1997)
(a )
’
'I
Pd68A'32
l
A
Pd52A'48
n
'
Pd48A'52
l
Pd4d'60
J
144-151
’
’
Pd,&JGaAs Pd,,Al,dGaAs Pd,$l,IGaAs
A
I
::
/
60 40
5 400
500
600
I 500
I
I
I
I
600
700
800
900
700
800
900
..”
A
200 t:: 27
2
500 600 700 800 TEMPERATURE ( “C )
900
The Pd,,Al,,/GaAs diodes retained their low H values smaller than 1.1 after annealing below 600°C. Above 6OO”C, the diodes significantly degraded. Although the annealed Pd,,Al,,/GaAs diodes exhibited rectifying characteristics in the linear I-V plot, all diodes were leaky and their log I-V curves did not show linear region for obtaining reliable ideality factor and barrier height.
_ (a)
400oc
(e)
(b)
500~~
(f) 9oo”c
(c)
600°C
(d)
700"~
10-I
-1
a
o-3
I
-
800
I
I
'C
5jfT%F‘VOLT, Fig. 10. The log I-V curves of Pd,,Ai,,/GaAs annealed at various temperatures.
0.9
$
Fig. 9. Variation of film resistivity as a function of annealing temperature.
I
> v
Schottky diodes
i?
0.8
Y 5
0.7
a 2
0.6
a 0.5 1 400
TEMPERATURE ( ’ C ) Fig. 11. Variations of the ideality factors and the barrier heights of the Pd-Al/n-GaAs Schottky diodes as a function of annealing temperature.
4. Discussion It is well established that thin Pd film reacts readily with GaAs substrate, even during deposition by electron-gun evaporation [1,20,21]. Final products of interfacial reactions after annealing in a closed system are PdGa and PdAs, and a layered structure of PdAs,/ PdGa/GaAs is reported. If annealing is carried out in a vacuum system or an open system, PdAs, decomposes at high temperatures and only PdGa is retained. On the other hand, Al/GaAs contact is relatively more inert [5>6]. In this study, we found that the thin Pd-Al alloy films are more stable than pure Pd films on GaAs and particularly Al-rich films exhibited superior thermal stability. The Pd-rich Pd68Al,, alloy lies in the composition range of p-Pd,Al phase. In contrast to Pd/GaAs contact, interfacial reactions were not observed in the as-deposited Pd,,Al,,/GaAs sample, however? Pd,Al compound phase still exhibited poor thermal stability on GaAs after annealing treatment. Phase formation sequences of interfacial reactions in annealed Pd,,Al,,/ GaAs were found to be similar to that of PdiGaAs.
150
T.S. Huwg,
J.G. Pang/Materials
Science
The compositions of Pd,,A&* and Pd48A1,, alloys are both within ,B-PdAl phase region. It is interesting that these two alloy films exhibited quite different stabilities on GaAs. The Pd-rich Pd,,AI,, films reacted with GaAs at 800°C whereas the Al-rich Pdd8Al,, films were stable on GaAs up to 1000°C anneal. It is well known that both the physical and the chemical properties of a compound phase with a large composition range are usually dependent on the stoichiometry within the phase region. The Al activity in the ,B-PdAI compound phase increased with increasing Al content and varied abruptly across the stoichiometric composition 122,231, we therefore speculate that the compositional dependence of the thermal stabilities of these two P-phase PdAl alloy films is caused by significant difference of AI (and/or Pd) activity in the Pd- and the Al-rich /3-PdAI. The higher Al activity (lower Pd activity) of Pd,,AIS, than that of Pd,,Al,, ensures its better interfacial stability on GaAs. The Pd,,Al,O alloy has a composition within 6Pd2Al, phase region, The chemical stability of the Pd,,Al,,/GaAs interface was also good and no apparent interfacial reaction was detected up to 1000°C anneal. However, interfacial diffusion was prominent resulting in a rough interface, as shown in Fig. 3. Therefore, the Schottky diodes started to degrade with increased ideality factor after annealing above 700°C. The large current, which might be due to the recombination component, appeared in the small forward bias voltage region, Fig. 10, for 800°C annealed diode. After 900°C anneal, the diode was leaky owing to rough interface. Besides the difference in the compositional dependence of thermal stabilities of the Pd-Al alloy contacts to GaAs, the behaviors of the interfacial diffusions and the oxidation at high temperatures were also found to be compositional dependent. The outer surface of Pd,,AI,, film, which has the largest Al content, oxidized and some oxygen diffused into the film after annealing above 800°C Fig. 2. The oxidation of Pd,,Al, film was very limited and nearly no oxygen diffused into the film up to 1000°C anneal, Fig. 4. In the mean time, the As outdiffused into the Al-rich Pd-Al alloy films and accumulated at the interface as shown in the Auger depth profiles of 1000°C annealed Pd,,Al,,/GaAs and Pd,,Al,,/GaAs contacts. On the contrary, the Ga outdiffused into the Pd-rich Pd,,Al,, film and distributed relatively uniformly in the film, Fig. 7. Since the Al-rich Pd-Al alloy film has higher Al activity, it is more easily oxidized and may attract As from the GaAs substrate during high temperature anneal, whereas the higher activity of Pd in the Pd-rich Pd-Al alloy film results in the Pd-Ga attraction. Furthermore, because the Ga diffusivity has been found to be higher than the As diffusivity in most metal films on the GaAs substrate [I], the outdiffused Ga would distribute more uniformly
and Engineering
349
(1997)
144-151
in the Pd-Al film, while the outdiffused As in the Pd-AI film might accumulate at the interface region for a rapid thermal annealing process. For all stable Pd-Al/GaAs diodes, the barrier heights of both Pd,,Al,,/GaAs and Pd,,Al,JGaAs Schottky diodes increased with annealing temperature, whereas the barrier heights of Pd,,Al,,/GaAs diodes decreased with annealing temperature. The increase of the barrier height after high temperature annealing has also been observed in NiAl/GaAs [12], Mo-Al/GaAs [14,1.5] and Ta-Al/GaAs [16] Schottky diodes. With the same Fermi level pinning position at the metal/semiconductor interface, because the band gap of Al,Ga, -,As is larger than that of GaAs, the Schottky barrier height of metal/Al,Ga, _ ,As/GaAs diode should be larger. The Schottky barrier height of Mo/Al,Ga, -,As has also been experimentally verified to be larger than that of Mo/GaAs diode [24]. The interfacial Al,YGa, -,YAs layer, which was formed epitaxially on GaAs, has been identified by high resolution XTEM analysis [23]. Therefore, the barrier height enhancement should be mainly due to the formation of Al,YGa, _ .As layer at the interface between the Al-rich Pd-Al alloy contacts to n-GaAs.
5. Conclusions
The most important results of this study are summarized as follows: (1) The Al-rich Pd,OAl,,/GaAs and Pd,,Al,JGaAs contacts are chemically stable up to 1000°C. However, interfacial diffusion is prominent in Pd,,A160/GaAs contact after annealing above 800°C. (2) Interfacial reactions are detected in the Pd-rich Pd,,Al,,/GaAs and Pd,,Al,,/GaAs contacts after annealing above 800 and 500°C respectively. The phase formations of interfacial reactions in these two contacts are similar to that of Pd/GaAs contact. (3) The compositional dependence of the interfacial stability of Pd-AI alloy contacts on GaAs can be correlated to the variation of the activities of Pd and Al in these alloys. (4) For all thermally stable Pd-Al/n-GaAs Schottky diodes, the barrier heights of Al-rich Pd,,Al,,/n-GaAs and Pd4pAl,Jn -GaAs diodes increased whereas the barrier heights of Pd-rich Pd,,Al,,/n-GaAs diodes decreased with annealing temperature. The Pd,,Al&GaAs diodes were thermally unstable and became leaky after annealing. (5) The promising metallurgical, electrical stabilities and low film resistivities of the Al-rich Pd-Al alloy contacts to n-GaAs indicate that these alloys may offer an alternative as improved gate metallization in selfaligned gate metal-semiconductor field effect transistor technologies.
T.S.
Huang,
J.G.
Pang/Materials
Science
Acknowledgements This work was supported by the Republic of China National Science Council through Grant No. NSC 86-2215-E-007-007.
References [l] T. Sands, Mater. Sci. Eng. Bl (1989) 289. [2] A.G. Lahav, C.S. Wu, F.A. Baiocchi, J. Vat. Sci. Technol. B6 (1988) 1785. [3] K.M. Yu, J.M. Jaklevic, E.E. Haller, SK. Cheung, S.P. Kwok, J. Appl. Phys. 64 (1988) 1284. [4] T.E. Haynes, W.K. Chu, C.C. Han, S.S. Lau, S.T. Picraus, Appl. Phys. Lett. 53 (1988) 2200. [5] M. Missous, E.H. Rhoderick, K.E. Singer, J. Appl. Phys. 59 (1986) 3189. [6] N.M. Johnson, T.J. Magee, J. Peng, J. Vat. Sci. Technol. 13 (1976) 838. [7] S.S. Lau, W.X. Chen, E.D. Marshal, C.S. Pai, W.F. Tseng, T.F. Kuech, Appl. Phys. Lett. 47 (1985) 1298. [8] R. Beyers, K.B. Kim, R. Sinclair, J. Appl. Phys. 61 (1987) 2195. [9] H. Nakamura, Y. Sano, T. Nonaka, T. Ishida, K. Kaminishi, IEEE GaAs IC Symp. Tech. Dig. IEEE, New Jersey, 1983, pp. 134.
and
Engineering
B49
(1997)
144-151
151
UOI S.P. Kwok, J. Vat. Sci. Technol. B4 (1986) 1383. [ill Y. Sekino, T. Kimura, K. Inokuchi, Y. Sano, M. Sakuta, Jpn. J. Appl. Phys. 27 (1988) L2183.
tw T. Sands, W.K. Chan, CC. Chang, E.W. Chase, V.K. Keramidas, Appl. Phys. Lett. 52 (1988) 1388. B. Blanpain, G.D. Wilk, J.O. Olowolafe, J.W. Mayer, Appl. Phys. Lett. 57 (1990) 392. [I41 T.S. Huang, J.G. Peng, C.C. Lin, Appl. Phys. Lett. 61 (1992) 1131
3017.
T.S. Huang, J.G. Peng, C.C. Lin, J. Vat. Sci. Technol. Bll (1993) 756. 1161T.S. Huang, SM. Jean, J. Appl. Phys. 75 (1994) 7519. s71 R. Hultgren, P.D. Desai, D.T. Hawkins, M. Gleiser, K.K. Kelley, Selected Values of the Thermodynamic Properties of Binary Alloys, American Physical Society, Metal Park, OH, 1973. [I81 A.J. McAlister, Bull. Alloy Phase Diagr. 7 (1986) 368. u91 E.H. Rhoderick, R.H. Williams, Metal-Semiconductor Contacts, 2nd ed. Clarendon Press, Oxford, 1988, pp. 38. PO1 J.O. Olowolafe, P.S. Ho, H.J. Hovel, J.E. Lewis, J.M. Woodall, J. Appl. Phys. 50 (1979) 955. VI T.S. Kuan, J.L. Freeouf, P.E. Batson, E.L. Wilkie, J. Appl. Phys. 58 (1985) 1519. WI M. Ettenberg, K.L. Komarek, E. Miller, Metall. Trans. 2 (1971) 1173. ~231 T.S. Huang, J.G. Pang, J. Appl. Phys. 78 (1995) 5739. 1241 M. Eizenberg, M. Heiblum, M.I. Nathan, N. Braslau, P. Mooney, J. Appl. Phys. 61 (1987) 1516. 1151
B
Materials Scienceand EngineeringB49 (1997) 144- 151
Thermal stability of the Pd-Al TX Department
of Materials
Science
alloy Schottky contacts to II-GaAs
Huang *, J.G. Pang
mm Engineering,
National
Tsing Hera lkicersity,
Hsinchu
30 033, Taiwan.
ROC
Received 4 April 1997;accepted 11 June 1997
Abstract Thermal stability of the Pd-Al alloy Schottky contacts to n-GaAs have been investigated by X-ray diffraction (XRD), cross-sectional transmission electron microscopy (XTEM) and Auger depth profiling. Electrical characteristics of the Schottky diodes were assessed by current-voltage measurement. Four different alloy compositions, i.e. PddOAl,“, Pd48A152,Pd52A138and Pd,8Al,,, were codeposited by dual electron-gun evaporation and then annealed by rapid thermal processing at temperatures 500-1000°C for 20 s. The Al-rich films were chemically stable on GaAs up to lOOO”C, however, the contacts degraded and exhibited poor diode characteristics when interfacial diffusion was prominent and the interface became rough. The Pd-rich films were chemically less stable and the interfacial reaction occurred at a lower temperature with increasing Pd content. After high-temperature annealing, more As outdiffused into the Al-rich films. On the other hand, more Ga outdiffused into the Pd-rich Nms resulting in the chemical reaction and the formation of PdGa compound. The Pd,,AI,, contact exhibited the best stability among all Pd-Al alloy metailizations on GaAs. A barrier height of 0.97 V and an ideality factor of 1.09 were obtained for Pd,,Al,,/n-GaAs Schottky diode annealed at 900°C for 20 s. 0 1997 Elsevier Science SA. Ke~~*urds:
Contact stability; P&Al alloy; GaAs
1, Introduction
Refractory metal silicides and nitrides, e.g. WSi, [2], [3] and TaSi, [4], with appropriate compositions have been reported to be thermally stable after high temperature annealing and have been demonstrated to yield good results. On the other hand, Al is the most common gate metal used in microwave FETs. However, because of the low melting point of Al (66O”Q Al/GaAs interface is only metallurgically stable below 500°C [5,6]. The thermal stability and lateral uniformity of Al/GaAs interface do not satisfy the requirements of SAG process. In principle, chemical stability of metal/GaAs interface can be well accounted for by using thermodynamic data associated with bulk compounds [7,8]. From a thermodynamic point of view, in order to assure a low chemical reactivity between metal and GaAs, one could select metallic compounds with strong metal-metal bond strength. Hence transition metal aluminides, which have higher melting temperatures than pure Al and favorably large heat of formations, are considered as new candidates for SAG process. The W-Al alloy has been successfully applied as self-aligned gate for GaAs MESFETs [9-l 11. Sands et al. [12] studied the NiAl Schottky metallization on GaAs and found that WN,
The high speed integrated circuits based on GaAs metal semiconductor field effect transistors (MESFETs) have been considered as one of the most promising devices for the next generation electronic devices. Many fabrication processes have been proposed for GaAs MESFETs. Among them, the self-aligned gate (SAG) process, which can reduce the unnecessary source, drain resistances and improve device performance, has been extensively employed for enhancement mode digital logic MESFETs. In the SAG process, the gate metal serves as the implantation mask for source and drain regions. The implanted GaAs wafer is then subjected to a high temperature ion activation annealing, either 800-850°C for lo-20 min for furnace annealing or 800-400°C for 5-20 s for rapid thermal annealing. The gate metal must maintain good Schottky characteristics after high temperature annealing. In searching for high temperature stable metallizations for GaAs SAG process, almost all pure metals were found to be not thermally stable on GaAs [l]. * Corresponding author. 0921-5107/97/$17.00Q 1997Elsevier ScienceS.A. All rights reserved. P11s0921-5107(97)00113-x
T.S. Humg,
J.G. Pang / Murerials
Stience
NiAl is more stable than pure Ni and Al, although it is still not stable enough to withstand the post-implantation annealing treatment. Blanpain et al. [13] have investigated the thermal stability of Pt-Al alloy films on GaAs and their results indicate that for the composition region between 50 and 67 at.“? Al, the alloy films fuhill the thermal stability requirements imposed by GaAs SAG technology. Our previous investigations on MO-AI [14,15] and Ta-Al [16] alloy Schottky contacts to fz-GaAs indicated that the MO-AI alloy metallizations with Al compositions between 25 and 73 at.% and the Ta-Al alloys with Al content between 63 and 70 at.O/o were stable after rapid thermal annealing up to 900°C. For these thermally stable Mo-Al/GaAs and Ta-Al/GaAs Schottky contacts, the barrier heights increased with annealing temperature. The barrier height enhancement is beneficial to the GaAs digital logic devices application based on the enhancement mode FETs. Ni, Pd and Pt belong to the same group of metals in the periodic table of elements. The PdAl compound has a large heat of formation (AH= - 92 kJ/g-atom), which is more negative than that of NiAl (AH= - 59 kJ/g-atom) and is similar to that of PtAl (AH= - 100 kJ/g-atom) [17]. Therefore, the Pd-Al alloy films may also possess good stability on GaAs as Pt-Al alloy metallizations. In this paper we report both the metallurgical and the electrical characteristics of electronbeam evaporated Pd-Al alloy contacts to n-GaAs after isochronous rapid thermal annealing at temperatures 500-1000°C. According to the Pd-Al binary phase diagram [18], there exist three intermediate phases with a wide composition range and melting temperature higher than 900°C. Therefore, we have investigated four alloy contacts, namely Pd,,Al,O, Pd48A152, Pd,,Al,, and Pd,*Al,,. The PddOAl,, alloy lies in the 6-Pd,Al, phase regime. The compositions of both Pd,,Al,, and Pd,,Al,, alloys are in the phase regime of /3-PdAl, one is Al-rich while the other is Pd-rich. The PdBBAl,, alloy lies in the p-Pd,Al phase regime.
2. Experimental
procedures
Si-doped (100) GaAs wafers (17= 2.2 x lOI cmA3) were degreased in boiling trichloroethylene, acetone and then etched with H2S04:H,0,:H,0 (5:l:l by volume) to remove surface damage layer and undesirable impurities. A short dip in NH,OH:H,O (1:l) was performed immediately before loading the wafers into the vacuum chamber to further remove native oxide. The thin Pd-Al alloy films were codeposited by a dual electron-gun evaporation system equipped with a cryopump under a base vacuum of 3 x 10 - 7 Torr. The deposition rate of Al was fixed at 0.2 nm s- * and the Pd deposition rate was varied according to the desired
and Engineering
B49 (1997)
144-151
145
composition. The film thickness was about 100 nm. A small piece of oxidized Si wafer was also placed next to the GaAs substrate as a control sample to facilitate the Rutherford back scattering analysis of the film compositions. The compositions of the analyzed alloy films were Pd,,Al,O, Pd,,Al,,, Pd,,Al,, and Pd,,A132. To prepare Schottky diodes, a layer of SiO, about 100 nm was deposited by electron-beam evaporation onto PdAl films as an annealing cap. The samples were annealed in a Heatpulse 410 incoherent lamp rapid thermal annealing furnace at temperatures 500- 1000°C for 20 s under flowing nitrogen atmosphere. After heat treatment, the samples for making Schottky diodes were etched with HF:H,O (1:l) to remove SiO, layer and then further processed by standard photolithography and aqua regia solution etch to make circular contact pads of 500 urn diameter. Backside ohmic contacts of Schottky diodes were formed by evaporation In followed by a 1 min annealing at 400°C. The thermal stability and interface structures of PdAl/GaAs contacts were investigated by X-ray diffraction (XRD), cross-sectional transmission electron microscopy (XTEM) and Auger depth profiling analysis. The electrical resistivity of the film was measured by collinear four-point probe in conjunction with the film thickness measurement using a surface profiler. The electrical properties of the Schottky diodes were characterized by dark current-voltage (I-V) measurement. All I-V measurements were performed by a HP4145B semiconductor parameter analyzer.
3. Results
3.1.1. P&AII,,lGcrAs
contact
Fig. 1 shows the XRD patterns for Pd,,Al,,/GaAs annealed at different temperatures. The as-deposited sample showed the characteristic diffraction pattern of amorphous phase. After 500°C anneal, the amorphous film crystallized into Pd,Al, phase. Pd,Al, was the dominant phase in the film up to 800°C. Additional peaks of /3-PdAl emerged after 900°C anneal. After annealing at lOOO”C, the XRD intensities of Pd,Al, peaks significantly decreased and ,L?-PdAl became the major phase. Although the XRD analysis did not show phases associated with the reaction between the film and GaAs substrate, the Auger depth profiles.- Fig. 2(a), revealed apparent interdiffusion near the interface at 900°C. The &lm also contained some oxygen and a layer of aluminum oxide covered the surface. At lOOO”C, a lot of As outdiffused into the film as shown in Fig. 2(b) and simultaneously more oxygen indiffused to the film. The interfacial diffusion at 900°C seems not to alter the original phases within the film. The XTEM
146
T.S. Hrrang, J.G. Pang/Materials
Science and Engitleering 849 (1997) 144- 151
analysis, Fig. 3, indicated that the grains of Pd,Al, and ,B-PdAl adjoined directly to the substrate and no other new phase was observed. However, the interface was not flat any more. 3.12. Pd~8Alj2/G~As contact The XRD analysis for Pd,,Al,,/GaAs annealed at different temperatures indicated that the P-PdAl phase formed in the as-deposited film and after annealing in the temperature range of 500-lOOO”C, only /?-PdAl existed and its diffraction intensity increased. Other phases associated with interfacial reactions were not detected by XRD analysis. The interfacial stability was further confirmed by Auger depth profiling and XTEM analysis, The Auger depth profiles of Pd,,Al,,/GaAs annealed at 900°C Fig. 4(a), indicated that the interface was still quite sharp. The bright field XTEM image, Fig. 5, shows a flat and smooth interface in 900°C annealed sample. In contrast to Pd,,Al,,/GaAs, the outdiffused As in 1000°C annealed Pd,,Al,,/GaAs accumulated in the interfacial region and the oxygen content in the film was negligible, as shown in Fig. 4(b). The Pd,,Al,,/GaAs contact exhibited the best stability among all samples prepared in this study. 3.1.3. Pd,,Al,,/GaAs contact The results of XRD analysis for Pd,,Al,,/GaAs annealed at various temperatures are shown in Fig. 6. The ,L?-PdAl was detected in the as-deposited sample and the contact interface was stable up to 700°C anneal. The diffraction intensity of (110) P-PdAl peak decreased after annealing above 800°C. Concurrently, a small
Al,o/GaA~, Pd
9OO OC, 20 S
{a’
Ga ,_ / /
I
0 I
5
IO
Pd40Also/GaAs,
15
20
25
1000 ‘C, OC, 20 s
b)
Pd
Ga /
.1=: s t; . ;.p Y .
\
I
1 ‘l’,’
E. 8
.
0
. . . . . .. .*....; . . . . . .. A.1 . . . . . . . . . . . . ._.-.-.-........ 1 I I I I 5 10 15 20 25 30
Sputter
time (min)
Fig. 2. Auger depth profiles for Pd,,Al,O/GaAs 900°C: and (b) 1000°C.
annealed at: (a)
amount of the interfacial reaction product PdGa was observed in 800°C annealed sample and the (210) PdGa diffraction intensity increased with annealing temperature above 800°C. Auger depth profiles, Fig. 7, indicated that Ga outdiffused into the PdAl film at 800°C whereas the amount of As in the film was insignificant.
20
30
40
50
60
70
80
28 (degree) Fig. 1. XRD patterns of Pd,,Al,,/GaAs: (a) as-deposited and annealed at: (b) 800°C; (c) 900°C; and (d) 1000°C.
3.1.4. Pd6,Ai,JGnAs contact Fig. 8 shows the XRD patterns for Pd,,Al,,/GaAs annealed at different temperatures. The as-deposited sample showed the (111) peak of Pd phase only. After 500°C anneal, the Pd,Al and a small amount of PdGa were detected. At 6OO”C, more PdGa was formed and some PdAs, also emerged. The Pd,Al disappeared at 700°C and the dominant phases in the film became
147
PdGa and PdAs,. At 800°C and higher temperatures, PdAs, decomposed and only PdGa was retained after 1000°C anneal.
Pd,,AI,,/GaAs,
900 OC
Pd,,AI,,/GaAs,
1000 OC
3.2. Film resisfivit]
Fig. 9 shows the variation of film resistivities with annealing temperatures. The resistivity of Pd,,Al,,/ GaAs was the lowest among all samples. It decreased from 5.3 to 1.3 1tR cm below 700°C and then increased with temperature. The increase of resistivity above 700°C may be due to the oxidation of the film as revealed in Auger depth profiling analysis. The resistivity of Pd4*A1,, film decreased monotonously with increasing temperature. After 900°C anneal, the film resistivity reduced to 23 I.IR cm. Because the interface of the sample was stable up to 900°C the decrease of resistivity with temperature would be a result of grain growth of the major ,8-PdAl phase in the film as detected in the XRD analysis. After annealing below 800°C the Mm resistivity of Pd,,Al,,/ GaAs exhibited a similar temperature dependence to that of Pd,,Al,,/GaAs. The film resistivity of the as-deposited Pd,,Al,,/GaAs was the highest among all samples. It decreased after annealing below 700°C and varied irregularly above 800°C. The interfacial reactions occurred extensively above 600°C resulting in the formation of several different phases in the reacted films, therefore, the variation of resistivity became irregular.
0
10 20 30 Sputter Time (min)
Fig. 4. Auger depth profiles for Pd,,Al,jGaAs 900°C; and (b) 1000°C.
40 annealed at: (a)
It is worth mentioning that the thermally stable Pd-Al alloy films on GaAs substrate have resistivities smaller than 60 ltQ cm, which is also much smaller than the resistivities of other refractory alloy films on GaAs, e.g. WSi, ( > 100 @2 cm) [2], WN, ( > 70 @2 cm) [3], WAl,Y (50- 100 Q cm) [9] and TaAl, ( b 200 uQ cm) [16]. The low electrical resistivity is one of the important requirements for the contact metals in the semiconductor devices applications. 3.3. Electrical clzaracteristics of Schottlcy diocles
PdAl
Pd2Ah
AlOx
Epoxy
GaAs
Fig. 3. XTEM bright field image and corresponding schematic drawing for Pd,,Al,,/GaAs annealed at 900°C for 20 s.
The typical log I-V curves of Pd,,Al,,/GaAs Schottky diodes are shown in Fig. 10. The forward log I-V curves of all stable diodes can be well analyzed with thermionic emission model [19]. The ideality factor, II
148
T.S. Humg,
J.G. Pung,‘M~rerials
S&we
and Engitzeeritlg
B49 (1997)
144- 151
Pd&I,,/GaAs,
800 “C
Ga
Fig. 5. XTEM brightfield image for Pd,,Al,,/GaAs 900°C for 20 s.
annealed at
and the barrier heights, q&,, extracted from the forward log I-V curves of the stable Pd-Al/GaAs diodes are plotted as a function of annealing temperature in Fig. 11. The Pd,,,Al,,/GaAs diodes annealed below 600°C exhibited good Schottky characteristics with n values smaller than 1.04. It started to degrade after 700°C anneal and the ideality factor increased abruptly. After 900°C anneal, the forward log I-V curve did not show linear region to be analyzed with thermionic emission model for obtaining reliable ideality factor and barrier height. The n values of Pd,,Al,,/GaAs diodes annealed below 500°C were also lower than 1.04. It increased slightly to 1.17 at 700°C and then decreased to 1.09
40
50
60
20 (degree) Fig. 6. XRD patterns of Pd,,Al,,/GaAs tures.
annealed at various tempera-
0
5
10
15
20
25
30
35
40
Sputter Time (min) Fig. 7. Auger depth profiles for Pd,,Al,,/GaAs
annealed at 800°C.
after 900°C anneal. A barrier height of 0.97 V with an ideality factor of 1.09 was obtained for 900°C annealed diode. The excellent thermal stability of Pd,,Al,JGaAs diodes can be correlated to the metallurgical stability of the contacts as revealed in the structure arnlysis.
20
30
40
50
60
70
80
28 (degree) Fig. 8. XRD patterns of Pd,Al,dGaAs: (a) as-deposited and annealed at: (b) 500°C; (c) 600°C; (d) 7OO“C; and Ie) 800°C.
T.S.
Huang,
J.G.
Pang
/Materials
Science
and
Engirrreriag
120rS-
~100 I I F180 I c 2 6 z I$
B49
(1997)
(a )
’
'I
Pd68A'32
l
A
Pd52A'48
n
'
Pd48A'52
l
Pd4d'60
J
144-151
’
’
Pd,&JGaAs Pd,,Al,dGaAs Pd,$l,IGaAs
A
I
::
/
60 40
5 400
500
600
I 500
I
I
I
I
600
700
800
900
700
800
900
..”
A
200 t:: 27
2
500 600 700 800 TEMPERATURE ( “C )
900
The Pd,,Al,,/GaAs diodes retained their low H values smaller than 1.1 after annealing below 600°C. Above 6OO”C, the diodes significantly degraded. Although the annealed Pd,,Al,,/GaAs diodes exhibited rectifying characteristics in the linear I-V plot, all diodes were leaky and their log I-V curves did not show linear region for obtaining reliable ideality factor and barrier height.
_ (a)
400oc
(e)
(b)
500~~
(f) 9oo”c
(c)
600°C
(d)
700"~
10-I
-1
a
o-3
I
-
800
I
I
'C
5jfT%F‘VOLT, Fig. 10. The log I-V curves of Pd,,Ai,,/GaAs annealed at various temperatures.
0.9
$
Fig. 9. Variation of film resistivity as a function of annealing temperature.
I
> v
Schottky diodes
i?
0.8
Y 5
0.7
a 2
0.6
a 0.5 1 400
TEMPERATURE ( ’ C ) Fig. 11. Variations of the ideality factors and the barrier heights of the Pd-Al/n-GaAs Schottky diodes as a function of annealing temperature.
4. Discussion It is well established that thin Pd film reacts readily with GaAs substrate, even during deposition by electron-gun evaporation [1,20,21]. Final products of interfacial reactions after annealing in a closed system are PdGa and PdAs, and a layered structure of PdAs,/ PdGa/GaAs is reported. If annealing is carried out in a vacuum system or an open system, PdAs, decomposes at high temperatures and only PdGa is retained. On the other hand, Al/GaAs contact is relatively more inert [5>6]. In this study, we found that the thin Pd-Al alloy films are more stable than pure Pd films on GaAs and particularly Al-rich films exhibited superior thermal stability. The Pd-rich Pd68Al,, alloy lies in the composition range of p-Pd,Al phase. In contrast to Pd/GaAs contact, interfacial reactions were not observed in the as-deposited Pd,,Al,,/GaAs sample, however? Pd,Al compound phase still exhibited poor thermal stability on GaAs after annealing treatment. Phase formation sequences of interfacial reactions in annealed Pd,,Al,,/ GaAs were found to be similar to that of PdiGaAs.
150
T.S. Huwg,
J.G. Pang/Materials
Science
The compositions of Pd,,A&* and Pd48A1,, alloys are both within ,B-PdAl phase region. It is interesting that these two alloy films exhibited quite different stabilities on GaAs. The Pd-rich Pd,,AI,, films reacted with GaAs at 800°C whereas the Al-rich Pdd8Al,, films were stable on GaAs up to 1000°C anneal. It is well known that both the physical and the chemical properties of a compound phase with a large composition range are usually dependent on the stoichiometry within the phase region. The Al activity in the ,B-PdAI compound phase increased with increasing Al content and varied abruptly across the stoichiometric composition 122,231, we therefore speculate that the compositional dependence of the thermal stabilities of these two P-phase PdAl alloy films is caused by significant difference of AI (and/or Pd) activity in the Pd- and the Al-rich /3-PdAI. The higher Al activity (lower Pd activity) of Pd,,AIS, than that of Pd,,Al,, ensures its better interfacial stability on GaAs. The Pd,,Al,O alloy has a composition within 6Pd2Al, phase region, The chemical stability of the Pd,,Al,,/GaAs interface was also good and no apparent interfacial reaction was detected up to 1000°C anneal. However, interfacial diffusion was prominent resulting in a rough interface, as shown in Fig. 3. Therefore, the Schottky diodes started to degrade with increased ideality factor after annealing above 700°C. The large current, which might be due to the recombination component, appeared in the small forward bias voltage region, Fig. 10, for 800°C annealed diode. After 900°C anneal, the diode was leaky owing to rough interface. Besides the difference in the compositional dependence of thermal stabilities of the Pd-Al alloy contacts to GaAs, the behaviors of the interfacial diffusions and the oxidation at high temperatures were also found to be compositional dependent. The outer surface of Pd,,AI,, film, which has the largest Al content, oxidized and some oxygen diffused into the film after annealing above 800°C Fig. 2. The oxidation of Pd,,Al, film was very limited and nearly no oxygen diffused into the film up to 1000°C anneal, Fig. 4. In the mean time, the As outdiffused into the Al-rich Pd-Al alloy films and accumulated at the interface as shown in the Auger depth profiles of 1000°C annealed Pd,,Al,,/GaAs and Pd,,Al,,/GaAs contacts. On the contrary, the Ga outdiffused into the Pd-rich Pd,,Al,, film and distributed relatively uniformly in the film, Fig. 7. Since the Al-rich Pd-Al alloy film has higher Al activity, it is more easily oxidized and may attract As from the GaAs substrate during high temperature anneal, whereas the higher activity of Pd in the Pd-rich Pd-Al alloy film results in the Pd-Ga attraction. Furthermore, because the Ga diffusivity has been found to be higher than the As diffusivity in most metal films on the GaAs substrate [I], the outdiffused Ga would distribute more uniformly
and Engineering
349
(1997)
144-151
in the Pd-Al film, while the outdiffused As in the Pd-AI film might accumulate at the interface region for a rapid thermal annealing process. For all stable Pd-Al/GaAs diodes, the barrier heights of both Pd,,Al,,/GaAs and Pd,,Al,JGaAs Schottky diodes increased with annealing temperature, whereas the barrier heights of Pd,,Al,,/GaAs diodes decreased with annealing temperature. The increase of the barrier height after high temperature annealing has also been observed in NiAl/GaAs [12], Mo-Al/GaAs [14,1.5] and Ta-Al/GaAs [16] Schottky diodes. With the same Fermi level pinning position at the metal/semiconductor interface, because the band gap of Al,Ga, -,As is larger than that of GaAs, the Schottky barrier height of metal/Al,Ga, _ ,As/GaAs diode should be larger. The Schottky barrier height of Mo/Al,Ga, -,As has also been experimentally verified to be larger than that of Mo/GaAs diode [24]. The interfacial Al,YGa, -,YAs layer, which was formed epitaxially on GaAs, has been identified by high resolution XTEM analysis [23]. Therefore, the barrier height enhancement should be mainly due to the formation of Al,YGa, _ .As layer at the interface between the Al-rich Pd-Al alloy contacts to n-GaAs.
5. Conclusions
The most important results of this study are summarized as follows: (1) The Al-rich Pd,OAl,,/GaAs and Pd,,Al,JGaAs contacts are chemically stable up to 1000°C. However, interfacial diffusion is prominent in Pd,,A160/GaAs contact after annealing above 800°C. (2) Interfacial reactions are detected in the Pd-rich Pd,,Al,,/GaAs and Pd,,Al,,/GaAs contacts after annealing above 800 and 500°C respectively. The phase formations of interfacial reactions in these two contacts are similar to that of Pd/GaAs contact. (3) The compositional dependence of the interfacial stability of Pd-AI alloy contacts on GaAs can be correlated to the variation of the activities of Pd and Al in these alloys. (4) For all thermally stable Pd-Al/n-GaAs Schottky diodes, the barrier heights of Al-rich Pd,,Al,,/n-GaAs and Pd4pAl,Jn -GaAs diodes increased whereas the barrier heights of Pd-rich Pd,,Al,,/n-GaAs diodes decreased with annealing temperature. The Pd,,Al&GaAs diodes were thermally unstable and became leaky after annealing. (5) The promising metallurgical, electrical stabilities and low film resistivities of the Al-rich Pd-Al alloy contacts to n-GaAs indicate that these alloys may offer an alternative as improved gate metallization in selfaligned gate metal-semiconductor field effect transistor technologies.
T.S.
Huang,
J.G.
Pang/Materials
Science
Acknowledgements This work was supported by the Republic of China National Science Council through Grant No. NSC 86-2215-E-007-007.
References [l] T. Sands, Mater. Sci. Eng. Bl (1989) 289. [2] A.G. Lahav, C.S. Wu, F.A. Baiocchi, J. Vat. Sci. Technol. B6 (1988) 1785. [3] K.M. Yu, J.M. Jaklevic, E.E. Haller, SK. Cheung, S.P. Kwok, J. Appl. Phys. 64 (1988) 1284. [4] T.E. Haynes, W.K. Chu, C.C. Han, S.S. Lau, S.T. Picraus, Appl. Phys. Lett. 53 (1988) 2200. [5] M. Missous, E.H. Rhoderick, K.E. Singer, J. Appl. Phys. 59 (1986) 3189. [6] N.M. Johnson, T.J. Magee, J. Peng, J. Vat. Sci. Technol. 13 (1976) 838. [7] S.S. Lau, W.X. Chen, E.D. Marshal, C.S. Pai, W.F. Tseng, T.F. Kuech, Appl. Phys. Lett. 47 (1985) 1298. [8] R. Beyers, K.B. Kim, R. Sinclair, J. Appl. Phys. 61 (1987) 2195. [9] H. Nakamura, Y. Sano, T. Nonaka, T. Ishida, K. Kaminishi, IEEE GaAs IC Symp. Tech. Dig. IEEE, New Jersey, 1983, pp. 134.
and
Engineering
B49
(1997)
144-151
151
UOI S.P. Kwok, J. Vat. Sci. Technol. B4 (1986) 1383. [ill Y. Sekino, T. Kimura, K. Inokuchi, Y. Sano, M. Sakuta, Jpn. J. Appl. Phys. 27 (1988) L2183.
tw T. Sands, W.K. Chan, CC. Chang, E.W. Chase, V.K. Keramidas, Appl. Phys. Lett. 52 (1988) 1388. B. Blanpain, G.D. Wilk, J.O. Olowolafe, J.W. Mayer, Appl. Phys. Lett. 57 (1990) 392. [I41 T.S. Huang, J.G. Peng, C.C. Lin, Appl. Phys. Lett. 61 (1992) 1131
3017.
T.S. Huang, J.G. Peng, C.C. Lin, J. Vat. Sci. Technol. Bll (1993) 756. 1161T.S. Huang, SM. Jean, J. Appl. Phys. 75 (1994) 7519. s71 R. Hultgren, P.D. Desai, D.T. Hawkins, M. Gleiser, K.K. Kelley, Selected Values of the Thermodynamic Properties of Binary Alloys, American Physical Society, Metal Park, OH, 1973. [I81 A.J. McAlister, Bull. Alloy Phase Diagr. 7 (1986) 368. u91 E.H. Rhoderick, R.H. Williams, Metal-Semiconductor Contacts, 2nd ed. Clarendon Press, Oxford, 1988, pp. 38. PO1 J.O. Olowolafe, P.S. Ho, H.J. Hovel, J.E. Lewis, J.M. Woodall, J. Appl. Phys. 50 (1979) 955. VI T.S. Kuan, J.L. Freeouf, P.E. Batson, E.L. Wilkie, J. Appl. Phys. 58 (1985) 1519. WI M. Ettenberg, K.L. Komarek, E. Miller, Metall. Trans. 2 (1971) 1173. ~231 T.S. Huang, J.G. Pang, J. Appl. Phys. 78 (1995) 5739. 1241 M. Eizenberg, M. Heiblum, M.I. Nathan, N. Braslau, P. Mooney, J. Appl. Phys. 61 (1987) 1516. 1151