- Email: [email protected]
Electronic levels and properties of the selfinterstitials in irradiated silicon
Physics Letters A 166 (1992) 40—42 North-Holland
PHYSICS LETTERS A
Electronic levels and properties of the selfinterstitials in irradiated silicon Kh.A. Abdullin, B.N. Mukashev, M.F. Tamendarov and T.B. Tashenov Physical—Technical Institute, Academy of Sciences of the Kazakh SSR, Alma-Ala 480082, Ka~akhszan Received 7 June 1991; accepted for publication 3 April 1992 Communicated by J.l. Budnick
DLTS studies of the electronic states and properties of irradiated p-type silicon are performed. The injection-enhanced annealing is observed for two traps E2(E~—0.39eV) and El(E~—0.26eV). The annealing of E2 and El are simultaneous with the growth of the hole traps connected with V, C, and Al~It is concluded that the observed electronic levels belong to the different 23p) /Si~(3~2) and El = Si? ( 3s23p2 ) /Si~(3s23p). The conclusion is confirmed charge of silicon selfinterstitial E2 =ofSi~ by the states differences of the cross-sections the(3s formation of Al, and C, as well as by the influence of the zero and reverse bias conditions on the introduction rate and optical annealing ofthe E2 state.
It is well established that the selfinterstitial in irradiated p-type silicon is unstable and migrates even at temperatures <4.2 K until it is trapped by impurities. As a result, the shallow acceptors as well as carbon are injected from substitutional to interstitial sites [1,21. This is why isolated silicon selfinterstitials have eluded detection. In this report on the basis of the injection-enhanced annealing and growth of defect states as well as of the influence of reverse bias conditions on the introduction rate and injection annealing of defects it is concluded that the observed E2 and El minority carrier traps belong to different charge states of the silicon selfinterstitial. The experimental details have been described in our previous paper [31, where we have presented data concerning the main properties of the E2 (E~—0.39 eV) center. This center has appeared in all a-irradiated (4.7 MeV) samples irrespective of the kind of acceptor impurities in both Czochralskigrown (CG) and float zone (FZ) crystals. The amplitude and the rate of annealingof the E2 (0.39) level depends on the minority carrier injection level as well as on the concentration of carbon and oxygen impurities. The annealing of E2 (0.39) (fig. 1) increases the concentration of the H2(E~+0.29.C,) state. It is supposed that E2 is positively charged and during the filling pulse captures electrons. The charge 40
transfer can take place also due to thermal transition of electrons from the valence band to the E2(0.39) level at temperatures >330 K, at which the temperature defect is annealed out. Therefore, it is not introduced at temperatures 330 K. It is suggested that the defect anneals due to the charge transfer mechanism or due to the saddle point (Bourgoin— Corbett) mechanism. The E2(0.39) defect is tentatively attributed to the Si~/Si~donor state. We have studied the origin of E2(0.39) in detail. It is found that optical injection greatly changes the DLTS spectra of irradiated samples (fig. 2). The new El (E~—0.25+0.05 eV) state is resolved by substraction of curve 6 (after annealing) from curve I (as irradiated) (fig. 2). The electrical injection annealing studies show that the El and E2 states are annealed out at the same time due to minority carrier injection. The annealing of El and E2 levels is simultaneous with the growth of H2 (C,) and H3 (Al (fig. 3). The H3 levels are observed only in irradiated Al-doped material. It is attributed to the Al~/Al~[4] state because the defect has the same annealing characteristic and temperature scale position as well as a small and temperature dependent hole-capture cross-section. Note that the cross-section for production of H3 (Al,) is approximately 100 times larger than that of H2 (C,). Indeed, in spite of ‘—
ElsevLer Science Publishers By.
Volume 166, number 1
PHYSICS LETTERS A
8 June 1992 100
‘150
I
200
I
17’K
U
/
S~-B-FZ-SB H2
.
a2St
~
fQ39
~I.
IR\ R
-n t’ 0
~
St
/
7
‘
/
~1
‘I
I
+ L
____
d
6 1~ It
_______________________
5
(f,
~
El,
1
-J
_J
2 (I)
/
I
_J 400
150
2
.-—
‘~g
200
-J
*
E2
Fig. 1. DLTS spectrum of the a-irradiated p-Si: B (*) under low level injection conditions. Shown also are successive injection annealings. The inset shows the proposed configuration coordinate diagram for different charge states of Si,.
Fig. 2. DLTS spectrum ofa-irradiated p-Si: B recorded under optical excitation through a window in the back contact. (1) Initial spectrum, (2)—(6) after forward bias injection at 190 K, (7) spectrum 1 after subtraction ofspectrum 6.
the fact that the concentration of Al in the initial samples is 20 times smaller than that of carbon, the amplitude the H3 state is 5 times larger than that of H2 (fig.of3).
____________________________ —CG — 4c~~3 SB NAe:9.1Ol N~=2lO16crrC3
sL-~e
The additional data about the properties ofthe E2
V-V
defect have been obtained by studies of low tern-
perature optical injection annealing and of the influence of the reverse bias on the introduction rate and annealing of this state. The five and twenty second optical injection at 175 K introduced the H4(E~+0.l3 eV) vacancy state, enhanced the forrnation of H2(C 1) and slightly decreased the Hl(VV) concentration (fig. 4). The E2 center did not anneal during optical injection and was not introduced at > 200 K, if the samples were under reverse bias, and finally the rate of its injection an nealing is the same over the temperature range 77—
300 K. We propose a model to explain these experimental
observations. According to this model there are three
J
100-____________________________ 150 200
T,K Fig. 3. (1) DLTS spectrum of a-irradiated p-Si: Al. (2) After injection at 250 K and (3) after the heat treatment at 300 K.
41
Volume 166, number 1
PHYSICS LETTERS A
________________________
St-B-CC-SB p=fl&4c~n3 ~ H2
v-v
i fl i
8 June 1992
between the cross-section of the formation of Al, and that of C, (GA= lOOo~-).The influence ofthe applied reverse bias on the optical annealing and the ternE2 state may be easily explained also. The reverse bias removes the minority carriers from the depletion region of the junction thereby preventing changing the charge state of E2. The charge transfer can take place due to the thermal generation of electrons perature and the transition dependence to the of the E2(0.39) introduction state atrate -..330 of the K. is reduced to 200 K if the samples are irradiated However, the temperature of the thermal transition by a-particles under reverse bias, because the E2 centers are filled by electrons and their transitions from the level to the conduction band begins from 200 K. At a low temperature K), in irradiated n-type silicon, immediately after(77 irradiation, a small E(0.12.
H’t
-~
£ 100
125
150
175
T,K Fig. 4. (1) DLTS spectrum ofa-irradiated p-Si: B. Shown are the annealings under the near-band-gap illumination 2 s during (2)light 5 sand (3) 20 5. with the fluence 1018 photon/cm
different charge states of the selfinterstitial in irradiated p-type silicon crystals: Si~~ (3s2), Si~(3s23p) and Si~(3s23p2) which correspond to the E2.( +/2+) and El (0/ +) electronic levels. At least two of them, Si~and Sit, are saddle points of each other (we do not have enough information to discuss the annealing mechanism of the Si, (0/+) state). Fig. I displays the configuration coordinate diagram which explains the intensive electrical and optical annealing of the E2 state via the athermal Bourgoin—Corbett mechanism. Note that the same rate of injection annealing in the temperature range 77—300 K confirms the athermal migration mechanism. Low ternperature optical injection causes the migration of selfinterstitials and their interaction with divacancies (Si,+VV—*V). As a result one can observe the appearance of the H4(V) state due to secondary processes in the samples, irradiated at 273 K. The Coulomb interaction between Si~ and substitutional Al~enhances the injection of Al~into the interstitial site. Therefore one can observe a large difference
42
C 1) signal is observed [5]. Moreover, the rate of the appearance of the state is injection-enhanced, growing significantly at 77 K when the diode is forward biased.confirmation We believe that theseexistence data are of additional indirect of the selfinterstitials in irradiated silicon. In summary, we have presented several experimental indications of the existence of selfinterstitials in irradiated silicon. We have concluded that the observed electronic levels belong to the different charge states of the selfinterstitial: E2(0.39, Si~(3s23p)/ Si~~ (3~2)) and El (0.25, Si~(3s23p2)/Si~ (3s23p)). However, further studies are required to get information about the symmetry of the defect and to obtam additional direct evidence of the existence ofthe isolated selfinterstitial in irradiated silicon.
References [11 G.D. Watkins, Radiation damage in semiconductors (1964) ~. 9~l. [2] GD. Watkins, Phys. Rev. B 12 (1975) 5824. [31Kii.A. Abdullin. B.N. Mukashev, M.F. Tamendarov and T.B. Tashenov, Phys. Lett. A 144 (1990) 198. 141 J.R. Troxell, A.P. Chatterjee, G.D. Watkins and L.C. Kimerling, Phys. Rev. B 19 (1979) 5336. [51L.C. Kimerling, P. Blood and W.M. Gibson, Inst. Phys. Conf. Ser. 46 (1979) 273.
PHYSICS LETTERS A
Electronic levels and properties of the selfinterstitials in irradiated silicon Kh.A. Abdullin, B.N. Mukashev, M.F. Tamendarov and T.B. Tashenov Physical—Technical Institute, Academy of Sciences of the Kazakh SSR, Alma-Ala 480082, Ka~akhszan Received 7 June 1991; accepted for publication 3 April 1992 Communicated by J.l. Budnick
DLTS studies of the electronic states and properties of irradiated p-type silicon are performed. The injection-enhanced annealing is observed for two traps E2(E~—0.39eV) and El(E~—0.26eV). The annealing of E2 and El are simultaneous with the growth of the hole traps connected with V, C, and Al~It is concluded that the observed electronic levels belong to the different 23p) /Si~(3~2) and El = Si? ( 3s23p2 ) /Si~(3s23p). The conclusion is confirmed charge of silicon selfinterstitial E2 =ofSi~ by the states differences of the cross-sections the(3s formation of Al, and C, as well as by the influence of the zero and reverse bias conditions on the introduction rate and optical annealing ofthe E2 state.
It is well established that the selfinterstitial in irradiated p-type silicon is unstable and migrates even at temperatures <4.2 K until it is trapped by impurities. As a result, the shallow acceptors as well as carbon are injected from substitutional to interstitial sites [1,21. This is why isolated silicon selfinterstitials have eluded detection. In this report on the basis of the injection-enhanced annealing and growth of defect states as well as of the influence of reverse bias conditions on the introduction rate and injection annealing of defects it is concluded that the observed E2 and El minority carrier traps belong to different charge states of the silicon selfinterstitial. The experimental details have been described in our previous paper [31, where we have presented data concerning the main properties of the E2 (E~—0.39 eV) center. This center has appeared in all a-irradiated (4.7 MeV) samples irrespective of the kind of acceptor impurities in both Czochralskigrown (CG) and float zone (FZ) crystals. The amplitude and the rate of annealingof the E2 (0.39) level depends on the minority carrier injection level as well as on the concentration of carbon and oxygen impurities. The annealing of E2 (0.39) (fig. 1) increases the concentration of the H2(E~+0.29.C,) state. It is supposed that E2 is positively charged and during the filling pulse captures electrons. The charge 40
transfer can take place also due to thermal transition of electrons from the valence band to the E2(0.39) level at temperatures >330 K, at which the temperature defect is annealed out. Therefore, it is not introduced at temperatures 330 K. It is suggested that the defect anneals due to the charge transfer mechanism or due to the saddle point (Bourgoin— Corbett) mechanism. The E2(0.39) defect is tentatively attributed to the Si~/Si~donor state. We have studied the origin of E2(0.39) in detail. It is found that optical injection greatly changes the DLTS spectra of irradiated samples (fig. 2). The new El (E~—0.25+0.05 eV) state is resolved by substraction of curve 6 (after annealing) from curve I (as irradiated) (fig. 2). The electrical injection annealing studies show that the El and E2 states are annealed out at the same time due to minority carrier injection. The annealing of El and E2 levels is simultaneous with the growth of H2 (C,) and H3 (Al (fig. 3). The H3 levels are observed only in irradiated Al-doped material. It is attributed to the Al~/Al~[4] state because the defect has the same annealing characteristic and temperature scale position as well as a small and temperature dependent hole-capture cross-section. Note that the cross-section for production of H3 (Al,) is approximately 100 times larger than that of H2 (C,). Indeed, in spite of ‘—
ElsevLer Science Publishers By.
Volume 166, number 1
PHYSICS LETTERS A
8 June 1992 100
‘150
I
200
I
17’K
U
/
S~-B-FZ-SB H2
.
a2St
~
fQ39
~I.
IR\ R
-n t’ 0
~
St
/
7
‘
/
~1
‘I
I
+ L
____
d
6 1~ It
_______________________
5
(f,
~
El,
1
-J
_J
2 (I)
/
I
_J 400
150
2
.-—
‘~g
200
-J
*
E2
Fig. 1. DLTS spectrum of the a-irradiated p-Si: B (*) under low level injection conditions. Shown also are successive injection annealings. The inset shows the proposed configuration coordinate diagram for different charge states of Si,.
Fig. 2. DLTS spectrum ofa-irradiated p-Si: B recorded under optical excitation through a window in the back contact. (1) Initial spectrum, (2)—(6) after forward bias injection at 190 K, (7) spectrum 1 after subtraction ofspectrum 6.
the fact that the concentration of Al in the initial samples is 20 times smaller than that of carbon, the amplitude the H3 state is 5 times larger than that of H2 (fig.of3).
____________________________ —CG — 4c~~3 SB NAe:9.1Ol N~=2lO16crrC3
sL-~e
The additional data about the properties ofthe E2
V-V
defect have been obtained by studies of low tern-
perature optical injection annealing and of the influence of the reverse bias on the introduction rate and annealing of this state. The five and twenty second optical injection at 175 K introduced the H4(E~+0.l3 eV) vacancy state, enhanced the forrnation of H2(C 1) and slightly decreased the Hl(VV) concentration (fig. 4). The E2 center did not anneal during optical injection and was not introduced at > 200 K, if the samples were under reverse bias, and finally the rate of its injection an nealing is the same over the temperature range 77—
300 K. We propose a model to explain these experimental
observations. According to this model there are three
J
100-____________________________ 150 200
T,K Fig. 3. (1) DLTS spectrum of a-irradiated p-Si: Al. (2) After injection at 250 K and (3) after the heat treatment at 300 K.
41
Volume 166, number 1
PHYSICS LETTERS A
________________________
St-B-CC-SB p=fl&4c~n3 ~ H2
v-v
i fl i
8 June 1992
between the cross-section of the formation of Al, and that of C, (GA= lOOo~-).The influence ofthe applied reverse bias on the optical annealing and the ternE2 state may be easily explained also. The reverse bias removes the minority carriers from the depletion region of the junction thereby preventing changing the charge state of E2. The charge transfer can take place due to the thermal generation of electrons perature and the transition dependence to the of the E2(0.39) introduction state atrate -..330 of the K. is reduced to 200 K if the samples are irradiated However, the temperature of the thermal transition by a-particles under reverse bias, because the E2 centers are filled by electrons and their transitions from the level to the conduction band begins from 200 K. At a low temperature K), in irradiated n-type silicon, immediately after(77 irradiation, a small E(0.12.
H’t
-~
£ 100
125
150
175
T,K Fig. 4. (1) DLTS spectrum ofa-irradiated p-Si: B. Shown are the annealings under the near-band-gap illumination 2 s during (2)light 5 sand (3) 20 5. with the fluence 1018 photon/cm
different charge states of the selfinterstitial in irradiated p-type silicon crystals: Si~~ (3s2), Si~(3s23p) and Si~(3s23p2) which correspond to the E2.( +/2+) and El (0/ +) electronic levels. At least two of them, Si~and Sit, are saddle points of each other (we do not have enough information to discuss the annealing mechanism of the Si, (0/+) state). Fig. I displays the configuration coordinate diagram which explains the intensive electrical and optical annealing of the E2 state via the athermal Bourgoin—Corbett mechanism. Note that the same rate of injection annealing in the temperature range 77—300 K confirms the athermal migration mechanism. Low ternperature optical injection causes the migration of selfinterstitials and their interaction with divacancies (Si,+VV—*V). As a result one can observe the appearance of the H4(V) state due to secondary processes in the samples, irradiated at 273 K. The Coulomb interaction between Si~ and substitutional Al~enhances the injection of Al~into the interstitial site. Therefore one can observe a large difference
42
C 1) signal is observed [5]. Moreover, the rate of the appearance of the state is injection-enhanced, growing significantly at 77 K when the diode is forward biased.confirmation We believe that theseexistence data are of additional indirect of the selfinterstitials in irradiated silicon. In summary, we have presented several experimental indications of the existence of selfinterstitials in irradiated silicon. We have concluded that the observed electronic levels belong to the different charge states of the selfinterstitial: E2(0.39, Si~(3s23p)/ Si~~ (3~2)) and El (0.25, Si~(3s23p2)/Si~ (3s23p)). However, further studies are required to get information about the symmetry of the defect and to obtam additional direct evidence of the existence ofthe isolated selfinterstitial in irradiated silicon.
References [11 G.D. Watkins, Radiation damage in semiconductors (1964) ~. 9~l. [2] GD. Watkins, Phys. Rev. B 12 (1975) 5824. [31Kii.A. Abdullin. B.N. Mukashev, M.F. Tamendarov and T.B. Tashenov, Phys. Lett. A 144 (1990) 198. 141 J.R. Troxell, A.P. Chatterjee, G.D. Watkins and L.C. Kimerling, Phys. Rev. B 19 (1979) 5336. [51L.C. Kimerling, P. Blood and W.M. Gibson, Inst. Phys. Conf. Ser. 46 (1979) 273.