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Heat pulse propagation in Si substrates after YBCO laser ablation
ICEC 15 Proceedings
Heat P u l s e P r o p a g a t i o n I n S1 S u b s t r a t e s A f t e r YBCO L a s e r A b l a t i o n
M.M.Bonch-Osmolovskll, T.I.Galklna, A.Yu.Klokov, E.E.Onlshchenko, A.I.Sharkov. Solld State Department, P.N.Lebedev Physical Institute, Academy of Sclences of Russia; 117924, Lenlnskli pr.53, Moscow, Russia The propagation of heat pulses produced in thln silicon wafers by photoexcltatlon is investigated before and after the YBCO laser ablation. The Monte-Carlo simulation and analysis of the heat pulse shape led to conclusion about the introducing into the Si substrate of the acceptor type additional centers in the process of laser ablation. These centers turn to be phonon scatterers and therefore change the heat transfer velocity.
INTRODUCTION
The procedure of constructing the discrete elements of hybrld electronics (HTS/slllcon) requires the knowledge of the influence of HTS-manufacturing process on the silicon substrate properties : in particular - on the velocity of heat transfer. The HTS-fllnVslllcon substrate manufacturing technology (laser ablation) involves the hlgh temperature annealing in the oxygen atmosphere. Such process is known to introduce into the crystal various imperfections, which could affect on the heat transfer through the Si substrate (e.g. when one needs to dissipate the energy stored by some way (light or Joule heating) in YBCO-fllm). In order to estimate this effect we have studied the heat pulses propagation in various [100] SI substrates: in pure (p -~ 8 kOhm.cm) and industrial crystals (p = 2.50hm.cm) before as well as after YBCO deposition onto the SI substrate by laser ablation.
EXPERIMENTS AND RESULTS. The geometry of experiment is shown on the Flg. I.
The
heat
pulses
Cryogenics 1994Vol 34 ICEC Supplement 855
ICEC 15 Proceedings
were generated in the process of photolnduced carriers cooling (the last being produced by the laser pulse, ~ = 0.34 mcm, At = 7.5 ns, f = 100 Hz), and registered by the thin fllm bolometer (size 0 . 3 5 x 0 . 5 0 m m 2) of granular AI [ 1 ] . Fig. 2 shows the bolometer response to the heat pulse for following experiments : curve I - the response for the case of heat pulse arrival across the sample of pure SI (thickness = 0.4 mm) at low level of incident energy. Curve 2 belongs to the industrial Si and the broadening of the Curve 2 as compared to the Curve 1 is connected with the increase of impurity concentration. Curve 3 belongs to the same sample (as for the curve 2), but exposed to the YBCO-flIm deposition. Later on the YBCO-flIm was removed. The broadening of the Curve 3 as compared to Curve 2 we relate to the additional phonon scattering by the impurities or thermodefects introduced in the substrate in the process of laser epltaxy. The drop of p occurs from 2.50hm.cm to 0.80hm, cm, that Is - about 1016 acceptors centers were introduced (the sample remained p-type). The technological process developed in [2] requires the temperature regime = 800°C in oxygen for a half of hour. The possible mechanisms of additional defects are as follows : first, the impurities can diffuse from the buffer YSZ layer; second, at high temperature thermoacceptors can be introduced in the Si sample due to diffusion of Cu from YBaCuO film. One should notice that the low level of excitation was realized in our experiments. So at low level of excitation the nonequillbrlum acoustic phonons take part in two processes - spontaneous an_harmonlc decay and elastic collision on the point defects.
MO}~EE-CARLO SIMULATION UslngMonte-Carlo method we develop the model for the semlquantatlve explanation describing the heat pulse (nonequilibrium acoustic phonon) propagation in our real experimental geometry and obtaining the tlme-resolved response. As a fitting parameter in our calculation a constant A serves for describing the crystal imperfection (the mean free time relative to the elastic scattering ~s depends on phonon frequency ~ in the case of point defect scattering as ~ I = A.~4). In the case of isotopic scattering the value of A is equal to 2.43,10-42s 3 [3]. If one has the additional scattering centers the magnitude of A increases and consequently the mean free lifetime decreases. For comparison with experiment the experimental bolometer 856 Cryogenics 1994Vo134 ICECSupplement
ICEC 15 Proceedings
1.0
0 ~D
~0.5
.ha o 0
0.0
)~
2~0'
'
4~0
Time
6~0
8i )0
ns
,
Flgure 2. The experimental bolometer response. I. Pure $1 (p = 8 kOhm.cm). Pulse energy E = 1.4.nJ; density P = 15 nJ/mm 2, the laser spot dlameter D = 2 0 0 m c m ~ the response FWHM At = 80 ns; 2. Industrial
SI
(p = 2 . 5 0 b m . c m ) .
Before
the
deposition
of
YBCO-flIm. E = 10 nJ; P = 400 nJ/mm2; D = 160 mcm; At = 300 ns; 3. Industrial SI. After the deposition of ~ C O bolometer (p of Sl substrate droped from 2.50hm,cm to 0 . 8 0 h m - c m ) . E = 10 nJ; P = 10 pJ/mm2; D = 40 mcm; At = 400 ns. t.0
~ 0.5 -
0.0 (~
260
460 Time
Figure 3. Calculated I. 2. f I. t 2.
The The The The
and
660 ,
experimental
experimental curve for experimental curve for calculated curve for A calculated curve for A
860
ns
bolometer
response.
pure Sl. industrial SI. = 2.43.10 -42 s 3. = 6.16.10 -42 s 3.
Cryogenics 1994Vol 34 ICEC Supplement 8 5 7
ICEC 15 Proceedings
responses are shown In the Flg. 3 for pure (1) and industrial S1 (p ~ 2.50hm.cm) before YBaCuO deposition (2), and wlth points - the t calculated bolometer responses for A = 2.43.10-42s 3 (I) and A = 6.16.10-42s 3 (imperfect silicon) (2'). One can see that the model taking into account acoustic phonon decay and elastic collision flt the experimental curve at least wlth respect to two maln response characteristics : full width at half maximum height and shift of the maximum due to increase of scattering centers.
CONCLUSION
The reason of change of heat transfer veloclty is the introduction of addltlonal defects in $1 substrate in the process of YBaCuO deposition. We thank E.Pechen and S.Krasnosvobotsev for preparation YBCO/YSZ/SI structures and P.Greenstein for useful remarks.
REFERENCES 1. Blinov A.Yu. Bonch-Osmolovskll M.M. Galklna T.I. Danllchenko B.A. Rozhko S.Kh. Superconductive thin film bolometers with controlled width of superconducting transition Kratkle Soobschenl~a 2£ Fizike FIAN (1989) [ 31-33 (Soy. Phys. Lebedev Inst. Reports (1989) 7 31-33)
2. Krasnosvobodtsev S.l. Pechen E.V. YBaOuO thin films on SI substrate P_~slca C (1991) 185-189 2097-2098 3. Tamura S. Shields J. Wolfe J.P. Lattice dynamics and elastic phonon scattering In silicon P_~s.Rev.B (1991) 44 7 3001-3011
l
Monitoring I system I Pream. PAR M-115I BOXCAR PARM-150J
Time
Nitrogen laser
I
o,,o
Figure 1. Experimental setup for heat pulse measurement. 858 Cryogenics 1994 Vo134 ICECSupplement
Heat P u l s e P r o p a g a t i o n I n S1 S u b s t r a t e s A f t e r YBCO L a s e r A b l a t i o n
M.M.Bonch-Osmolovskll, T.I.Galklna, A.Yu.Klokov, E.E.Onlshchenko, A.I.Sharkov. Solld State Department, P.N.Lebedev Physical Institute, Academy of Sclences of Russia; 117924, Lenlnskli pr.53, Moscow, Russia The propagation of heat pulses produced in thln silicon wafers by photoexcltatlon is investigated before and after the YBCO laser ablation. The Monte-Carlo simulation and analysis of the heat pulse shape led to conclusion about the introducing into the Si substrate of the acceptor type additional centers in the process of laser ablation. These centers turn to be phonon scatterers and therefore change the heat transfer velocity.
INTRODUCTION
The procedure of constructing the discrete elements of hybrld electronics (HTS/slllcon) requires the knowledge of the influence of HTS-manufacturing process on the silicon substrate properties : in particular - on the velocity of heat transfer. The HTS-fllnVslllcon substrate manufacturing technology (laser ablation) involves the hlgh temperature annealing in the oxygen atmosphere. Such process is known to introduce into the crystal various imperfections, which could affect on the heat transfer through the Si substrate (e.g. when one needs to dissipate the energy stored by some way (light or Joule heating) in YBCO-fllm). In order to estimate this effect we have studied the heat pulses propagation in various [100] SI substrates: in pure (p -~ 8 kOhm.cm) and industrial crystals (p = 2.50hm.cm) before as well as after YBCO deposition onto the SI substrate by laser ablation.
EXPERIMENTS AND RESULTS. The geometry of experiment is shown on the Flg. I.
The
heat
pulses
Cryogenics 1994Vol 34 ICEC Supplement 855
ICEC 15 Proceedings
were generated in the process of photolnduced carriers cooling (the last being produced by the laser pulse, ~ = 0.34 mcm, At = 7.5 ns, f = 100 Hz), and registered by the thin fllm bolometer (size 0 . 3 5 x 0 . 5 0 m m 2) of granular AI [ 1 ] . Fig. 2 shows the bolometer response to the heat pulse for following experiments : curve I - the response for the case of heat pulse arrival across the sample of pure SI (thickness = 0.4 mm) at low level of incident energy. Curve 2 belongs to the industrial Si and the broadening of the Curve 2 as compared to the Curve 1 is connected with the increase of impurity concentration. Curve 3 belongs to the same sample (as for the curve 2), but exposed to the YBCO-flIm deposition. Later on the YBCO-flIm was removed. The broadening of the Curve 3 as compared to Curve 2 we relate to the additional phonon scattering by the impurities or thermodefects introduced in the substrate in the process of laser epltaxy. The drop of p occurs from 2.50hm.cm to 0.80hm, cm, that Is - about 1016 acceptors centers were introduced (the sample remained p-type). The technological process developed in [2] requires the temperature regime = 800°C in oxygen for a half of hour. The possible mechanisms of additional defects are as follows : first, the impurities can diffuse from the buffer YSZ layer; second, at high temperature thermoacceptors can be introduced in the Si sample due to diffusion of Cu from YBaCuO film. One should notice that the low level of excitation was realized in our experiments. So at low level of excitation the nonequillbrlum acoustic phonons take part in two processes - spontaneous an_harmonlc decay and elastic collision on the point defects.
MO}~EE-CARLO SIMULATION UslngMonte-Carlo method we develop the model for the semlquantatlve explanation describing the heat pulse (nonequilibrium acoustic phonon) propagation in our real experimental geometry and obtaining the tlme-resolved response. As a fitting parameter in our calculation a constant A serves for describing the crystal imperfection (the mean free time relative to the elastic scattering ~s depends on phonon frequency ~ in the case of point defect scattering as ~ I = A.~4). In the case of isotopic scattering the value of A is equal to 2.43,10-42s 3 [3]. If one has the additional scattering centers the magnitude of A increases and consequently the mean free lifetime decreases. For comparison with experiment the experimental bolometer 856 Cryogenics 1994Vo134 ICECSupplement
ICEC 15 Proceedings
1.0
0 ~D
~0.5
.ha o 0
0.0
)~
2~0'
'
4~0
Time
6~0
8i )0
ns
,
Flgure 2. The experimental bolometer response. I. Pure $1 (p = 8 kOhm.cm). Pulse energy E = 1.4.nJ; density P = 15 nJ/mm 2, the laser spot dlameter D = 2 0 0 m c m ~ the response FWHM At = 80 ns; 2. Industrial
SI
(p = 2 . 5 0 b m . c m ) .
Before
the
deposition
of
YBCO-flIm. E = 10 nJ; P = 400 nJ/mm2; D = 160 mcm; At = 300 ns; 3. Industrial SI. After the deposition of ~ C O bolometer (p of Sl substrate droped from 2.50hm,cm to 0 . 8 0 h m - c m ) . E = 10 nJ; P = 10 pJ/mm2; D = 40 mcm; At = 400 ns. t.0
~ 0.5 -
0.0 (~
260
460 Time
Figure 3. Calculated I. 2. f I. t 2.
The The The The
and
660 ,
experimental
experimental curve for experimental curve for calculated curve for A calculated curve for A
860
ns
bolometer
response.
pure Sl. industrial SI. = 2.43.10 -42 s 3. = 6.16.10 -42 s 3.
Cryogenics 1994Vol 34 ICEC Supplement 8 5 7
ICEC 15 Proceedings
responses are shown In the Flg. 3 for pure (1) and industrial S1 (p ~ 2.50hm.cm) before YBaCuO deposition (2), and wlth points - the t calculated bolometer responses for A = 2.43.10-42s 3 (I) and A = 6.16.10-42s 3 (imperfect silicon) (2'). One can see that the model taking into account acoustic phonon decay and elastic collision flt the experimental curve at least wlth respect to two maln response characteristics : full width at half maximum height and shift of the maximum due to increase of scattering centers.
CONCLUSION
The reason of change of heat transfer veloclty is the introduction of addltlonal defects in $1 substrate in the process of YBaCuO deposition. We thank E.Pechen and S.Krasnosvobotsev for preparation YBCO/YSZ/SI structures and P.Greenstein for useful remarks.
REFERENCES 1. Blinov A.Yu. Bonch-Osmolovskll M.M. Galklna T.I. Danllchenko B.A. Rozhko S.Kh. Superconductive thin film bolometers with controlled width of superconducting transition Kratkle Soobschenl~a 2£ Fizike FIAN (1989) [ 31-33 (Soy. Phys. Lebedev Inst. Reports (1989) 7 31-33)
2. Krasnosvobodtsev S.l. Pechen E.V. YBaOuO thin films on SI substrate P_~slca C (1991) 185-189 2097-2098 3. Tamura S. Shields J. Wolfe J.P. Lattice dynamics and elastic phonon scattering In silicon P_~s.Rev.B (1991) 44 7 3001-3011
l
Monitoring I system I Pream. PAR M-115I BOXCAR PARM-150J
Time
Nitrogen laser
I
o,,o
Figure 1. Experimental setup for heat pulse measurement. 858 Cryogenics 1994 Vo134 ICECSupplement