effect of melt stoichiometry on carrier concentration profiles of silicon diffusion in undoped lec...

J. Electrochem. Soc., Vol. 136, No. 4, Apr i l 1989 �9 The Electrochemical Society, Inc. 1165

0.67-0.68 eV. Their n-values are 1.03-1.04. Therefore, barrier height for NiSi is independent of Si orientation.

The forward characteristics of the NiSi Schottky barrier diode formed by heat-treating Ni on Si(100) in UHV, and of the NiSi2 Schottky barrier diode formed by codepositing Ni and Si onto Si(100), are shown in Fig. 6. The n-value is 1.04-1.06 for both diodes. The barrier height is 0.67-0.68 eV. Similar results are obtained for Si(l l!) . The XPS spectra for clean Si(100) and codeposited NiSi2 surfaces are shown in Fig. 7. The Si2p signal shifts to higher binding energy in the NiSi2. The XPS intensity ratio Ni3JSi2p = 0.29, which corresponds to the NiSi2 identified by x-ray diffraction. Similar XPS spectra are obtained for clean Si(111), and codeposited NiSi2 surfaces are obtained as shown in Fig. 8. That is, it is found that Si substrate orientation and silicide phase do not affect barrier height for thicker (->50 nm) Ni silicide. These effects differ from the case of thinner (<-30 nm) type-B NiSi2/Si(lll) and may be due to Fermi- pinning resulting from lattice mismatch between Ni silicide and Si (6).

The present results show that variations in silicide characteristics apparently do not affect barrier height within experimental accuracy:

Summary Silicide phase and Si substrate orientation were found

not to affect Ni silicide barrier height.

Acknowledgments The author is grateful to T. Ishiba for performing the

x-ray diffraction analyses, and to T. Ohshima and T. Ikezu for preparing the samples.

Manuscript submitted March 14, 1988; revised manuscript received Aug. 19, 1988.

REFERENCES 1. K. N. Tu, G. Ottaviani, U. Gosele, and H. F611, J. Appl.

Phys., 54, 758 (1983). 2. C. Canali, F. Catellani, G. Ottaviani, and M. Prudenziati,

Appl. Phys. Lett., 33, 187 (1978). 3. P .E. Schmid, P. S. Ha, H. F61t, and T. Y. Tan, Phys. Rev.

B, 28, 4593 (1983). 4. G. Ottaviani and K. N. Tu, ibid., 24, 3354 (1981). 5. R. Tung, Phys. Rev. Lett., 52, 461 (1984). 6. A. Kikuchi, T. Ohshima, and Y. Shiraki, To be pub-

lished. 7. A. Ishizaka, Unpublished results.

Effect of Melt Stoichiometry on Carrier Concentration Profiles of Silicon Diffusion in Undoped LEC SI-GaAs

Dadang Sudandi and Satoru Matsumoto* Department of Electrical Engineering, Keio University, Hiyoshi, Yokohama 223, Japan

ABSTRACT

The effect of melt stoichiometry on the carrier concentration profiles resulting from silicon diffusion has been investigated for undoped LEC semi-insulating GaAs. Thermal furnace processing is used to diffuse silicon into GaAs from a thin elemental source in the temperature range of 950~176 Most carrier concentration profiles are subject to a gaussian dis- tribution, showing that the silicon film is behaving as a finite diffusant source. The diffusion junction depths for Ga-rich substrates are shallower than those of near-stoichiometric and As-rich substrates, indicating low Si diffusivity in a Ga-rich substrate. The diffusion coefficients and an activation energy are obtained for this process, and this activation energy is in the range associated with substitutional diffusion.

Many processes for forming n-type layers in GaAs in- volve the growth of epitaxial layers (1, 2), diffusion of Si from solid sources (3, 4), and Si ion implantation (6-12). Heavily doped n-type layers with free carrier concentrations in the range of 5-6.5 • 1018 cm -3 were obtained (3, 4) as a result of Si diffusion in GaAs from the surface. Greiner et al. (4, 5) observed that the Si concentration was as high as 2 • 102o cm 3 at the surface. In their work a diffusion model was proposed for Si diffusion in GaAs under heavy doping conditions. This model involves the formation of compensating acceptors and neutral donor-acceptor pairs which produce the free carrier concentration saturation. However, Si pairs dominate the diffusion flux at high Si concentrations and a concentration dependent diffusion coefficient is predicted by this model. Therefore it is im- portant to know the constant diffusion coefficient in low concentration diffusion as a reference when such a high concentration diffusion is discussed.

Diffusion of Si into GaAs from the surface resulting in lightly doped n-type layers has not been reported. Many publications have reported Si ion implantation to form n-type layers in GaAs (6-12). However, annealing of ion implanted layers is usually performed at temperatures rang- ing from 800~176 Thus, there is a lack of information about fundamental physical quantities such as an activation energy of Si diffusion in GaAs.

Recently, the effect of melt stoichiometry on the activation uniformity of Si-implanted undoped semi-insulating (SI) GaAs has been studied (13, 14). The most uniform car-

*Electrochemical Society Active Member.

rier profile was obtained in crystals grown from melts of nearly stoichiometric to As-rich substrates. Then it is es- sential to study the effect of melt stoichiometry on the carrier concentration profiles in order to clarify the mechanism of low concentration Si diffusion in GaAs.

In this work, we study the effect of melt stoichiometry on the carrier concentration profiles of silicon diffusion into GaAs from different melt compositions. Diffusion of Si into GaAs substrates is accomplished by furnace annealing at temperatures from 950 ~ to 1100~ for various annealing times. Low carrier concentration profiles are investigated for this process, and most carrier concentration profiles are gaussian in shape.

Experimental The samples used in this study were <100> oriented un-

doped SI-GaAs, grown in a high-pressure puller using B203 liquid encapsulant and pyrolitic boron nitride (PBN) crucibles. The arsenic atom fraction (X = [As]/[As + Ga]) are 0.484 (Ga-rich); 0.499 (near-stoichiometric); and 0.508 (As-rich), which are estimated by weight-in, weight-out method. The samples were cleaned with trichloroethylene, acetone, ethanol, and DI water. They were subsequently free-etched in 3H2SO4:1H202:1H20 solution for 90s. A thin film of Si approximately 200A thick was deposited onto GaAs surface at room temperature by an RF (13.6 MHz) sputtering system using a target of floating zone refined single-crystal silicon.

Prior any heat-treatment, samples were coated with 15001k of SiO2 on both sides by chemical vapor deposition at 420~ The SiO2 acted as an encapsulant, preventing de-

) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 129.108.9.184Downloaded on 2014-11-08 to IP

http://ecsdl.org/site/terms_use

1166 J. Electrochem. Soc., Vol. 136, No. 4, Apr i l 1989 �9 The Electrochemical Society, Inc.

?'~1018

8 lo17

(J

bOQOl

O Q

e�9 �9 �9

As-rich

1000"(:. lOmin

I I iIIi I! I i i �9 I

i I a , I , , , , I

2000 4000

DEPTH ( A )

104

Fig. I, Carrier concentration and mobility profiles plotted as o function of depth for Si diffusion into As-rich substrate at 1000~ 10 min.

compos i t ion of the semiconduc to r at the surface dur ing heat- t reatment . In this study, the samples were annea led in quar tz capsule wi th d imens ion of 1 cm in d iam and 2.5 cm in length, to min imize arsenic vaporizat ion. The sample was baked at 300~176 for 5 rain to dr ive off water and oxygen. The capsule was opened dur ing baking and c losed dur ing annealing. Si l icon was diffused by conven- t ional furnace anneal ing in f lowing n i t rogen gas over a t empera tu re range of 950~176 for several minutes .

Electr ical contact was m a d e with ind ium dots on the four corners of the square-shaped (typically 5 x 5 mm) sample surface which was hea ted at 430~ for 1 rain in f lowing n i t rogen gas. Dep th profiles of carr ier concentra- t ion and mobi l i ty were made th rough the combined use of the anodic s t r ipping t echn ique and s tandard van der Pauw-Hal l measurements . In in terpre t ing Hall effect and resis t ivi ty measurement s , the Hall factor was t aken as unity. The e lect rolyte of H20:H3PO4 (2.6 -< pH -< 3.0) solu- t ion (15) and a P t e lec t rode as a ca thode were used, and the sys tem was opera ted wi th cons tan t appl ied voltage. A thin ox ide layer was g rown successful ly by apply ing a cons tant vo l tage source of 40V. The anodic oxide was r e m o v e d pr ior to the Hall m e a s u r e m e n t us ing a 15H20:1N H4OH so- lut ion.

Results and Discussion Figure 1 shows a typical examp le of the carr ier concen-

t ra t ion and mobi l i ty profiles for difffusion of Si into an As- r ich substra te at a t empera tu re of 1000~ for 10 rain. The intr insic e lec t ron concent ra t ion ni of GaAs at the diffusion t empera tu re s be tween 950 ~ and l l00~ is in the range of

I S t oich__.___~_ ,~E 1018 o 950~ 60min

[3 1000~ 10min

~ F ~ 1050~ 5min I- o 1100~ 5rain

I ,I I I I I I I I I [ ' ' ' '

2000 4000

DEPTH (A)

Fig. 3. Carrier concentration profiles plotted as a function of depth for Si diffusion into near-stoichiometric substrate at 950 ~ 1100~

1 x 1017 to 1 x 10 TM cm -3 (16). For extr ins ic condi t ions the surface dopant concent ra t ion C, is larger than ni (Cs > ni). U n d e r this condi t ion it is recognized that the diffusion coefficient depends on concent ra t ion as a resul t of the inter- nal electric field, point defec t generat ion, and o ther effects (17). In the p resen t work, surface concent ra t ions are lower than 1018 cm -3 (Cs < ni). Therefore the present resul ts are ob ta ined f rom near intr insic condit ions.

The average mobi l i ty va lue as shown in the mobi l i ty profile is near ly cons tant throughout , and shows a sl ight in- crease near the bulk region. When this mobi l i ty va lue is corre la ted to the carrier concent ra t ion value, it is m u c h be low the theoret ical va lue for zero compensa t ion condition. For impur i ty concent ra t ion exceed ing 101~ cm -3, the mobi l i ty becomes sensi t ive to ionized impur i ty scattering, pe rmi t t ing the de te rmina t ion of the compensa t ion ratio in the mater ia l (18). When a compensa t ion ratio of 0.5 has been taken into account , the theoret ica l resul t is in good a g r e e m e n t wi th the expe r imen ta l value. F igure 2, 3, and 4 show the carr ier concent ra t ion profiles f rom Si diffusion into Ga-rich, near-s toichiometr ic , and As-rich substrates, respect ively . The actual plots of carrier concent ra t ion and Hal l mobi l i ty v s . dep th as shown in Fig. 1 p roduce curves that are cons iderably erratic. We have shown the average curve for the data in Fig. 2, 3, and 4 to compare the difference in each diffusion profile. I r respec t ive of the stoichi- omet ry of substrates, the saturat ion of carrier concentra- t ion profiles near surface is observed clearly for diffusion

,•io 18 v

.o

~1017

Go- rich n 1000~ 10rain

�9 z~ 1050~ 5min o " ~ n

1 ' ' I I I I I I �9 , , , ,

2000 4000

DEPTH (A)

Fig. 2. Carrier concentration profiles plotted as a function of depth for Si diffusion into Go-rich substrate at 1000~176

As- rich

o 950~162 ~1018 n 1000~

~ z~ 1050 oC o ~ ." ~ o 1100 C

8

~ 1017

60rain 10min 5min 5rain

I I I I I I I i I 1 I i i I

2000 4000 DEPTH (A)

Fig. 4. Carrier concentration profiles plotted as a function of depth for Si diffuslan into As-rich substrata at 950~176



J. Electrochem. Soc., Vol. 136, No. 4, April 1989 �9 The Electrochemical Society, Inc.

Table I. Summary of electrical measurements for Si diffusion into LEC semi-insulating GaAs

1167

Annealing Sheet Sheet carrier Hall Diffusion Temperature Time resistivity concentration mobility coefficient

Sample T(~ t(min) p~ (~/[~) N~ (cm -2) UH (cm2/V �9 s) D (cm2/s)

Ga-rich 100O 10 175.7 5.6 x 1012 3600 3.1 x 10 -14 1050 5 195.8 5.9 x 1012 2700 1.0 x 10 -13 1100 10 240.1 4.1 • 1012 2700 6.4 x 10 13

Stoich 950 60 291.7 1.1 x 10 ~2 3500 3.8 x 10 TM

100O 10 128.9 9.7 x 1012 3200 1.8 x 10 -13 1050 5 216.5 5.1 x 1012 3000 3.4 x 10 -13 1100 5 178.7 8.2 x 1012 2600 9.6 x 1O -13

As-rich 950 60 257.5 2.3 x 10 ~2 3100 3.0 x 10 ~4 1000 10 149.5 7.4 x 1012 3000 1.6 x 10 -13 1050 5 91.4 1.2 x 1013 3100 3.9 x 10 13 1100 5 196.6 6.0 x 1012 2700 8.9 x l0 -1~

t e m p e r a t u r e of 1100~ D u r i n g h i g h t e m p e r a t u r e d i f fus ion , As a t o m s are d i s soc ia t ed to l e a v e As vacanc ies , w h i c h can b e o c c u p i e d by Si a toms . T h e e x i s t e n c e of c o m p e n - sa t ing accep to r s a n d t he f o r m a t i o n of n e u t r a l donor - a c c e p t o r pa i rs c an p r o d u c e t he s a t u r a t i o n of ca r r i e r con- c e n t r a t i o n profiles.

I n th i s work , t he sur face ca r r ie r c o n c e n t r a t i o n s Cs are in t h e r a n g e of 1 x 1017-1 x 1018 c m 3 as s h o w n in Fig. 2, 3, a n d 4, a n d as d e s c r i b e d be fo re t h e s e are nea r in t r ins i c condi- t ions . T h e m o b i l i t y va lues are a b o u t 3000 cm2/V - s. Da ta f rom Si d i f fus ion in to GaAs are s u m m a r i z e d in Tab le I. The shee t ca r r i e r c o n c e n t r a t i o n s at t he s a m e t e m p e r a t u r e a n d d i f fus ion t i m e are def in i te ly la rger in As- r ich s u b s t r a t e t h a n in Ga- r i ch subs t ra te . T h a t is, a t t he s ame t e m p e r a t u r e a n d d i f fus ion t ime, s i l icon d i f fuses d e e p e r in As- r ich sub- s t r a t e t h a n in Ga- r i ch subs t ra t e . However , a n u n c e r t a i n t y in t h e su r face Si layer t h i c k n e s s m a y exist . I t appea r s in t h e d i f f e rence in t he to ta l q u a n t i t y o b t a i n e d b y i n t e g r a t i n g ca r r ie r c o n c e n t r a t i o n profiles. T h e r e f o r e t h e i n t e r p r e t a t i o n of t h e d i f fus ion re su l t s m u s t invo lve t he ca l cu la t ion of diffu s ion coeff icient .

F igu re s 5 a n d 6 s h o w t he ca r r ie r c o n c e n t r a t i o n profi les as a f u n c t i o n of the s q u a r e of d e p t h for Ga- r ich a n d As- r ich subs t r a t e s , respec t ive ly . At l l00~ dev i a t i on f rom a s t r a igh t l ine is obse rved , b u t m o s t of the p ro f i l e s are" al- m o s t g a u s s i a n in shape. D e v i a t i o n f rom a g a u s s i a n distr i - b u t i o n nea r sur face sugges t s t h a t some accep to r s ta tes are i n t r o d u c e d in t he n e a r sur face reg ion at h i g h e r t e m p e r a - t u r e of l l00~ T h e s e accep to r s are r e s p o n s i b l e for t he re- d u c t i o n of ca r r i e r c o n c e n t r a t i o n a n d mob i l i t y values . The s a m e t e n d e n c y is also o b s e r v e d in n e a r - s t o i c h i o m e t r i c subs t r a t e .

T h e in t r in s i c d i f fus ion c o n d i t i o n impl i e s t h a t d i f fus ion coeff ic ient is c o n s t a n t for a g iven t e m p e r a t u r e . U s i n g t he e q u a t i o n of a g a u s s i a n d i s t r ibu t ion , d i f fus ion coeff ic ient

,~"~" 1018 I

8 .~ 1017

Ga-rich n 1000~ 10min z~ 1050~ 5min o 1100~C 10rain

i i i i I i i i i I i , i i

I 2

(DEPTH) 2 (x107~ 2)

Fig. 5. Carrier concentration profiles plotted as a function of the square of depth for Si diffusion into Ga-rich substrate at 1000 ~ 1100~

for Si d i f fus ion in GaAs can be d e t e r m i n e d b y t a k i n g the s lope of a gaus s i an d i s t r i b u t i o n at e ach t e m p e r a t u r e . The re su l t s of ca lcu la t ion are p lo t t ed in Fig. 7 a n d an ac t iva t ion e n e r g y Ea was d e t e r m i n e d f rom the s lope of d i f fus ion coeff ic ient p lo t t ed as a f u n c t i o n of r ec ip roca l t e m p e r a t u r e . As s ta ted before, t he s a tu ra t i on of ca r r i e r c o n c e n t r a t i o n prof i les nea r sur face is o b s e r v e d c lear ly at 1100~ i r respec- t ive of the s t o i c h i o m e t r y of subs t ra tes . S ince s o m e accep to r s i n t r o d u c e d d u r i n g t he h e a t - t r e a t m e n t m a y affec t t he d i f fus ion of sil icon, t he da ta of t he d i f fus ion coeffi- c ien t s at 1100~ for all s u b s t r a t e s are e x c l u d e d for t he de- t e r m i n a t i o n of t he ac t iva t ion energy. Sol id l ines in Fig. 7 s h o w the fi t ted e x p o n e n t i a l e q u a t i o n D = Doexp(-Ea/kT), a n d t he r e su l t i ng ca lcu la ted d i f fus ion coeff ic ient for Ga- r i ch is

D = 8.6 x 10 -1 exp ( -3 .4 eV/kT)

a n d for the ave rage b e t w e e n n e a r - s t o i c h i o m e t r i c a n d As- r i ch s u b s t r a t e s is

D = 0.7 exp ( -3 .2 eV/kT)

T h e a b o v e ac t iva t ion ene rg ies are a l m o s t the s a m e for b o t h Ga- r i ch a n d n e a r - s t o i c h i o m e t r i c a n d As- r ich sub- s t ra tes , s u g g e s t i n g t he s a m e d i f fus ion m e c h a n i s m a m o n g t h e s e subs t r a t e s . T h e s e ac t i va t i on ene rg ies are l a rger t h a n t he ac t i va t i on ene rg ies of 2.5 eV (4) a n d 2.65 eV (2) for h i g h Si d o p i n g concen t r a t i ons . This sugges t s a d i f fe rence in t he d i f fus ion m e c h a n i s m b e t w e e n low a n d h i g h d o p i n g concen t ra t ions . At h i g h concen t r a t i ons , t he d o m i n a n t diffus- ing spec ies are be l i eved to be Si pairs . On t he o the r h a n d , at Si c o n c e n t r a t i o n s less t h a n 10 TM c m -3 w h e r e t he d i f f u s i o n coeff ic ient is cons t an t , t he d o m i n a n t d i f fus ing species are s ingle Si a toms . This m e c h a n i s m for Si d i f fus ion invo lves t he m o t i o n of i so la ted Si a t o m s t h r o u g h Ga v a c a n c i e s a n d

•E1018 c .o

8

~ 3017

~.)

As-rich o 950~ 60rain [] 1000~C 10rain z~ 1050~ 5min o~ 1100~ 5min

, , , , I I i I i I ,

I 2

(DEPTH) 2 (x107~, 2)

I t I

Fig. 6. Carrier concentration profiles plotted as a function of the square of depth for Si diffusion into As-rich substrate at 950~176



1168 J. Electrochem. Soc., Vol. 136, No. 4, Apr i l 1989 �9 The Electrochemical Society, Inc.

[] Ga- rich o Stoich

1s 12 ~ A As-rich

r~1513

1614 I . I 0.7 0.75 0.8 0.85

~O00~T (oK -~)

Fig. 7. Diffusion coefficients plotted as a function of reciprocal of temperature for Si diffusion into Ga-rich, near-stoichiometric and As- rich GoAs substrates at 950 ~ 1100~

require the formation of Ga vacancies. It would presum- ably produce very low diffusion coefficients.

In addition, a difference in diffusion mechanism is also suggested by the diffusion coefficient value. At high Si doping condition, Greiner e t at. (4) obtained diffusion coefficient for Si pairs of 1.4 • 10:1' em 2/s at 1000~ At the same temperature, this value is about a hundred times greater than the result in the present study (1.6 x 10 ,3 cm2/s). Apparently, it is believed that the diffusion of near- est neighbor Si pairs through Ga and As vacancies is much faster than the moving of single Si atoms via Ga vacancies.

For the same diffusion time and temperature, the diffusion junction depth for Ga-rich is shallower than for As- rich substrates as stated previously. Figure 7 also clearly shows that the diffusion coefficient of Si diffusion is smaller into Ga-rich than into As-rich (DGa < DAs) substrates. This is interpreted that in As-rich substrates the Ga vacancy concentration increases, leading to enhanced oc- cupancy of Si atoms in Ga-sites. This increases the proba- bility of Si diffusion into the substrate via Ga vacancies.

Conclusions The effect of melt stoichiometry on the carrier concen-

tration profiles in the diffusion of Si has been investigated for undoped LEC semi-insulating GaAs.

1. Low carrier concentration profiles are obtained as a result of the furnace processing Si diffusion into GaAs from a thin elemental source for several minutes.

2. The diffusion junction depth for Ga-rich is shallower than for As-rich substrates, showing that low Si diffusivity in Ga-rich substrates.

3. Diffusion coefficients and an activation energy are de- termined for this process and these values are in the range associated with substitutional diffusion in GaAs.

Acknowledgment The authors would like to thank S. Chichibu for the ap-

paratus installation.

Manuscript submitted Dec. 30, 1987; revised manuscript received Aug. 31, 1988.

Keio University assisted in meeting the publicqtion costs of this article.

REFERENCES 1. Y. G. Chat, R. Chow, and C. E. C. Wood, Appl. Phys.

Lett., 39, 800 (1981). 2. K. L. Kavanagh and J. W. Mayer, ibid., 47, 1208 (1985). 3. G. R. Antell, Solid State Electron., 8, 943 (1965). 4. M. E. Greiner and J. F. Gibbons, Appl. Phys. Lett.i 44,

750 (1984). 5. M. E. Greiner and J. F. Gibbons, J. Appl. Phys., 57, 5181

(1985). 6. J. Kasahara and N. Watanabe, in "Semi-Insulating

III-V Materials," p. 115, Shiva, England (1982). 7. J. Kasahara, Y. Kato, M. Arai, and N. Watanabe, This

Journal, 130, 2275 (1983). 8. T. Onuma, T. Hirao, and T. Sugawa, ibid., 129, 837

(1982). 9. Y. K. Yeo and R. L. Hengehold, J. Appl. Phys., 58, 4083

(1985). 10. S. H. Xin, W. J. Schaff, C. E. C. Wood, and L. F. East-

man, Appl. Phys. Lett., 41, 742 (1982). 11. T. Egawa, Y. Sano, H. Nakamura, T. Ishida, and K. Ka-

minishi, Jpn. J. Appl. Phys., 24, L35 (1985). 12. J. M. Woodall, H. Rupprecht, and R. J. Chicotka, Appl.

Phys. Lett., 38, 639 (1981). 13. T. Safo, K. Terashima, H. Emori, S. Ozawa, M. Naka-

jima, T. Fukuda, and K. Ishida, Jpn. J. Appl. Phys., 24, L488 (1985).

14. A. R. Von Neida, S. J. Pearton, M. Stavola, and R. Ca- ruso, Appl. Phys. Lett., 49, 1708 (1986).

15. W. C. Niheaus and B. Schartz, Solid State Electron., 19, 175 (1976).

16. J. S. Blakemore, J. Appl. Phys., 53, 520 (1982). 17. H. C. Casey, Jr., "Atomic Diffusion in Semicon-

ductors," p. 311, Plenum Press, London (1973). 18. W. Walukiewicz, L. Lagowski, L. Jastrzebski, M.

Litchtensteiger, and H. C. Gatos, J. Appl. Phys., 50, 899 (1979).



effect of melt stoichiometry on carrier concentration profiles of silicon diffusion in undoped lec...

Documents