- Email: [email protected]
Threshold voltage for damage in Si under electron bombardment
Scriuta "
HETALLURGICA
V o l , 11, p~, 47-49~ 1977 Printed in the United States
Pergamon P r e s s ,
Inc
THRESHOLD VOLTAGE FOR DAMAGE IN Si UNDER ELECTRON BOMBARDMENT Hamish L. Fraser Department of Metallurgy and Mining Engineering and the Materials Research Laboratory, University of Illinois, Urbana, IL 61801
( R e c e i v e d O c t o b e r 28,
1976)
Introduction Radiation da-~ge in s e m i c o n d u c t i n g m a t e r i a l s has been the subject of much study (for a review, see Nelson (1)). In the case of electron irradiation Mathews and Ashby (2) have carried out experiments on Si using high voltage electron microscopy and performed irradiations at 173*K, room temperature and between 673°-823*K. Apparently, dislocation loops were formed during the two lower temperature irradiations although they were not able to deduce the nature of these defects (i.e. vacancy or interstitial); linear defects were produced during the higher temperature irradiations. These auth~rs~report an u n e x p e c t o d l y h i g h value of the displacement threshold of between 450 and 500 keY. F511 (3) has more recently carried out a study of electron damage in Si, and has concluded that those clusters formed during cold irradiation using electrons having energies betwee~ 400 and 650 kev which could be a n a l y m e d w e r e interstitial in nature. In both these sets of experiments it seems that the threshold voltage for electron damage in Si is between 400-500 keV. Such a value is indeed unexpectedly high, and implies a displacement energy Dr about 50 ev. This note presents some preliminary observations of electron dAmAge in Si where the threshold voltage for displacement is shown to be ~ 200 keY. Experimental
Details
The Si used in these experiments was kindly supplied by Dr. I. Sanders of Plessey Co., Ltd. (U.K.): the crystals were grown epitaxially by vapor deposition. The single crystals discs possessed a normal parallel to and the defect density (both dislocations and stacklng-faults lying on [iii]) was very lew. A JSEM 200 scanning transmission electron microscope was operated at the following accelerating voltages: i00, 150, 175 and 200 kV, and the instrument was operated in both the transmission and the scanning transmission modes. All irradiations were performed at ambient temperatures. Results and Discussion Prolonged irradiation (~50 minutes) at accelerating voltages of 100, 150 and 175 k V produced no observable damage in the foils. However, at an accelerating voltage of 200 kV, defects were observed associated with the stackingfaults. Fig. I shows the result of such an irradiation, and images of defects are clearly visible lying on the projection of the fault planes. The contrast behavior exhibited by the defects indicate that they are small dislocation loops. Furthermore, stereo microscopy reveals that the planes of these loops are inclined to the fault planes and are neither contained in nor parallel to
47
48
ELECTRON DAMAGE
IN Si
Yol, iI, N o
the fault planes. Increasing the irradiation time causes both the number and size of the loops to increase. The preliminary results of the present work imply that the threshold voltage for damage in Si ~ 200 kV, considerably lower than that reported previously (2). One possibility is that the observed damage is caused by an ion beam produced in the electron gun b y ionization of contamination in the vacuum system° The following control experiment was performed to eliminate this possibility. An area was selected using an accelerating voltage of i00 kV to avoid the production of damage. The accelerating voltage was then increased to 200 kV, and the electron beam was deflected above the specimen by electromagnetic coils so that the area was not irradiated by electrons. After 30 minutes of possible ion damage, the accelerating voltage was reduced to i00 kV and a detailed examination revealed no observable damage. The accelerating voltage was again increased to 200 k V and the given area exposed to electron irradiation. Within ten minutes defects were observed associated with the faults. It is reasonable, then, to suppose that the defects are produced by electron damage and not ion damage. If one assumes that at the threshold voltage for displacement, Eth, the maximum energy Em, transferred between an atom of atomic mass M and a fast electron just exceeds the energy for displacement, Ed, then the following expression (4) can b e used to estimate Ed: 2Eth(Eth + 2 m c 2) E
= m
Mc 2
where m is the electron rest mass and c is the velocity of light. Substituting a value of Eth = 200 k V (as determined in this experiment), E~ ~ E m ~ 18 eVo This is a very reasonable value for the displacement energy, and as typical of displacement energies of a number of elements. A detailed report of the defect analysis and role of the stacking faults in cluster formation will be published shortly. This work was supported in part b y the USERDA under contract E(ii-I)-1198 and the University of Illinois. Many useful discussions with Dr. M. H. Loretto are gratefully acknowledged° References i. 2. 3. 4.
R . S . Nelson, Radiation Damage and Defects in Semiconductors, ed. J. E. Whitehouse, p. 140, The Institute of Physics, London and Bristol (1972). M.J. Mathews and S.Jo Ashby, Phil. Mag., 27, 1313 (1972). H. FSII, Inst. Phys. Conf. Sero No. 23, p. 233, 1975. M . J . Makin and J. V. Sharp, J. Mat. Sci., ~, 360 (1968).
1
Vol.
II, No.
1
ELECTRON
DAMAGE
IN Si
(a)
(b) Figure I. Damage p r o d u c e d b y electron irradiation at 200 kV for 20 minutes. Note that the defects lie on the p r o j e c t i o n s of the stacking faults, a) Bright field micrograph, b e a m d i r e c t i o n close to [Iii], gvector as shown, w ( = s - ~g) - 0. b) Same as a) except that w - 2 . 2 . A c c e l e r a t i n g voltage 200 kV.
49
HETALLURGICA
V o l , 11, p~, 47-49~ 1977 Printed in the United States
Pergamon P r e s s ,
Inc
THRESHOLD VOLTAGE FOR DAMAGE IN Si UNDER ELECTRON BOMBARDMENT Hamish L. Fraser Department of Metallurgy and Mining Engineering and the Materials Research Laboratory, University of Illinois, Urbana, IL 61801
( R e c e i v e d O c t o b e r 28,
1976)
Introduction Radiation da-~ge in s e m i c o n d u c t i n g m a t e r i a l s has been the subject of much study (for a review, see Nelson (1)). In the case of electron irradiation Mathews and Ashby (2) have carried out experiments on Si using high voltage electron microscopy and performed irradiations at 173*K, room temperature and between 673°-823*K. Apparently, dislocation loops were formed during the two lower temperature irradiations although they were not able to deduce the nature of these defects (i.e. vacancy or interstitial); linear defects were produced during the higher temperature irradiations. These auth~rs~report an u n e x p e c t o d l y h i g h value of the displacement threshold of between 450 and 500 keY. F511 (3) has more recently carried out a study of electron damage in Si, and has concluded that those clusters formed during cold irradiation using electrons having energies betwee~ 400 and 650 kev which could be a n a l y m e d w e r e interstitial in nature. In both these sets of experiments it seems that the threshold voltage for electron damage in Si is between 400-500 keV. Such a value is indeed unexpectedly high, and implies a displacement energy Dr about 50 ev. This note presents some preliminary observations of electron dAmAge in Si where the threshold voltage for displacement is shown to be ~ 200 keY. Experimental
Details
The Si used in these experiments was kindly supplied by Dr. I. Sanders of Plessey Co., Ltd. (U.K.): the crystals were grown epitaxially by vapor deposition. The single crystals discs possessed a normal parallel to
47
48
ELECTRON DAMAGE
IN Si
Yol, iI, N o
the fault planes. Increasing the irradiation time causes both the number and size of the loops to increase. The preliminary results of the present work imply that the threshold voltage for damage in Si ~ 200 kV, considerably lower than that reported previously (2). One possibility is that the observed damage is caused by an ion beam produced in the electron gun b y ionization of contamination in the vacuum system° The following control experiment was performed to eliminate this possibility. An area was selected using an accelerating voltage of i00 kV to avoid the production of damage. The accelerating voltage was then increased to 200 kV, and the electron beam was deflected above the specimen by electromagnetic coils so that the area was not irradiated by electrons. After 30 minutes of possible ion damage, the accelerating voltage was reduced to i00 kV and a detailed examination revealed no observable damage. The accelerating voltage was again increased to 200 k V and the given area exposed to electron irradiation. Within ten minutes defects were observed associated with the faults. It is reasonable, then, to suppose that the defects are produced by electron damage and not ion damage. If one assumes that at the threshold voltage for displacement, Eth, the maximum energy Em, transferred between an atom of atomic mass M and a fast electron just exceeds the energy for displacement, Ed, then the following expression (4) can b e used to estimate Ed: 2Eth(Eth + 2 m c 2) E
= m
Mc 2
where m is the electron rest mass and c is the velocity of light. Substituting a value of Eth = 200 k V (as determined in this experiment), E~ ~ E m ~ 18 eVo This is a very reasonable value for the displacement energy, and as typical of displacement energies of a number of elements. A detailed report of the defect analysis and role of the stacking faults in cluster formation will be published shortly. This work was supported in part b y the USERDA under contract E(ii-I)-1198 and the University of Illinois. Many useful discussions with Dr. M. H. Loretto are gratefully acknowledged° References i. 2. 3. 4.
R . S . Nelson, Radiation Damage and Defects in Semiconductors, ed. J. E. Whitehouse, p. 140, The Institute of Physics, London and Bristol (1972). M.J. Mathews and S.Jo Ashby, Phil. Mag., 27, 1313 (1972). H. FSII, Inst. Phys. Conf. Sero No. 23, p. 233, 1975. M . J . Makin and J. V. Sharp, J. Mat. Sci., ~, 360 (1968).
1
Vol.
II, No.
1
ELECTRON
DAMAGE
IN Si
(a)
(b) Figure I. Damage p r o d u c e d b y electron irradiation at 200 kV for 20 minutes. Note that the defects lie on the p r o j e c t i o n s of the stacking faults, a) Bright field micrograph, b e a m d i r e c t i o n close to [Iii], gvector as shown, w ( = s - ~g) - 0. b) Same as a) except that w - 2 . 2 . A c c e l e r a t i n g voltage 200 kV.
49