THERMAL CONDUCTIVITY MEASUREMENTS OF IMPLANTED Si

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THERMAL CONDUCTIVITY MEASUREMENTS OF IMPLANTED Si

T. Papa, F. Scudieri, M. Marinelli, U. Zammit, G. Cembali

To cite this version:

T. Papa, F. Scudieri, M. Marinelli, U. Zammit, G. Cembali. THERMAL CONDUCTIVITY MEA- SUREMENTS OF IMPLANTED Si. Journal de Physique Colloques, 1983, 44 (C5), pp.C5-73-C5-76.

�10.1051/jphyscol:1983510�. �jpa-00223090�

JOURNAL DE PHYSIQUE

Colloque C5, supplement au nO1O, Tome 44, octobre 1983 page C5-73

THERMAL CONDUCTIVITY MEASUREMENTS OF IMPLANTED Si

T . Papa, F . s c u d i e r i r , M . ~ a r i n e l l i " , U . zarnmiprand G. cembalirr

I s t i t u t o d i F i s i c a , Fac. d i I n g e g n e r i a , l h z i v e r s i t d d i Roma m d GNSM-CNR, I t a l y + ~ s t i t u t o d i F i s i c a , Fac. d i I n g e g n e r i a , 2a Universita' d i Roma and GNEQP-CNR, I t a l y

r r ~ ~ A B ~ - ~ ~ ~ , Bologna, I t a l y

Resume

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Abstract - We have measured photoacoustic-signal phase of ion implanted and evaporated Si. The values of thermal conductivity of amorphous layers have been determined by a two layer

theoretical model.

The purpose of the present work is to investigate the thermal

conductivity of ion implanted and evaporated silicon layers, because this quantity has an important role in the annealing processes of such layers by laser radiation. In fact our previous works demonstrate that the photoacousticeffect enables usto obtain a direct measureof such a quantity by studying the photoacoustic behaviour of implanted and evaporated silicon in comparison with the crystalline one /1,2/.

The experimental set-up is the conventional one, that is the gas- microphone method. The sample, placed inside a photoacoustic cell is illuminated, through a quartz window, by an ~ r + laser radiation. The radiation beam whose power is a few tens of mW, is modulated by a mechanical chopper at frequencies ranging from 400 to 1000 Hz and then focused on the sample surface. The photoacoustic signal is detected by a Bruel and Kjaer 4166 microphone and processed by means a Brookdeal 9505 lock-in amplifier.

In order to compare the photoacoustic response from implanted silicon with the crystalline one we used samples which present an unimplanted

zone. In this way by scanning the sample with a radiation beam we have, at the boundary between two parts, a sudden change in the signal

amplitude andin the phase. We observe that the signal amplitude does not show a remarkable dependence on the thermal conductivity. Its behaviour is shown in Fig.1 where we observe, in log-log plot a slope like u-' in the low frequency range. The phase, on the contrary, is very sensitive to the thermal properties, and consequently we prefer to study this parameter; moreover it is independent from the reflec- tivity and from the efficiencies of non radiative processes in both implanted and crystalline material. The phase behaviour vs.frequency relatively to fig.1 is shown in fig.2. We note furthermore that in our experiment no absolute calibration for the phase lag is necessary since the phase difference between crystalline and amorphous regions has to be measured.

We have studied the following samples:

i) Si (110) , B doped, 9 > 2000 fl cm, implanted at room temperature with Si ions at a total dose of 10I5/cm2 and 5. 10I5/cm2. The ions

energies were 80 KeV, 150 KeV, 230 KeV to which correspond implanted

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983510

C5-74 JOURNAL DE PHYSIQUE

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layer thicknesses of 1560 A, 3100 A, 4700 A respectively.

ii) A sample implanted with various ions energies: 80 KeV, 150 KeV and 230 KeV at a total dose of 10I5/cm2 and 5.10I5/cm2 in order to obtain a more uniform damaged layer.

iii)A samgle of amorphous Si evaporated onto a Si (111) substrate 7400 A thick.

frequency (HZ)

Fig. 1 - Photoacous$ic signal amplitude vs. modulation frequency for a sample with 3100 A thick implanted layer.

frequency ( H Z ) Pig. 2 - Phase vs. modulation frequency relatively to Fig.1.

fn the context of theory of Mandelis et a1./3/ which gives the photo- acoustic response for two layer samples with different optical and thermal properties, we have compared our experimental results using eq. (18) of ref. (3) assu ing the following numerical values: density !?

Yc

T

optrcal absofptioy- coef f icient at 2,5 eV zadiaeion energy. 6 = 10 cm ;

f i = 1,5.10 cm , thermal conductivity kc = 1 .49 W/cm K. ~ g e

subscript c and a refer to crystalline and amorphous samples respec- a tively. We have assumed that the optical absorption coefficient value of implanted layers is that of amorphous silicon.

In general it depends on the method of preparation, but in any case, at the used laser wavelength it is surely an order of magnitude higher than the crystalline silicon.

The obtained results at 400 and 1000 Hz of chopping frequency are sum- marized in table 1.

TABLE I

Ion energy 80 KeV 150 KeV

Implanted o o

thickness 1 5 6 0 A 3 1 0 0 A

230 XeV 80+150+230 ~ o s e (cm-2) f (Hz) KeV

0

Evaporated (7400 A) at 250°C: Ka = -018 W/cmK at 400 Hz and 1000 Hz.

From the results we can note that, at a fixed dose, the difference between the data at different frequencies lies within the

experimental errors. This means that at the used chopping frequencies the implanted layer appears homogeneous and the presence of the backing is not relevant. Concerning the dependence on the dose we note that the Ka values at 5.10I5/cm2 are always smaller than the ones at 10I5/cm2: this means that the higher dose produces a better amorphized layer. Thebehavior ofKa valuesvs. the energyof theions shows that increasing the implantation energy the damaged layer cannot be considered homogeneous. Such a result is also confirmed in the case of the sample ii) where the multiple implantation (every one at 0,33 of the total dose) is not sufficient to give a completely amorphous layer.

Finally of a particular intere3t is the comparison of the K values obtained for 80 KeV, 5. lo1 5/cm , and the evaporated one: thg implanted sample exhibits a type of damage more intense than in the case of evaporated layer; therefore it may be considered as a better amorphous than the evaporated one.

In order to check the quality of the amorphous layer, we have examined the samples by electron diffraction. The diffraction pattern for a230 KeV irradiated sample presents some evident crystalline aspects. This circumstance can explain the higher value of measured thermal conduc- tivity. For the sample irradiated at various energy ions (80+150+230 KeV) the electron diffraction pattern shows a good amorphous aspect, but a deeper crystalline contribution exists. In fact the diffraction pattern refers only to a very thin surface layer than is entirely amorphous (deeper layers one not amorphized because the used dose at each energy is too low).

In conclusion, the photoacoustic effect represents a simple and useful technique for measuring thermal conductivity of thin layers and, in particular, to check the quality of amorphous materials. In addition our results allows us to assert that the values found for such layers

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are similar t o thermal conductivity of vitreous materials.

REFERENCES

1 . T.Papa,F.Scudieri

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2. T.Papa,D.Sette,F.Scudieri

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g ,

3 . A.Mandelis, Y.C.Teng and B.S.H.Royce - J.Appl.Phys. 50, (1979)7138.