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Processing effects on the electrical properties of defects in silicon
Materials ,~,cieme and Engim'erittg. B4 ( 19N 9 ) 343- 346
343
Processing Effects on the Electrical Properties of Defects in Silicon A. CASTALI)INI, D. CAVALCOI.I and A. CAVA LLINI
l)epartment of l'hysics. Univ('rsiO"oI Bologna, Via lrnerio 4(J, 1-40126 Bologna (Italy) IReceived May 31, 19St))
Abstract Minori O, carrier l!fetime and Hall effect measurements, as well as capacitance transient spectroscopy, have been used to characterize the defects induced by thermal and stress treatments in ,[loat zone n-O'pe silicon, lhe minori O, carrier l!fi,time was analysed with re,spect to the processing conditions by charge collection scanning electron mic'roscopy. Parameters related to the role of the introduced defects were evahmted from the lifetime values by a fitting procedure based on the Shockley-ReadHall theor)'; namely, the energy level, capture cross-section, trap density and dMocation occupation fi~ctor were obtained. 7he same parameters were also deduced from deep level transient spectroscopy (DL 7:S) and Hall effect measurements. In comparing the results, it was taken into account that by the l!fetime analysis only the energy level related to the most e.ficient minori O, carrier caprare centre could be found, while from DL ZS" arm Hall effect measurements several deep levels were dewcted. A hypothesis is advanced on the nature of the defects and signifi'cant d!lferences, according to the ,specimen history, are pointed out.
1. Introduction The electrical behaviour of semiconductors is greatly affected by the presence of a variety of defects introduced in the starting material by the processing required for device production. It is therefore important to analyse the influence of such defects on the electrical performance of the devices and their dependence on the characteristics of the processes. For this purpose the minority charge carrier lifetime is often chosen as a significant parameter because of its strong dependence on the type and concentration of electrically active defects. In this paper we con0921-5107/89/$3.50
sider minority carrier lifetime as well as junction spectroscopy and Hall effect measurements to characterize defect levels produced by different treatments in float zone n-type silicon. On the basis of the results obtained by the different techniques, a hypothesis on the defect parameters is advanced. 2. Experimental details Float zone silicon (5 x 10/3 P atoms cm ~) samples with (111) crystallographic orientation were bent along the [1121 direction under a stress of 47 MPa. Plastic deformation was performed at 650, 700 and 750 °C in an argon atmosphere in such a way as to produce different strains. For the lifetime measurements, performed by charge collection scanning electron microscopy, Schottky diodes were obtained by gold evaporation. Ohmic contacts were produced by an Hg-ln alloy on freshly cleaned samples to avoid high temperature alloying processes. Hall effect measurements were carried out between 15 K and room temperature. I - V and C - V characterization was used to select the Schottky diodes for electron-beaminduced current {EBIC), deep level transient spectroscopy (DLTS) and Hall effect investigations. Undeformed samples, heated under the same time and temperature conditions as the deformed ones, were also analysed. 3. Results Minority charge carrier diffusion lengths were calculated as the reciprocal slope of the semilogarithmic plot of the electron-beam-induced current I~ vs. the beam-junction distance x, by the expression 1~,~ exp( - &/L). For a direct comparison between our results and those in the literature referring to the recombination probability at traps in terms of capture time, we will © Elsevier Sequoia/Printed in The Nelherlands
344
refer to the lifetime values r calculated from L = ( rD )l/: where D is the diffusion coefficient. Defect parameters were evaluated from the lifetime values; namely, the energy level, minority carrier capture cross-section, state density and occupation factor f of the dislocation level with the dominant contribution were obtained. The influence of plastic deformation on the lifetime is shown in Fig. 1, where the experimental values of the minority carrier lifetime r referring to samples deformed at ?~t~J=650 and 700°C are reported against the dislocation density N~p obtained by etch pit counting. It is noteworthy that, while the shape of the two curves is the same, the 7~.~ effect corresponds to a shift towards lower values by increasing the bending temperature. The r trend was fitted by a procedure [1] based on the Shockley-Read-Hall (SRH) model [2] of the recombination process and the Landsberg model [3] of the doping-dependent defect concentration. A deep centre energy level at 0.18 eV from the conduction band (c.b.) resulted from the lifetime analysis. However, by this procedure the values of the occupation factors f obtained were too high to be reliable, probably because of an underestimation of the electrically active dislocation density. Moreover, the strong effect of the deformation temperature on the lifetime cannot be entirely justified. Thus we hypothesized the presence of another deep level, whose presence has been investigated by Hall effect and junction spectroscopy measurements. To point out Td~~ effects, we also examined samples processed at 750°C. DLTS spectra confirmed our hypothesis (Fig. 2) about the presence of two levels ED''li and / ~ l j 2) a t 0.18 and 0.40 eV from the c.b. respectively in highly dislocated samples bent at 700 and 750 °C. whereas only a level at 0.40 eV was observed for the lowest deformation temperature.
Diagrams of the carrier concentralion ~:n obtained by Hall effect measurements are shown in Figs. 3 and 4 as a function of 7 ~ and dislocation density resulting from etch pit counting. These curves could not bc analysed in terms of the Schr/3ter-Labusch model [41 since our specimen characteristics do not obey all its validity conditions [5]; namely, the mean free path l ol' . . . . . . . . . . . . . . .
T
-
i
Def
/50
f~J-
'/
I
\
<
100
i
200
300
T E M P E R A TUNE
fK)
Fig. 2, D L T S spectrum of a sample deformed at ?50°C, The
dislocation densityis 10~cm
__
_
AS-GRO~fN
. . . . eso"c
.... 7oo'c -750°c
!I
?
-\
[
0
10
20
30
40
IO00/T
-,50 60 (K-~
Fig. 3. Electron density vs. reciprocal temperature of asgrown material and of samples deformed at different temperatures (etch pit density greater than 10 7 cm 2).
10 [. ]
.
.
. ---
]I ]
500
--
a S-g~0~g,v U,qo. 7 ~ ac Nep l ~ c m ~ N e p 5~t~'cwf
-~, 1o 400
-~
aoo
ld' aoo i Ioo
t0 4
10 5
10 6
10 7
Nep {cm-2]
Fig. 1. Lifetime vs. etch pit density. The deformation t e m p e r a t u r e s are 650 °C (--*--) and 700 °C (--o--).
0
10
20
30 40 50 60 IO00/T [ K - Q
Fig. 4. Electron density vs. reciprocal temperature of asgrown ( - - - ) , undeformed (. . . . . ) and differently dislocated samples (treatment temperature T = 750 °C).
345
free charge carriers is larger than the Debye screening length 2. Thus the concentration diagrams have been analysed with the Putley treatment [6] for systems containing more than one type of centre by straightforward fitting in the high and low temperature ranges. A shallow level E s = 4 meV from the c.b. resulted from the low temperature data, while different deep level energy values were obtained by the fitting procedure in the high temperature range for specimens differently processed, as reported in Table 1. Here N*, the factor multiplying the exponential in the expression
/<-'%/ k T ]"
(n>=N*exp
curves, eqn. (2) can be approximated in the high temperature range by 1 N~ ND'" exp
<">=2 ,,,
N* =
[
<-<'I
1 N~ N,~Iexp
kT
+~)
kT
exp
]
J
kT
where N~, is the effective state density in the c.b., n 0 is the concentration of free charge carriers in the unperturbed crystal, and N is the concentration and E the energy of chemical donors, shallow and deep centres induced by deformation, indexed by cd, s and D respectively. Under the working conditions N~d, N , ~ N D ~c' and E c - E~ <{E c - EDc~' (E c is the bottom of the c.b.), justified by the best fitting of the experimental TABLE l
i3)
1 N~,
i, 1 N~,N~.pg ND 2 n, 2 no s
(4)
g can thus be obtained by eqn. (4) as a function of the best-fitting value of the factor N*. By taking into account this correction factor in the lifetime data analysis, f i s now equal to 0.018 and 0.01 for the 650 and 700°C deformed samples respectively, in good agreement with the data reported in the literature. 4. Discussion and conclusions
_ < - E, / + ,5, exp
]
By combining eqn. (3) with eqn. ( 1), N* is given as a function of ND'*), which is equal in turn to the ratio between the effective dislocation density Nd~ and the dangling bond distance s. Ndi~lcan be related to N~p by N,u <= N~pg (g is a correction factor, specific to the sample, which accounts for the underestimation of the dislocation density by optical observation of etch pits). Therefore
(1)
is also reported, where E d is the energy level value inferred for a good fitting. To match the lifetime measurements with the above results, we assumed the presence of two deep levels E t ) l~ and ED e', resulting in different findings depending on the material defective state. Following Sumino's model [7] applied to our experimental findings, we can write the detailed balance equation as follows:
E~,- El)" )
Halleffect data
(°C)
',eV)
(cm ')
(cm -~)
650 7(t0
().IN 0.40
2 x 1017 10:'
750
0.40
10-'"
7 x 1(1 II 4 x 10 H 4 x 1014
We can summarize our results as follows. (1) Lifetime measurements revealed the presence of a deep level at 0.18 eV from the c.b. for the 650 and 700 °C deformed samples. (2) DLTS spectra detected one level (0.40 eV) in samples deformed at 650°C and two levels (0.18 and 0.40 eV) in the 700 and 750°C deformed samples with equal dislocation density. (3) Free charge carrier concentration diagrams gave a level at 0.18 eV in the 650°C deformed samples and at 0.40 eV in the others. The differences between the lifetime results and the results of the other techniques can be explained by the fact that the first investigates minority carrier trapping while the second and third concern the majority carrier behaviour. Thus the level at 0.18 eV is more efficient as a trap for minority carriers ( O p = 7 × 10 ~4 cm-" from r data) than for majority carriers (o, = 1 x 10 ~~ cm 2 from DLTS spectra). Since lifetime and transport data have been, up to now, analysed by a one-deep-level model for limiting the fitting parameter number, information can be deduced only on the recombination centre dominant in the specific physical mechanism involved
346
in the technique. This explains quite well the transport measurement results, except for the 650 °C bent samples. The absence of the 0.18 eV level in the DLTS spectra of the 650 °C deformed specimens can be attributed to the concentration of this centre (7 x l0 II cm ~ from Hall effect data fitting), which is too low with respect to the sensitivity limit of the junction spectroscopy. Acknowledgments The authors want to thank Drs. E. Gombia and R. Mosca for carrying out the DLTS measurements.
References I A. Castaldini, D. Cavalcoli and A. Cavallini, m l-i, Mi>'era ,cd). l)~(/~'cts i, ('ryslals, We!rid Scientific, Siugapore~ 198~), pp. 3 3 5 - 3 4 0 . 2 W. Shockle\ and W. T. Read ,It., l'hvs. HEY., v7 1952 835 842. 3 t'. 1. Landsberg and G. S. Kousik. ,1. Appl. l'h3:s., 5(~ :~ 1984) 1 6 9 6 - 1 7 0 0 , 4 W. Schr6ter and R. Labusch, I'h;*. 5tatu.s Solidi, ,¢r~ ( 19691 539 550. 5 Vv~ Schriiter, ./. Phys. (t'arisi, ('olloq. ((~, ,5uppl, N6. 40 ,197t))6 51. 6 E. H. Pulley, The ttall l:[]bct atld Rehlled PhenomeH, Bulterworlhs, L o n d o n , 1 9 6 0 7 H. O n o and K. Sumino, ,1, ,qM~l. I'hy.~., 54 (,~I (1983 H 2 6 - 4432.
343
Processing Effects on the Electrical Properties of Defects in Silicon A. CASTALI)INI, D. CAVALCOI.I and A. CAVA LLINI
l)epartment of l'hysics. Univ('rsiO"oI Bologna, Via lrnerio 4(J, 1-40126 Bologna (Italy) IReceived May 31, 19St))
Abstract Minori O, carrier l!fetime and Hall effect measurements, as well as capacitance transient spectroscopy, have been used to characterize the defects induced by thermal and stress treatments in ,[loat zone n-O'pe silicon, lhe minori O, carrier l!fi,time was analysed with re,spect to the processing conditions by charge collection scanning electron mic'roscopy. Parameters related to the role of the introduced defects were evahmted from the lifetime values by a fitting procedure based on the Shockley-ReadHall theor)'; namely, the energy level, capture cross-section, trap density and dMocation occupation fi~ctor were obtained. 7he same parameters were also deduced from deep level transient spectroscopy (DL 7:S) and Hall effect measurements. In comparing the results, it was taken into account that by the l!fetime analysis only the energy level related to the most e.ficient minori O, carrier caprare centre could be found, while from DL ZS" arm Hall effect measurements several deep levels were dewcted. A hypothesis is advanced on the nature of the defects and signifi'cant d!lferences, according to the ,specimen history, are pointed out.
1. Introduction The electrical behaviour of semiconductors is greatly affected by the presence of a variety of defects introduced in the starting material by the processing required for device production. It is therefore important to analyse the influence of such defects on the electrical performance of the devices and their dependence on the characteristics of the processes. For this purpose the minority charge carrier lifetime is often chosen as a significant parameter because of its strong dependence on the type and concentration of electrically active defects. In this paper we con0921-5107/89/$3.50
sider minority carrier lifetime as well as junction spectroscopy and Hall effect measurements to characterize defect levels produced by different treatments in float zone n-type silicon. On the basis of the results obtained by the different techniques, a hypothesis on the defect parameters is advanced. 2. Experimental details Float zone silicon (5 x 10/3 P atoms cm ~) samples with (111) crystallographic orientation were bent along the [1121 direction under a stress of 47 MPa. Plastic deformation was performed at 650, 700 and 750 °C in an argon atmosphere in such a way as to produce different strains. For the lifetime measurements, performed by charge collection scanning electron microscopy, Schottky diodes were obtained by gold evaporation. Ohmic contacts were produced by an Hg-ln alloy on freshly cleaned samples to avoid high temperature alloying processes. Hall effect measurements were carried out between 15 K and room temperature. I - V and C - V characterization was used to select the Schottky diodes for electron-beaminduced current {EBIC), deep level transient spectroscopy (DLTS) and Hall effect investigations. Undeformed samples, heated under the same time and temperature conditions as the deformed ones, were also analysed. 3. Results Minority charge carrier diffusion lengths were calculated as the reciprocal slope of the semilogarithmic plot of the electron-beam-induced current I~ vs. the beam-junction distance x, by the expression 1~,~ exp( - &/L). For a direct comparison between our results and those in the literature referring to the recombination probability at traps in terms of capture time, we will © Elsevier Sequoia/Printed in The Nelherlands
344
refer to the lifetime values r calculated from L = ( rD )l/: where D is the diffusion coefficient. Defect parameters were evaluated from the lifetime values; namely, the energy level, minority carrier capture cross-section, state density and occupation factor f of the dislocation level with the dominant contribution were obtained. The influence of plastic deformation on the lifetime is shown in Fig. 1, where the experimental values of the minority carrier lifetime r referring to samples deformed at ?~t~J=650 and 700°C are reported against the dislocation density N~p obtained by etch pit counting. It is noteworthy that, while the shape of the two curves is the same, the 7~.~ effect corresponds to a shift towards lower values by increasing the bending temperature. The r trend was fitted by a procedure [1] based on the Shockley-Read-Hall (SRH) model [2] of the recombination process and the Landsberg model [3] of the doping-dependent defect concentration. A deep centre energy level at 0.18 eV from the conduction band (c.b.) resulted from the lifetime analysis. However, by this procedure the values of the occupation factors f obtained were too high to be reliable, probably because of an underestimation of the electrically active dislocation density. Moreover, the strong effect of the deformation temperature on the lifetime cannot be entirely justified. Thus we hypothesized the presence of another deep level, whose presence has been investigated by Hall effect and junction spectroscopy measurements. To point out Td~~ effects, we also examined samples processed at 750°C. DLTS spectra confirmed our hypothesis (Fig. 2) about the presence of two levels ED''li and / ~ l j 2) a t 0.18 and 0.40 eV from the c.b. respectively in highly dislocated samples bent at 700 and 750 °C. whereas only a level at 0.40 eV was observed for the lowest deformation temperature.
Diagrams of the carrier concentralion ~:n obtained by Hall effect measurements are shown in Figs. 3 and 4 as a function of 7 ~ and dislocation density resulting from etch pit counting. These curves could not bc analysed in terms of the Schr/3ter-Labusch model [41 since our specimen characteristics do not obey all its validity conditions [5]; namely, the mean free path l ol' . . . . . . . . . . . . . . .
T
-
i
Def
/50
f~J-
'/
I
\
<
100
i
200
300
T E M P E R A TUNE
fK)
Fig. 2, D L T S spectrum of a sample deformed at ?50°C, The
dislocation densityis 10~cm
__
_
AS-GRO~fN
. . . . eso"c
.... 7oo'c -750°c
!I
?
-\
[
0
10
20
30
40
IO00/T
-,50 60 (K-~
Fig. 3. Electron density vs. reciprocal temperature of asgrown material and of samples deformed at different temperatures (etch pit density greater than 10 7 cm 2).
10 [. ]
.
.
. ---
]I ]
500
--
a S-g~0~g,v U,qo. 7 ~ ac Nep l ~ c m ~ N e p 5~t~'cwf
-~, 1o 400
-~
aoo
ld' aoo i Ioo
t0 4
10 5
10 6
10 7
Nep {cm-2]
Fig. 1. Lifetime vs. etch pit density. The deformation t e m p e r a t u r e s are 650 °C (--*--) and 700 °C (--o--).
0
10
20
30 40 50 60 IO00/T [ K - Q
Fig. 4. Electron density vs. reciprocal temperature of asgrown ( - - - ) , undeformed (. . . . . ) and differently dislocated samples (treatment temperature T = 750 °C).
345
free charge carriers is larger than the Debye screening length 2. Thus the concentration diagrams have been analysed with the Putley treatment [6] for systems containing more than one type of centre by straightforward fitting in the high and low temperature ranges. A shallow level E s = 4 meV from the c.b. resulted from the low temperature data, while different deep level energy values were obtained by the fitting procedure in the high temperature range for specimens differently processed, as reported in Table 1. Here N*, the factor multiplying the exponential in the expression
/<-'%/ k T ]"
(n>=N*exp
curves, eqn. (2) can be approximated in the high temperature range by 1 N~ ND'" exp
<">=2 ,,,
N* =
[
<-<'I
1 N~ N,~Iexp
kT
+~)
kT
exp
]
J
kT
where N~, is the effective state density in the c.b., n 0 is the concentration of free charge carriers in the unperturbed crystal, and N is the concentration and E the energy of chemical donors, shallow and deep centres induced by deformation, indexed by cd, s and D respectively. Under the working conditions N~d, N , ~ N D ~c' and E c - E~ <{E c - EDc~' (E c is the bottom of the c.b.), justified by the best fitting of the experimental TABLE l
i3)
1 N~,
i, 1 N~,N~.pg ND 2 n, 2 no s
(4)
g can thus be obtained by eqn. (4) as a function of the best-fitting value of the factor N*. By taking into account this correction factor in the lifetime data analysis, f i s now equal to 0.018 and 0.01 for the 650 and 700°C deformed samples respectively, in good agreement with the data reported in the literature. 4. Discussion and conclusions
_ < - E, / + ,5, exp
]
By combining eqn. (3) with eqn. ( 1), N* is given as a function of ND'*), which is equal in turn to the ratio between the effective dislocation density Nd~ and the dangling bond distance s. Ndi~lcan be related to N~p by N,u <= N~pg (g is a correction factor, specific to the sample, which accounts for the underestimation of the dislocation density by optical observation of etch pits). Therefore
(1)
is also reported, where E d is the energy level value inferred for a good fitting. To match the lifetime measurements with the above results, we assumed the presence of two deep levels E t ) l~ and ED e', resulting in different findings depending on the material defective state. Following Sumino's model [7] applied to our experimental findings, we can write the detailed balance equation as follows:
E~,- El)" )
Halleffect data
(°C)
',eV)
(cm ')
(cm -~)
650 7(t0
().IN 0.40
2 x 1017 10:'
750
0.40
10-'"
7 x 1(1 II 4 x 10 H 4 x 1014
We can summarize our results as follows. (1) Lifetime measurements revealed the presence of a deep level at 0.18 eV from the c.b. for the 650 and 700 °C deformed samples. (2) DLTS spectra detected one level (0.40 eV) in samples deformed at 650°C and two levels (0.18 and 0.40 eV) in the 700 and 750°C deformed samples with equal dislocation density. (3) Free charge carrier concentration diagrams gave a level at 0.18 eV in the 650°C deformed samples and at 0.40 eV in the others. The differences between the lifetime results and the results of the other techniques can be explained by the fact that the first investigates minority carrier trapping while the second and third concern the majority carrier behaviour. Thus the level at 0.18 eV is more efficient as a trap for minority carriers ( O p = 7 × 10 ~4 cm-" from r data) than for majority carriers (o, = 1 x 10 ~~ cm 2 from DLTS spectra). Since lifetime and transport data have been, up to now, analysed by a one-deep-level model for limiting the fitting parameter number, information can be deduced only on the recombination centre dominant in the specific physical mechanism involved
346
in the technique. This explains quite well the transport measurement results, except for the 650 °C bent samples. The absence of the 0.18 eV level in the DLTS spectra of the 650 °C deformed specimens can be attributed to the concentration of this centre (7 x l0 II cm ~ from Hall effect data fitting), which is too low with respect to the sensitivity limit of the junction spectroscopy. Acknowledgments The authors want to thank Drs. E. Gombia and R. Mosca for carrying out the DLTS measurements.
References I A. Castaldini, D. Cavalcoli and A. Cavallini, m l-i, Mi>'era ,cd). l)~(/~'cts i, ('ryslals, We!rid Scientific, Siugapore~ 198~), pp. 3 3 5 - 3 4 0 . 2 W. Shockle\ and W. T. Read ,It., l'hvs. HEY., v7 1952 835 842. 3 t'. 1. Landsberg and G. S. Kousik. ,1. Appl. l'h3:s., 5(~ :~ 1984) 1 6 9 6 - 1 7 0 0 , 4 W. Schr6ter and R. Labusch, I'h;*. 5tatu.s Solidi, ,¢r~ ( 19691 539 550. 5 Vv~ Schriiter, ./. Phys. (t'arisi, ('olloq. ((~, ,5uppl, N6. 40 ,197t))6 51. 6 E. H. Pulley, The ttall l:[]bct atld Rehlled PhenomeH, Bulterworlhs, L o n d o n , 1 9 6 0 7 H. O n o and K. Sumino, ,1, ,qM~l. I'hy.~., 54 (,~I (1983 H 2 6 - 4432.