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ULTRASONIC DETECTION OF THE VACANCY IN
BORON-DOPED SILICON
W. Johnson, A. Granato
To cite this version:
W. Johnson, A. Granato.
ULTRASONIC DETECTION OF THE VACANCY IN
JOURNAL D E PHYSIQUE
Colloque CIO, supplément
au
no
12, Tome46,
décembre 1985 page CIO-537ULTRASONIC DETECTION O F T H E VACANCY I N BORON-DOPED SILICON
W.L.
JOHNSON AND A.V. GRANATODepartment of Physics and Materials Research Laboratory, University of Illinois at Urbana- Champaign, Urbana ,Illinois 61801, U.S.A.
Abstract -Ultrasonic attenuation and velocity measurements at 15.4 and 75.0 MHz were performed on electron-irradiated boron-doped silicon. A defect relaxation was observed which, from comparison with EPR data, is identified as the positively charged vacancy
.
1
-
INTRODUCTIONThe vacancy in electron-irradiated p-type silicon has previously been studied with electron paramagnetic resonance (EPR) /1-5/ and junction capacitance techniques
13-71. Watkins /1,2/, who first associated the vacancy with a photogenerated EPR spectrum, presented a model which has remained essentially intact over the past twenty years. He suggested that the spectrum is generated by a positively charged metastable state (v+) which arises from a three-fold degenerate (tZ) electronic level split by coupling to Jahn-Teller tetragonal (e) distortions of the surrounding lattice.
Watkins observed that, once generated with light, the state is thermally depopulated (the spectrum decays) when the temperature is raised above
-
22 K. On applying <100> uniaxial stress, he observed a preferential realignment of distortions, and, by studying spectral line broadening at 14-21 K, deduced a relaxation rate of the formFrom the viewpoint of the present study, it is most useful to interpret Watkins' model not as an electronic level split by distortions, but as a vibronic state which includes both electronic and lattice coordinates and has the same t2 symmetry as the uncoupled electronic state
/a/.
Under this interpretation, since each of the three degenerate states contributes different amounts to the two e-type strains, repopulation between the states will result in relaxation of strain for any applied stress with an e component. One expects, therefore, an ultrasonic C'-type wave to exhibit a temperature-dependent relaxation of the elastic constant and corresponding attenuation.JOURNAL DE PHYSIQUE
II
-
EXPERIMENTAL TECHNIQUEOur sample was float-zone silicon, boron doped to a concentration of .3 (+/-.l) ppm. Ultrasonic faces were oriented within .2 degrees of the <110> direction and mechanically polished flat and parallel to .5p. Sample dimensions were .94 x 2.2 x 2.2 cm., the shorter dimension k i n g the direction of wave propagation (<110>).
A 15 MHz AC-cut (shear) quartz transducer was used to generate and receive <llO> (C') and <100> (Ck4) polarized waves. A . 3 p gold/chromium film was evaporated onto the sample face to act as a ground for the RF pulse. The transducer was bonded to the sample with an organic fluid, 3-methylpentane (Aldrich Chem. Co.), which freezes at 120 K and is useful for measurements below 70 K.
Irradiations were performed at 4.2-12 K using 2.8 MeV electrons. velocity and attenuation at 15.4 and 75.0 MHz were measured from 1.3 to 70 K using a periodically interrupted pulse superposition technique /9/.
III
-
RESULTSImmediately after irradiation we observe an attenuation peak for C' waves, as shown in fig. 1. After heating to 25 K, this peak disappears. (The curves displayed in fig. 1 are the difference between the signal immediately after irradiation and that after heating to 25 K.) The 'peak can then be regenerated with light or with a few seconds exposure to the electron beam.
The relaxation rate deduced from EPR studies (eq. 1) gives peak positions within .5 K of Our results. It does not, however, give nearly so large an attenuation below -15 K. If we assume that the peak is due to a single relaxation process, then Our data indicate that below 15 K the relaxation rate is greater than that given by eq. 1. In fact, eq. 1 was never meant to be applicable below 14 K. Although Watkins could not perform detailed measurements at lower temperatures, he did indicate that eq. 1 is inadequate in that region, sinCe "alignment- occurs even at 2 K" 121.
The attenuation is cut off abruptly as the temperature is increased above -22 K, as is most obvious from 75 MHz in the figure. The time constant for this decay is measured to be 165 seconds at 22.5 K. Within about .5 K, this agrees with EPR results /2/ for depopulation of the V+ state.
Although we have so far not attempted to perform detailed annealing studies because of deterioration of the bond above 120 'K, we have found that the defect anneals in the range 150-200 K, as expected for V+ Il/.
Measurements of C44 (t2) waves exhibit no corresponding relaxation, in agreement with the symmetry described in section 1.
The velocity measurements (fig. 2) show a dispersion corresponding to the attenuation peak. They also show the presence of other defects in the sample. The background, which has been subtracted to produce the curves of fig. 2, has a steep 1 / ~ dependence which has been attributed to a resonance of the neutral substitutional boron 110-121. The remaining low temperature 1/T component in fig. 2 suggests that, in the process of generating @, holes are trapped at compensated boron, as previously observed with EPR /4,5/.
PERATURE (K)
I I
Figure 1. Log decrement vs. temperature for 15.4 and 75 .O MHz C' waves after a dose of 1.5~1017 electrons/cm2. Background after heating to 25 K is subtracted.
t 15.4 MHz
0 75. O MHZ
Figure 2. Fractional change of elastic constant vs. temperature for 15.4 and 75 .O MHz C' waves after a dose of 1.5~1017 electrons/cm2. Background after heating to
25 K is subtracted.
N
A TEMPERATURE (K)
JOURNAL DE PHYSIQUE
I V
-
DISCUSSIONConsidering t h e c l o s e agreement w i t h EPR r e s u l t s , we conclude t h a t t h e r e l a x a t i o n
i s due t o
v+.
From r e p o r t e d damage r a t e s 121, we e x p e c t t h e volume averaged c o n c e n t r a t i o n of v a c a n c i e s i n t h i s s t u d y t o be l e s s t h a n a few hundredths of a ppm. Large Jahn-Teller d i s t o r t i o n s have allowed d e t e c t i o n of s u c h s m a l l c o n c e n t r a t i o n s and p r e s e n t t h e p o s s i b i l i t y of o b s e r v i n g r a d i a t i o n d e f e c t s i n s i l i c o n which have t h u s f a r evaded d e t e c t i o n w i t h o t h e r t e c h n i q u e s . I n t h i s r e g a r d , w e n o t e t h a t , s i n c e t h e r e is no requirement of an unpaired e l e c t r o n , u l t r a s o n i c measurements can d e t e c t d e f e c t s which would be i n v i s i b l e t o EPR.We may, i n f a c t , a l r e a d y be s e e i n g a n o t h e r d e f e c t i n what has been p r e s e n t e d h e r e . The a t t e n u a t i o n peak shape is s u g g e s t i v e of two superimposed peaks, but we have s o f a r n o t been a b l e t o c o n c l u s i v e l y show t h a t t h i s is t h e case. The f a c t t h a t t h e peak is g e n e r a t e d , depopulated and annealed a s a u n i t s u g g e s t s t h a t , i f t h e r e a r e two r e l a x a t i o n p r o c e s s e s o c c u r i n g , they b o t h a r i s e from s t a t e s of t h e vacancy. We hope t o r e s o l v e t h i s q u e s t i o n w i t h f u t u r e s t u d i e s .
We would l i k e t o thank E. Johnsdn, M. Brophy, and K. Maschoff f o r a s s i s t a n c e w i t h t h e measurements and E. K r a s i c k a f o r h e r i n v a l u a b l e a d v i c e and a s s i s t a n c e i n developing t r a n s d u c e r bonding t e c h n i q u e s . T h i s work was s u p p o r t e d by t h e U.S. Dept. of Energy, D i v i s i o n of M a t e r i a l s S c i e n c e s , C o n t r a c t DE-AC02-76-ER091198
REFERENCES
Watkins, G.D., J o u r . Phys. Soc. J a p a n g , Suppl. 2, (1963) 22.
Watkins, G.D., L a t t i c e D e f e c t s i n Semiconductors 1974 ( I n s t . Phys. Conf. S e r . 23) (1975) 1.
Watkins, G.D., T r o x e l l , J.R. and C h a t t e r j e e , A.P., D e f e c t s and R a d i a t i o n E f f e c t s i n Semiconductors 1978 ( I n s t . Phys. Conf. Ser. 46) (1979) 16. Watkins, G.D., C h a t t e r j e e , A.P. and H a r r i s , R.D., D e f e c t s and R a d i a t i o n E f f e c t s i n Semiconductors 1980 ( I n s t . Phys. Conf. Ser. 59) (1981) 199. Newton, J.L., e t a l . , P h y s i c s (1982) 219.
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