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The comparative analysis of S and Se in an (NH4)2(S,Se)1.08-treated GaAs (100) surface
ELSEVIER
Surface and Coatings Technology 100-101 ( 1998) 222-228
The comparative analysis of S and Se in an -treated GaAs ( 100) surface wH,),@~w1.0, Seung-Hoon
Sa a, Min-Gu
Kang a, Hyung-Ho
Park a,*, Kyung-Hui
Oh b
a Department of Ceramic Engineering. Yonsei Unioersity, 134 Shinchon-dong, Seodaemtm-krt, Seoul 120-749. South Korea b National Institute of Tech. and Quality, Jutlang-dorlg 2, Kwachun. Kyungki-do 427-011, South Korea
Abstract A GaAs surface was passivated simultaneously with S and Se using an optimized (NH&(S.Se),,,, solution. X-ray photoelectron spectroscopy was used to analyze the surface composition and bonding states after several treatments of GaAs. It was revealed that the passivated surface mainly contained As-(&Se) bonds. The passivating effect between S and Se could be compared by exposure of the passivated surface to air. During the exposure, the degradation of Se-bond was found to be faster than that of S-bond. However, during in situ annealing under ultra high VdCUUUI conditions, a decrease of S/Se ratio with the temperature was observed. It was revealed that during the anneal, the bond exchange from As-(S,Se) to Ga-(S.Se) occured. Also, high vaporization pressure of S induced the decrease of S content. Because of this evaporation and bonds exchanging reaction, the apparent thermal stability of Se(Ga,As) was found to be higher than that of S-(Ga,As). 0 1998 Published by Elsevier Science S.A. Keywords:
GaAs; (NH&(
&Se),,,,
solution;
Passivation
effect; X-ray photoelectron
1. Introduction The GaAs surfaces have been studied for many years in order to investigate a passivation effect which could reduce the surface state. The GaAs surfaces are known to be plagued by high density of surface state which causes the pinning of surface Fermi level within the band gap of semiconductor and is detrimental to the performance of microelectronic and optoelectronic devices [ 11. To overcome this problem, several attempts have been made in recent years [2-41 to passivate the GaAs surface by sulfide treatment with Na,S, (NH&S, and (NH&S, solutions. Among them, the sulfidation with (NH,),& solution has shown to be most effective in reducing surface state density and surface recombination velocity. Lee et al. [5] reported that with (NH&&-treatment, breakdown characteristic of GaAs power MESFET, could be improved. Passivation with Se gas flux, Na,Se and the other solutions containing Se, has also shown to be a promising one. Selenium-treated GaAs has demonstrated dramatic photoluminescence gain and stability compared with photo-oxidation of the sulfide compartments [6* Corresponding author. Tel: 82-2-361-2853; Fax: 82-2-365-5882; e-mail: [email protected]
81. However, the effects of S- or Se-passivation are being explored. Up to now, very little work has been focused on how they differ from their passivation effect. In this study, we compared the passivation effect of S and Se on GaAs surface using the combined solution of both elements. The chemical bonding state and the stability of (S,Se)-treated GaAs using (NH&( S,Se)l.,, aqueous solution was investigated by X-ray photoelectron spectroscopy (XPS). The difference in passivation effect of (&Se) could be examined through monitoring the changes of surface composition and bonding characteristics through air exposure or in situ anneal treatment under ultra high vacuum (UHV) condition.
2. Experimental procedures The samplesused in this study were undoped GaAs (100) substrate. The wafers were degreasedby immersing in boiling acetone for 10 min, in methanol for 5 min, in deionized water (DIW) for 1 min, and successively dried by flow nitrogen. The wafers were then cleaned by HCl-DIW treatment. After the cleaning, (S,Se) was passivated on GaAs surface using the combined solution
0257-8972/98/$19.00 0 1998 Published by Elsevier Science B.V. All rights reserved. PII
SO257-8972(
97)00618-X
spectroscopy
S.-H.
Sa et al. / Surjke
and Coatings
of both S and Se, which was prepared by adding excess 8% of Se powder (Aldrich) in (NH&S solution at 60 “C. A desiccator maintaining constant humidity was adopted to investigate the oxidation behavior of (&Se) passivated surface in air. To observe the thermal stability of (S,Se), the passivated surface was also annealed from 150 to 550 “C with a step of 50 “C in UHV condition. The XPS measurement was carried out to characterize the surface state of GaAs after each treatment. The excitation was accomplished using monochromated Al kcc radiation source on VG Scientific ESCALAB220i-XL, giving an overall resolution of 0.47 eV for the Ag 3d,,, line. Narrow scan spectra of all
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regions of interest were recorded with 20 eV passenergy in order to identify the bonding states of each element. A take-off angle of 30” between sample surface and detector was mainly used to investigate near the surface.
3. Resultsand discussion (&Se)-passivated GaAs surface was investigated just after the treatment with (NH,),(S,Se),.Os solution by XPS analysis under the vacuum of 2 x 10-i’ Torr. Fig. 1 corresponds to Ga 3d, As 3d, S 2p, and Se 3d core level spectra obtained at 30” of take-off angle. In Ga 3d core
GaAs
Se-As
17
15
19
Binding
21
energy
23
after treatment
(eV)
As-Ga
(b) As 3d As-(S,Se) 1
37
39
43
41
Binding
energy
45
(eV)
before treatment
Ga 3s 50
I
I
I
I
53
56
59
62
Binding
I
153
I
I
156
Binding Fig. 1. XPS spectra
159
energy
I
162
energy
(eV)
I
165
(eV)
of (a) Ga 3d; (b) As 3d: (c) S 2p, and (d) Se 3d from
GaAs
surface just after
(NH,),(S,Se),,,,-treatment.
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level spectrum, due to the spin-orbital coupling, Ga 3d peak is composed of two orbital energy levels as 3d,,, and 3d,,, with binding energies of 18.80 and 19.25 eV which correspond to the bonding state of Ga-As. No other bonding state was observed. However, As shows two kinds of bonding states; one is AssGa bond and the other is As-( S,Se) bond at 42.8 eV of binding energy. As-S and As-Se bonds show very similar binding energy due to the similar chemical nature of S and Se. For example, electronegativities of S and Se are 2.5 and 2.4, respectively. For simplicity, spin-orbital coupling states of 3d,,, and 3d,,, for As-( S,Se) bond were not resolved. The observation of (We) bond with only As was predictable, because As-rich surface with elemental As was obtained after wet cleaning with acid-based solution and only S-As bond was observed during the sulfidation treatment using (NH&S in our early study [ 111. Also in this experiment, only this elemental As, formed after wet cleaning of GaAs, seems to participate in the bonding with (SSe). However, the quantity of As which bound to (S,Se) was less than that of elemental As formed during wet cleaning. In S 2p narrow scan region (Fig. lc), S 2p peak of S-As bond was observed at
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161.5 eV in overlapping with Ga 3s peak of 159.5 eV and As plasmon loss peak of 156.5 eV binding energy. Fig. Id corresponds to the spectra of Se 3d binding energy region before and after the passivation. This also confirms that (S,Se) binds with only excess As of elemental state and this binding layer corresponds to the passivating layer of GaAs surface. Se 3d spectrum after the passivation treatment only shows one chemical bonding state as same as S and this corresponds to AsSe bond of 54.3 eV. Sandroff et al. [7] reported that Se could easily present several valence states ranging from -2 to +4. However, in this experiment, Se shows only one valence state of -2. This means that all of S and Se observed in GaAs surface constitute the passivating layer as anions, negatively charged, and repel oxygen effectively for preventing the passivated GaAs surface from oxidation in air. To evaluate the oxidation behavior of (S,Se)-passivated surface, the treated sample was exposed to air for a certain interval of time in a desiccator under controlled humidity, Fig. 2 represents the spectra of Ga 3d, As 3d, S 2p, and Se 3d, obtained after exposure of passivated GaAs to air for 2 weeks. In Ga 3d and As 3d peaks,
GaAs
GaAs
(4 s 2p
(4 Ga 3d Ga-0
OP
J
16
16 Binding
20
22
energy
24
1.53
159
156
(eV)
162
Binding energy
165
(eV)
As-Ga Se-As I
I
39
41
Binding Fig. 2. XPS spectra
I
I
I
43
45
47
energy
I
I
I
I
50
53
56
59
(eV)
of (a) Ga 3d; (b) As 3d: (c) S 2p. and (d) Se 3d obtained
Binding after exposure
of ( NH,)2(
energy
I
62
(eV)
S,Se),.,,-treated
C&As surface
to air for 2 we&s.
S.-H.
Sa et al. / Surface
and Coatings
Technology
the additional bonding states as Ga-0, As-O, and especially elemental As with higher binding energies than Ga-As were observed. In S 2p and Se 3d peaks region, a slight decrease of S and Se was observed. From the above results, the oxidation of passivated GaAs surface can be explained as follows. As-(&Se) bond seems to cover partial surface area of GaAs because (&Se) binds with elemental state of As. The nonpassivated GaAs surface area reacts with oxygen to form Ga-0 and this induced the formation of elemental As. The elemental As successively reacts with oxygen but partially remains as elemental state. The oxidation may proceed to the passivated surface area and induce the decrease of passivating element such as S and Se. This is why the decrease of S and Se could be observed only after 2 days’ exposure as shown in Fig. 3. Fig. 3a and b represent the changes of surface composition and the ratio of S to Se according to air exposure time, respectively. In Fig. 3a, due to gradual increase of 0, a relative decrease of the other elements was clearly seen. In Fig. 3b, S/Se ratio varied from 1.777 to 2.092 with air exposure time. This degradation rate of Se is larger
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than that of Sandroff et al. [7], who reported that S-passivated surface degraded faster than Se-passivated surface during the exposure to air and Se-passivated surface was more stable than S-passivated one against photo-oxidation. This is in contradiction with our result. But the formation of the passivation layer depends on the quantity of generated elemental states of As and Ga during wet cleaning [ 121, which could be easily varied on every wet cleaning step. This can cause different passivating surface coverages between S- and Se-treated samples. From this, it can be concluded that direct comparison of the passivating effect between S and Se samples is not possible. using two independent Concerning our result, the difference in bond strength between S or Se and GaAs may explain why As-Se bond degrades faster than As-S bond. Bond strength of As-S is 379.5 +6.3 kJ/mol and larger than that of As-Se, 96.0 kJ/mol [9]. Thermal stabilty of (NH&( S,Se),.O,-treated GaAs was examined by in situ anneal in UHV-XPS chamber. Fig. 4 shows the spectra of Ga 3d, As 3d, S 2p, and Se 3d after in situ anneal at 200 “C. The decrease and
al tij
t
30--uGa -o--As -o0
z B
.zs
-a--
20 -
s
-X-Q
g
__z____c_o
.
E HV 8
lo-
Ed------.-* x-x-x.--x 0
I 0
X *
I.
1
2
4
I
I. 6
Air exposure
I
I.
X 1
12
14
Il.
6
10
time (day)
2.5 w A
LO&
'. 0
'. 2
'. 4
'. 6
Air exposure Fig. 3. The variations
of (a) composition
and (b) S/Se ratio
" 6
" 10
--A--
S/Se
' 12
"
-
.
14
time (day)
of (NH,),(S.Se)l.O,-treated
GaAs
surface
with air exposure
time.
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ad
Coatings
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Ga-As Ga 3s
-1
1-1
I
15
17 Binding
19 energy
21
23
153
159
Binding
(eV)
I
I
162
165
I
I
166
energy
(eV)
As-Ga Se-(Ga, As)
u
4
37
39 Binding
Fig. 4. XPS spectra
41 energy
43
45
50
(eV)
of (a) Ga 3d; (b) As 3d; (c) S Zp, and (d) Se 3d obtained
increase of peak intensifies at 42.8 and 19.7 eV of binding energies, observed in As 3d and Ga 3d spectra, respectively. This means that the passivating agents, S and Se, changed the bonds from As to Ga. It can be explained by thermodynamic stability of each bond. Heat of formation (or bond energy) of Ga-S (Ga,S,) and Ga-Se bonds (Ga,Se,) are - 122.8 + 3.0 and -97.0 +4.0 kcal/ mol, respectively, and these values are negatively larger than those of As-S (As,&) and As-Se bonds ( AS,Se3) as -40.0 f 5.0 and -24.5i4.0 kcal/mol [lo]. As pointed out above, As-( S,Se) bonds were formed during the passivation treatment. Even though Gaa( &Se) bonds are more stable than As-(S,Se) bonds, Ga-(S,Se) bond is not formed because GaAs does not dissociate to form Ga-(We) bond. However, with a higher temperature, S and Se may effectively react with GaAs to form the bonds with Ga. In Fig. 4b, the appearance of elemental As state is clearly seen due to the dissociation of As-(&Se) bonds. Furthermore, in Fig. 4c and d, broadening of S 2p and Se 3d peaks was slightly observed due to the formation of the bonds with Ga and As. Normally, S-Ga and S-As bonds show a difference in binding energy of about 1 eV [4]. Especially in Fig. 4c,
53
Binding after in-situ
anneal
56
energy
59
62
(eV)
of (NH,)?(S.Se),
OR-treated
GaAs
surface
at 200 “C.
the peak binding energy of S 2p was slightly moved to low binding energy. Fig. 5a and b represent the variation of the surface composition and the ratio of S to Se according to in situ anneal temperature, respectively. The contents of S, Se and 0 decreased, while that of Ga in surface relatively increased. After anneal at 300 C, As-rich surface, just after the treatment, was changed to Ga-rich surface due to the high volatility of As. In Fig. 5b, S/Se ratio decreased from 3.716 to 1.937, and rapid decrease of the ratio was observed after anneal at 200 “C. By the anneal at 200 C, S and Se break the bonds with As to bind with Ga. During this exchange period, the vaporization of S happened easily, due to the high vaporization pressure of S (about 200 times larger than Se at a given temperature) [9]. After the anneal at 200 “C, even though the anneal temperature was higher, the ratio of S/Se slightly decreased. But with the anneal at 550 “C, a decrease of S and Se was observed, as shown in Fig. 5a, due to the thermal decomposition of (S,Se))Ga bonds. From the above analysis, it can be stated that the apparent thermal stability of Se-( As,Ga) bond is higher than that of S-(As,Ga) bond. However, it should
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and
Coatings Technology
100-101
(1998)
222-228
1
I
I
I
I
I 0
, 100
t 200
I 300
I
I
400
500
221
I
-
600
Anneal temperature (C) 4.5 r (b)
::;
,
i
\
--A--
S/Se
.o H
3.0 -
aB
2.5 -20 1.5 1.0. -
\ A--&---
’ 0
100
\-
200
300
A-----A
400
-
500
-1 600
Anneal temperature (c) Fig. 5. The variations of (a) composition and (b) S/Se ratio of (NH,),(S,Se),,,,-treated
be remembered that this apparent low thermal stability of S compared with Se resulted from the high volatility of S during the bonds exchanging reaction.
4. Conclusions XPS analyses of an (NH,),(S,Se),,,,-treated GaAs surface showed that the passivation layer mainly contains As-(&Se) bonds. During the air exposure of (S,Se)passivated GaAs, the degradation of the Se-As bond was observed to be explained by larger bond strength of the S-As bond, 379.5 + 6.3 kJ/mol than that of Se-As bond, 96.0 kJ/mol. An exchange of the bonds from As-(&Se) to Ga-(‘&Se) was found at 200 “C anneal under UHV condition. Large heat of formation of Ga-(&Se) bonds seemed to cause the dissociation of As-(&Se) bonds to react with GaAs and form Ga-(&Se) bonds. Decrease of S content was observed during this exchanging reaction due to the high vaporization pressure of S. Then, even though the bond energy (heat of formation) of S-(As,Ga) is larger than that of
GaAs surface with in-situ anneal temperature.
Se-(As,Ga), Se-( As,Ga) bonds show apparently high thermal stability compared with S-( As,Ga) bonds.
Acknowledgement This research was performed under the auspices of KOSEF (contract no. 961-0804-034-2).
References [ I] W.E. Spicer, 2. L-Weber, N. Newman, T. Kendelewieg, R. Cao, C. McCant, P. Mahowald, K. Miyano, I. Lindau, J. Vat. Sci. Technol. B 6 (1988) 1245. [2] C.J. Spindt, W.E. Spicer, Appl. Phys. Lett. 55 (1985) 1653. [ 31 C.J. Sandroff, M.S. Hegde, CC. Chang, J. Vat. Sci. Technol. B 7 (1989)841.
K. Sato, M. Sakata, H. Ikoma, Jap. 3. Appl. Phys. 32 (1993) 3354. [S] J.L. Lee, D. Kim, S.J. Maeng, H.H. Park, J.Y. Kang, Y.T. Lee, J. Appl. Phys. 54 (1983) 2533. [6] T. Scimeca, Y. Watanabe, F. Maeda, R. Berrigan, M. Oshima, Appl. Phys. Lett. 58 (1991) 1167. [4]
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and Coatings
[7] C.J. Sandroff, MS. Hegde, L.A. Farrow, R. Bhat. J.P. Harbison, C.C. Chang, J. Appl. Phys. 54 ( 1983) 2533. [8] B.A. Kurubilla. S.V. Ghaisas, A. Datta, S. Banerjee. S.K. Kulkami, J. Appl. Phys. 73 ( 1993) 4384. [9] D.R. Ride, J.A. Kerr, Handbook of Chemistry and Physics, 76th ed., 1996, pp. 9-51.
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[IO] 0. Kubaschwski, C.B. Alcock, MetahurgicdJ Thermochemistry. 5th ed.. 1979, pp. 2617323. [I I ] M.G. Kang. H.H. Park. K.S. Suh, J.L. Lee, Thin Solid Films 290 (1996)
328.
[ 121 M.G.
Kang.
H.H.
Park,
submitted
to J. Vat. Sci. Technol.
A.
Surface and Coatings Technology 100-101 ( 1998) 222-228
The comparative analysis of S and Se in an -treated GaAs ( 100) surface wH,),@~w1.0, Seung-Hoon
Sa a, Min-Gu
Kang a, Hyung-Ho
Park a,*, Kyung-Hui
Oh b
a Department of Ceramic Engineering. Yonsei Unioersity, 134 Shinchon-dong, Seodaemtm-krt, Seoul 120-749. South Korea b National Institute of Tech. and Quality, Jutlang-dorlg 2, Kwachun. Kyungki-do 427-011, South Korea
Abstract A GaAs surface was passivated simultaneously with S and Se using an optimized (NH&(S.Se),,,, solution. X-ray photoelectron spectroscopy was used to analyze the surface composition and bonding states after several treatments of GaAs. It was revealed that the passivated surface mainly contained As-(&Se) bonds. The passivating effect between S and Se could be compared by exposure of the passivated surface to air. During the exposure, the degradation of Se-bond was found to be faster than that of S-bond. However, during in situ annealing under ultra high VdCUUUI conditions, a decrease of S/Se ratio with the temperature was observed. It was revealed that during the anneal, the bond exchange from As-(S,Se) to Ga-(S.Se) occured. Also, high vaporization pressure of S induced the decrease of S content. Because of this evaporation and bonds exchanging reaction, the apparent thermal stability of Se(Ga,As) was found to be higher than that of S-(Ga,As). 0 1998 Published by Elsevier Science S.A. Keywords:
GaAs; (NH&(
&Se),,,,
solution;
Passivation
effect; X-ray photoelectron
1. Introduction The GaAs surfaces have been studied for many years in order to investigate a passivation effect which could reduce the surface state. The GaAs surfaces are known to be plagued by high density of surface state which causes the pinning of surface Fermi level within the band gap of semiconductor and is detrimental to the performance of microelectronic and optoelectronic devices [ 11. To overcome this problem, several attempts have been made in recent years [2-41 to passivate the GaAs surface by sulfide treatment with Na,S, (NH&S, and (NH&S, solutions. Among them, the sulfidation with (NH,),& solution has shown to be most effective in reducing surface state density and surface recombination velocity. Lee et al. [5] reported that with (NH&&-treatment, breakdown characteristic of GaAs power MESFET, could be improved. Passivation with Se gas flux, Na,Se and the other solutions containing Se, has also shown to be a promising one. Selenium-treated GaAs has demonstrated dramatic photoluminescence gain and stability compared with photo-oxidation of the sulfide compartments [6* Corresponding author. Tel: 82-2-361-2853; Fax: 82-2-365-5882; e-mail: [email protected]
81. However, the effects of S- or Se-passivation are being explored. Up to now, very little work has been focused on how they differ from their passivation effect. In this study, we compared the passivation effect of S and Se on GaAs surface using the combined solution of both elements. The chemical bonding state and the stability of (S,Se)-treated GaAs using (NH&( S,Se)l.,, aqueous solution was investigated by X-ray photoelectron spectroscopy (XPS). The difference in passivation effect of (&Se) could be examined through monitoring the changes of surface composition and bonding characteristics through air exposure or in situ anneal treatment under ultra high vacuum (UHV) condition.
2. Experimental procedures The samplesused in this study were undoped GaAs (100) substrate. The wafers were degreasedby immersing in boiling acetone for 10 min, in methanol for 5 min, in deionized water (DIW) for 1 min, and successively dried by flow nitrogen. The wafers were then cleaned by HCl-DIW treatment. After the cleaning, (S,Se) was passivated on GaAs surface using the combined solution
0257-8972/98/$19.00 0 1998 Published by Elsevier Science B.V. All rights reserved. PII
SO257-8972(
97)00618-X
spectroscopy
S.-H.
Sa et al. / Surjke
and Coatings
of both S and Se, which was prepared by adding excess 8% of Se powder (Aldrich) in (NH&S solution at 60 “C. A desiccator maintaining constant humidity was adopted to investigate the oxidation behavior of (&Se) passivated surface in air. To observe the thermal stability of (S,Se), the passivated surface was also annealed from 150 to 550 “C with a step of 50 “C in UHV condition. The XPS measurement was carried out to characterize the surface state of GaAs after each treatment. The excitation was accomplished using monochromated Al kcc radiation source on VG Scientific ESCALAB220i-XL, giving an overall resolution of 0.47 eV for the Ag 3d,,, line. Narrow scan spectra of all
Technology
100~101
(1998)
223
222-228
regions of interest were recorded with 20 eV passenergy in order to identify the bonding states of each element. A take-off angle of 30” between sample surface and detector was mainly used to investigate near the surface.
3. Resultsand discussion (&Se)-passivated GaAs surface was investigated just after the treatment with (NH,),(S,Se),.Os solution by XPS analysis under the vacuum of 2 x 10-i’ Torr. Fig. 1 corresponds to Ga 3d, As 3d, S 2p, and Se 3d core level spectra obtained at 30” of take-off angle. In Ga 3d core
GaAs
Se-As
17
15
19
Binding
21
energy
23
after treatment
(eV)
As-Ga
(b) As 3d As-(S,Se) 1
37
39
43
41
Binding
energy
45
(eV)
before treatment
Ga 3s 50
I
I
I
I
53
56
59
62
Binding
I
153
I
I
156
Binding Fig. 1. XPS spectra
159
energy
I
162
energy
(eV)
I
165
(eV)
of (a) Ga 3d; (b) As 3d: (c) S 2p, and (d) Se 3d from
GaAs
surface just after
(NH,),(S,Se),,,,-treatment.
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Sa et al. 1 Swfhce
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level spectrum, due to the spin-orbital coupling, Ga 3d peak is composed of two orbital energy levels as 3d,,, and 3d,,, with binding energies of 18.80 and 19.25 eV which correspond to the bonding state of Ga-As. No other bonding state was observed. However, As shows two kinds of bonding states; one is AssGa bond and the other is As-( S,Se) bond at 42.8 eV of binding energy. As-S and As-Se bonds show very similar binding energy due to the similar chemical nature of S and Se. For example, electronegativities of S and Se are 2.5 and 2.4, respectively. For simplicity, spin-orbital coupling states of 3d,,, and 3d,,, for As-( S,Se) bond were not resolved. The observation of (We) bond with only As was predictable, because As-rich surface with elemental As was obtained after wet cleaning with acid-based solution and only S-As bond was observed during the sulfidation treatment using (NH&S in our early study [ 111. Also in this experiment, only this elemental As, formed after wet cleaning of GaAs, seems to participate in the bonding with (SSe). However, the quantity of As which bound to (S,Se) was less than that of elemental As formed during wet cleaning. In S 2p narrow scan region (Fig. lc), S 2p peak of S-As bond was observed at
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161.5 eV in overlapping with Ga 3s peak of 159.5 eV and As plasmon loss peak of 156.5 eV binding energy. Fig. Id corresponds to the spectra of Se 3d binding energy region before and after the passivation. This also confirms that (S,Se) binds with only excess As of elemental state and this binding layer corresponds to the passivating layer of GaAs surface. Se 3d spectrum after the passivation treatment only shows one chemical bonding state as same as S and this corresponds to AsSe bond of 54.3 eV. Sandroff et al. [7] reported that Se could easily present several valence states ranging from -2 to +4. However, in this experiment, Se shows only one valence state of -2. This means that all of S and Se observed in GaAs surface constitute the passivating layer as anions, negatively charged, and repel oxygen effectively for preventing the passivated GaAs surface from oxidation in air. To evaluate the oxidation behavior of (S,Se)-passivated surface, the treated sample was exposed to air for a certain interval of time in a desiccator under controlled humidity, Fig. 2 represents the spectra of Ga 3d, As 3d, S 2p, and Se 3d, obtained after exposure of passivated GaAs to air for 2 weeks. In Ga 3d and As 3d peaks,
GaAs
GaAs
(4 s 2p
(4 Ga 3d Ga-0
OP
J
16
16 Binding
20
22
energy
24
1.53
159
156
(eV)
162
Binding energy
165
(eV)
As-Ga Se-As I
I
39
41
Binding Fig. 2. XPS spectra
I
I
I
43
45
47
energy
I
I
I
I
50
53
56
59
(eV)
of (a) Ga 3d; (b) As 3d: (c) S 2p. and (d) Se 3d obtained
Binding after exposure
of ( NH,)2(
energy
I
62
(eV)
S,Se),.,,-treated
C&As surface
to air for 2 we&s.
S.-H.
Sa et al. / Surface
and Coatings
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the additional bonding states as Ga-0, As-O, and especially elemental As with higher binding energies than Ga-As were observed. In S 2p and Se 3d peaks region, a slight decrease of S and Se was observed. From the above results, the oxidation of passivated GaAs surface can be explained as follows. As-(&Se) bond seems to cover partial surface area of GaAs because (&Se) binds with elemental state of As. The nonpassivated GaAs surface area reacts with oxygen to form Ga-0 and this induced the formation of elemental As. The elemental As successively reacts with oxygen but partially remains as elemental state. The oxidation may proceed to the passivated surface area and induce the decrease of passivating element such as S and Se. This is why the decrease of S and Se could be observed only after 2 days’ exposure as shown in Fig. 3. Fig. 3a and b represent the changes of surface composition and the ratio of S to Se according to air exposure time, respectively. In Fig. 3a, due to gradual increase of 0, a relative decrease of the other elements was clearly seen. In Fig. 3b, S/Se ratio varied from 1.777 to 2.092 with air exposure time. This degradation rate of Se is larger
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than that of Sandroff et al. [7], who reported that S-passivated surface degraded faster than Se-passivated surface during the exposure to air and Se-passivated surface was more stable than S-passivated one against photo-oxidation. This is in contradiction with our result. But the formation of the passivation layer depends on the quantity of generated elemental states of As and Ga during wet cleaning [ 121, which could be easily varied on every wet cleaning step. This can cause different passivating surface coverages between S- and Se-treated samples. From this, it can be concluded that direct comparison of the passivating effect between S and Se samples is not possible. using two independent Concerning our result, the difference in bond strength between S or Se and GaAs may explain why As-Se bond degrades faster than As-S bond. Bond strength of As-S is 379.5 +6.3 kJ/mol and larger than that of As-Se, 96.0 kJ/mol [9]. Thermal stabilty of (NH&( S,Se),.O,-treated GaAs was examined by in situ anneal in UHV-XPS chamber. Fig. 4 shows the spectra of Ga 3d, As 3d, S 2p, and Se 3d after in situ anneal at 200 “C. The decrease and
al tij
t
30--uGa -o--As -o0
z B
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1
2
4
I
I. 6
Air exposure
I
I.
X 1
12
14
Il.
6
10
time (day)
2.5 w A
LO&
'. 0
'. 2
'. 4
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Air exposure Fig. 3. The variations
of (a) composition
and (b) S/Se ratio
" 6
" 10
--A--
S/Se
' 12
"
-
.
14
time (day)
of (NH,),(S.Se)l.O,-treated
GaAs
surface
with air exposure
time.
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Ga-As Ga 3s
-1
1-1
I
15
17 Binding
19 energy
21
23
153
159
Binding
(eV)
I
I
162
165
I
I
166
energy
(eV)
As-Ga Se-(Ga, As)
u
4
37
39 Binding
Fig. 4. XPS spectra
41 energy
43
45
50
(eV)
of (a) Ga 3d; (b) As 3d; (c) S Zp, and (d) Se 3d obtained
increase of peak intensifies at 42.8 and 19.7 eV of binding energies, observed in As 3d and Ga 3d spectra, respectively. This means that the passivating agents, S and Se, changed the bonds from As to Ga. It can be explained by thermodynamic stability of each bond. Heat of formation (or bond energy) of Ga-S (Ga,S,) and Ga-Se bonds (Ga,Se,) are - 122.8 + 3.0 and -97.0 +4.0 kcal/ mol, respectively, and these values are negatively larger than those of As-S (As,&) and As-Se bonds ( AS,Se3) as -40.0 f 5.0 and -24.5i4.0 kcal/mol [lo]. As pointed out above, As-( S,Se) bonds were formed during the passivation treatment. Even though Gaa( &Se) bonds are more stable than As-(S,Se) bonds, Ga-(S,Se) bond is not formed because GaAs does not dissociate to form Ga-(We) bond. However, with a higher temperature, S and Se may effectively react with GaAs to form the bonds with Ga. In Fig. 4b, the appearance of elemental As state is clearly seen due to the dissociation of As-(&Se) bonds. Furthermore, in Fig. 4c and d, broadening of S 2p and Se 3d peaks was slightly observed due to the formation of the bonds with Ga and As. Normally, S-Ga and S-As bonds show a difference in binding energy of about 1 eV [4]. Especially in Fig. 4c,
53
Binding after in-situ
anneal
56
energy
59
62
(eV)
of (NH,)?(S.Se),
OR-treated
GaAs
surface
at 200 “C.
the peak binding energy of S 2p was slightly moved to low binding energy. Fig. 5a and b represent the variation of the surface composition and the ratio of S to Se according to in situ anneal temperature, respectively. The contents of S, Se and 0 decreased, while that of Ga in surface relatively increased. After anneal at 300 C, As-rich surface, just after the treatment, was changed to Ga-rich surface due to the high volatility of As. In Fig. 5b, S/Se ratio decreased from 3.716 to 1.937, and rapid decrease of the ratio was observed after anneal at 200 “C. By the anneal at 200 C, S and Se break the bonds with As to bind with Ga. During this exchange period, the vaporization of S happened easily, due to the high vaporization pressure of S (about 200 times larger than Se at a given temperature) [9]. After the anneal at 200 “C, even though the anneal temperature was higher, the ratio of S/Se slightly decreased. But with the anneal at 550 “C, a decrease of S and Se was observed, as shown in Fig. 5a, due to the thermal decomposition of (S,Se))Ga bonds. From the above analysis, it can be stated that the apparent thermal stability of Se-( As,Ga) bond is higher than that of S-(As,Ga) bond. However, it should
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1
I
I
I
I
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, 100
t 200
I 300
I
I
400
500
221
I
-
600
Anneal temperature (C) 4.5 r (b)
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,
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100
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200
300
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400
-
500
-1 600
Anneal temperature (c) Fig. 5. The variations of (a) composition and (b) S/Se ratio of (NH,),(S,Se),,,,-treated
be remembered that this apparent low thermal stability of S compared with Se resulted from the high volatility of S during the bonds exchanging reaction.
4. Conclusions XPS analyses of an (NH,),(S,Se),,,,-treated GaAs surface showed that the passivation layer mainly contains As-(&Se) bonds. During the air exposure of (S,Se)passivated GaAs, the degradation of the Se-As bond was observed to be explained by larger bond strength of the S-As bond, 379.5 + 6.3 kJ/mol than that of Se-As bond, 96.0 kJ/mol. An exchange of the bonds from As-(&Se) to Ga-(‘&Se) was found at 200 “C anneal under UHV condition. Large heat of formation of Ga-(&Se) bonds seemed to cause the dissociation of As-(&Se) bonds to react with GaAs and form Ga-(&Se) bonds. Decrease of S content was observed during this exchanging reaction due to the high vaporization pressure of S. Then, even though the bond energy (heat of formation) of S-(As,Ga) is larger than that of
GaAs surface with in-situ anneal temperature.
Se-(As,Ga), Se-( As,Ga) bonds show apparently high thermal stability compared with S-( As,Ga) bonds.
Acknowledgement This research was performed under the auspices of KOSEF (contract no. 961-0804-034-2).
References [ I] W.E. Spicer, 2. L-Weber, N. Newman, T. Kendelewieg, R. Cao, C. McCant, P. Mahowald, K. Miyano, I. Lindau, J. Vat. Sci. Technol. B 6 (1988) 1245. [2] C.J. Spindt, W.E. Spicer, Appl. Phys. Lett. 55 (1985) 1653. [ 31 C.J. Sandroff, M.S. Hegde, CC. Chang, J. Vat. Sci. Technol. B 7 (1989)841.
K. Sato, M. Sakata, H. Ikoma, Jap. 3. Appl. Phys. 32 (1993) 3354. [S] J.L. Lee, D. Kim, S.J. Maeng, H.H. Park, J.Y. Kang, Y.T. Lee, J. Appl. Phys. 54 (1983) 2533. [6] T. Scimeca, Y. Watanabe, F. Maeda, R. Berrigan, M. Oshima, Appl. Phys. Lett. 58 (1991) 1167. [4]
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and Coatings
[7] C.J. Sandroff, MS. Hegde, L.A. Farrow, R. Bhat. J.P. Harbison, C.C. Chang, J. Appl. Phys. 54 ( 1983) 2533. [8] B.A. Kurubilla. S.V. Ghaisas, A. Datta, S. Banerjee. S.K. Kulkami, J. Appl. Phys. 73 ( 1993) 4384. [9] D.R. Ride, J.A. Kerr, Handbook of Chemistry and Physics, 76th ed., 1996, pp. 9-51.
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[IO] 0. Kubaschwski, C.B. Alcock, MetahurgicdJ Thermochemistry. 5th ed.. 1979, pp. 2617323. [I I ] M.G. Kang. H.H. Park. K.S. Suh, J.L. Lee, Thin Solid Films 290 (1996)
328.
[ 121 M.G.
Kang.
H.H.
Park,
submitted
to J. Vat. Sci. Technol.
A.