Hyperfine Interactions 35 (1987) 715-718

715

PAC S T U D I E S OF ION I M P L A N T E D SILICON D. FORKEL, F. MEYER, W. WITTHUHN and H. WOLF

Physikalisches Institut der Universitdt Erlangen-Niirnberg, Erlangen, W-Germany

Erwin-Rommel-Strafle 1, D-8520

M. DEICHER

Physikalisches lnstitut der Universitat Konstanz, Biicklestrafle 13, D-7750 Konstanz, W-Germany M. UHRMACHER

Physikalisches Institut der Universitdt G6ttingen, W-Germany The annealing behaviour of radiation induced defects in ion implanted s i l i c o n is studied by the perturbed angular correl a t i o n method (PAC). Between 700 K and 1000 K the trapping and detrapping of vacancy-oxygen complexes is observed. In annealed p-Si a well defined, a x i a l l y symmetric e l e c t r i c f i e l d gradient (EFG) appears at low temperatures. This EFG is oriented to the surface and not to any crystallographic direction. The size of the EFG depends strongly on the surface charge. I.

INTRODUCTION

In the l a s t years ion implantation has become a common technique for doping semiconductors. Hereby radiation induced defects are produced which may act as e l e c t r i c a l l y active centres, i . e . acceptors or donors. Therefore i t is of technological importance to know the annealing behaviour and the electronic structures of these defects. Despite numerous investigations concerning t h i s physical topic there are s t i l l several problems of great controversy. Here, the PAC method promises to yield additional information. In this paper we present recent results concerning the study of oxygen-vacancy complexes in silicon. In annealed p-Si a distinct EFG is observed at low temperatures which is correlated to the surface charge. 2

EXPERIMENT

In the following experimeQts the PAC probe atom 111Cd was used which is populated by the 2.8 d EC decay of H J l n . Using a conventional four detector set-up 12 coincidence time spectra are obtained. Combining these spectra properly an i n t e n s i t y r a t i o R(t~ is obtained which is proportigo~l to the angular correlation c o e f f i cient A~ t and the perturbation factor G ~ ) ( t ) . The following function was f i t t e d to the data: .eff zf(i)G~)(t) F(t) = ~22 l

9

with

3

9

G~)(t) = n!oSzn(n(1)) cos(an(n (

(I)

i)

(i)t)

) w0

exp(-an( n

(i~i)

t)

(2)

The fraction f ( i ) of probe atoms is exposed to an EFG characterized by the quadrupole interaction frequency m~i) and the asymmetry parameter ~ l ) . For more details concerning the theory of angular correlation measurements and the more complex treatment of single crystals see / I / . The silicon single crystals were doped by implantations with 350 keV or 500 keV 1111n ions. The implantation doses were varied between 2 9 1012 In+/cm2 and 5 9 1013 In+/cm2. 9 J.C. Baltzer A.G., Scientific Publishing Company

D Forkel et al., P A t studies o f ion imp~anted Si

716

2.

RESULTSAND DISCUSSION

2.1. Radiation Induced Defects in S i l i c o n In t h i s section the r e s u l t s f o r defect annealing in p-type Si, undoped CZ-Si, and FZ-Si are given. a) Indium defect complexes Fig. I . Typical PAC time spectra f o r '11Cd in Si f o r d i f f e r e n t annealing temperatures

0.00

R(t) -0.05 as

-0.05

implanted

J,

-0.10 -0.05

S'

-0.10

T

'n =

/

I

i

I

100

o-,t

I

Z15 0i 0 < n,-1 0 14.

I

1050 K

I

I

I

__

200 300 TIME (ns) i

l

I

o n-Si Deicher et at /2/ 9 p-Si present

CZ-Si

9

~,,,\

28 MHz

k 142 MHz "q=0.42

/ ' \ ll=O o

t--~

o

5

I

I

500

700

900

A

Typical spectra obtained f o r e p i t a x i a l p-Si ([B]3.1015 at/cm 3) are shown in f i g u r e I. A f t e r the implantation a l l probe atoms are located in a s t r o n g l y disturbed surrounding. In an isochronal annealing programme an In defect complex is observed at T = 700 K; i t dissolves again at T = 1000 K (see f i g u r e 2). This complex is characterized by a quadrupole coupling constant eQVzz/h=142(1 ) MHz and an asymmetry parameter q =0,42. In undoped CZ-Si the same EFG is observed. A d d i t i o n a l l y in t h i s type of Si another a x i a l l y symmetric EFG is present at T = 510 K. The corresponding quadrupole coupling constant is 28 MHz. Both defect complexes are observed also in n-Si / 2 / . We assume that both defect induced EFGs are related to d i f f e r e n t vacancy-oxygen complexes. This conc l u s i o n is based on the f o l l o w i n g observation: a) Both defects appear in n - , ptype, and undoped Si, thus i t can be excluded t h a t the dopants phosphorus or boron are i n v o l v e d ; b) The f r a c t i o n s of probe atoms exposed to these EFGs depend on the oxygen content of the samples. They are s l i g h t l y enhanced in (oxygen r i c h ) CZ-Si. For t h i s reason oxygen atoms are expected to be components of the defects; c) The d i f f e r e n t annealing temperatures of both defects are In agreement with EPR measurements concerning vacancy-oxygen complexes / 3 / .

i

11 O0

TEMPERATURE/K

Fig.2. Isochronal annealing behav i o u r of the oxygen-vacancy complexes

D. Forkel et aL, PAC studies of ion implanted Si

717

b) The influence of d i s t a n t defects In high p u r i t y FZ-Si at remarkable high temperatures an annealing steo is observed: a f t e r annealing the sample f o r two hours at T = 1270 K the f r a c t i o n f~o) of probe atoms with a cubic surrounding is about 30 % at 293 K. The remaining Cd atoms are exposed to a broad d i s t r i b u t i o n of EFGs. At hiqher temperatures f~o) increases nearly to 100 %. By decreasing the temperature-the f r a c t i o n f ( o ) is reduced again. On the other hand a f t e r f u r t h e r annealing (one hour at T = 1400 K) a l l probe atoms are in a cubic surrounding at T = 293 K. These experimental r e s u l t s can be explained by the reduction of conduction electron density due to r a d i a t i o n induced defects. According to the model developed f o r 111Cd in Ge (see / 4 / ) the broad d i s t r i b u t i o n of EFGs which is observed for the i n completely annealed sample is due to the delocalized defect electrons of the Cd acceptor, These EFGs are switched o f f by the a n n i h i l a t i o n of the defect electrons by conduction electrons. Since r a d i a t i o n defects represent deep trapping centres f o r conduction electrons the p r o b a b i l i t y for a n n i h i l a t i o n is reduced. At higher temperatures the number of electrons and therefore f ( o ) increases. This model can be applied successfully to experimental data f o r p-Si as reported by Deicher et a l .

/5/.

2.2. Surface e f f e c t s in s i l i c o n

..... ,,,,,,,,~t,~ill ~lh

,&,/•,

0.00

0.00

,j

R(t)

I

I

-0.05

R(t) -0.05

1500 ~, TI

as implanted

,' ~176176 -o ~,~e'~

''~'~''''''''t''~l -o~o~

o.ooF. ,,

v,c_

'.

_oo

0

100

200

300 TIME (ns)

Fig.3. PAC time spectra f o r 111Cd in p-Si

0

100

200

300 TIME (ns)

Fig.4. PAC time spectra f o r 111Cd in Si f o r d i f f e r e n t materials in contact to the surface

D. Forkel et aL, PAC studies of ion implanted Si

718

Epitaxial p-Si ( [ B ] = 3.1015 at/cm 3) was implanted with 1111n ions. Typical PAC spectra for a< 100~single crystal are given figure 3. After annealing the sample (30 minutes at T : 1070 K) a l l probe atoms are located in a cubic surrounding. At T = 4.2 K, however, a well defined, a x i a l l y symmetric EFG is observed. The size of the EFG as well as the fraction f of probe atoms exposed to this EFG is d i f f e r e n t for< 100 ~ and < 111 ~single crystals (see table I ) . Table I Experimental results for 111Cd in ~O0>and ~11> single crystals for d i f f e r e n t materials in contact to the surface f <100> <~11> <100> <100>

30(2) 16(3) 27(2) 25(9)

eOVzz/h % % % %

67.0(2) 35.6(7) 25.4(5) 67(1.5)

MHz MHz MHz MHz

n

~)/mo

0 0 0 0

2(3) 10(4) 8(I) 19(6)

surface contact % % % %

l i q u i d helium l i q u i d helium vacuum titanium

For <100>- as well as for <111>-single crystals t h i s EFG is not oriented to any crystallographic direction but to the surface of the c r y s t a l s . Furthermore the size of the EFG and the fraction f depend on the material which is in contact with the Si surface (see figure 4). Sealing the sample into an evacuated quartz tube results in a decrease ofothe quadrupole coupling constant and the fraction (tablel). A titanium f i l m of 1500 A thickness on the Si surface causes also a reduction of the fraction (table I ) . By removing the metal the fraction increases again. The probe atoms are located 1000 to 2000 ~ below the surface. Nevertheless the experimental results give clear evidence that t h i s EFG is governed by the surface, most probabl~ by the surface charge. This charge is t y p i c a l l y in the order of 1011 to 1013 e/cm and causes a mirror charge in the bulk/6/.The width of t h i s depletion region depends on the impurity concentration and increases with decreasing temperature. For the material used in the present investigation the width of t h i s zone is in the order of 0.6 pm at room temperature / 7 / . In our samples t h i s zone is terminated by the In atoms. The EFG which can be produced d i r e c t l y by the space charge is at least f i v e orders of magnitude too small. Therefore the data indicate an enhancement due to local polarization of the Cd-defect-electron complex. ACKNOWLEDGEMENTS We would l i k e to thank Professor Dr. R. Helbig for encouraging discussions. This work has been sopported f i n a n c i a l l y by the Bundesministerium fur Forschung und Technologie. REFERENCES / I / H. Frauenfelder and R.M. Steffen in: Alpha-, Beta-, and Gamma-Ray Spectroscopy, VoI. 2, ed. K. Siegbahn (North-Holland, Amsterdam, 1968) /2/ M. Deicher, G. Gruebel, E. Recknagel, Th. Wichert, Proc. lon Beam Modification of Materials, Catania, 1986, to be published / 3 / J. Bourgoin, M. Lannoo in:Point Defects in Semiconductors I I , Springer series in Solid State Sciences, Vol. 22 (Springer, Berlin, Heidelberg, New York, 1981) p. 269 /4/ H. Wolf, D. Forkel, M. lwatschenko-Borho, S. Malzer, F. Meyer, and W. Witthuhn, t h i s conference /5/ M. Deicher, G. Gruebel, E. Recknagel, Th. Wichert, and D. Forkel, Nucl. I n s t r . and Meth. in Phys. Rev. B13 (1986) /6/ A.S. Grove in: Physics and Technology of Semiconductor Devices, John Wiley and Sons, New York, 1967, p. 266 /7/ S.M. Sze in: Physics of Semiconductor Devices, John Wiley and Sons, New York, 1981, p. 373