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An acceptorlike electron trap in GaAs related to Ni
Solid State Communications, Vol. 59, No. 7, pp. 4 6 9 - 4 7 1 , 1986. Printed in Great Britain.
0 0 3 8 - 1 0 9 8 / 8 6 $3.00 + .00 Pergamon Journals Ltd.
AN ACCEPTORLIKE ELECTRON TRAP IN GaAs RELATED TO Ni S. Brehme* and R. Pickenhain Karl Marx University Leipzig Section o f Physics, Department o f Semiconductors Physics, 7010 Leipzig, Linn6 str. 5, GDR
(Received 18 November 1985; in revised form l OMarch 1986 by M. Balkanski) We have investigated an emission level at E e -- 0.40 eV in GaAs b y DLTS. The photo cross section o ° reveals a resonance due to an excited state of the involved impurity centre. Tentatively we identify the level as related to the 3d 9 configuration of the isolated Nioa impurity. Two deep levels in the lower half of the band gap observed b y ODLTS are discussed as possible candidates for the single acceptor level of Nioa. As an alternative model a Ni-related complex is discussed. BY CAPACITIVE LOCK-IN DLTS we investigated unintentionally doped VPE GaAs which was from n-type with a net doping level of about 1 • 10 is cm -3 . We used semitransparent Schottky barriers made by Au evaporation, In the thermal DLTS spectra we often found an electron-emitting level with an effective activation energy E t = 0.48 -+ 0.02 eV. The concentration reached some 1014 cm -3 in several samples. F r o m the cross-over of the Arrhenius plot of the thermal emission data with the ordinate we obtained On exp (AS/k) = (4 -+ 2)" 10 -14 cm 2 with k = Boltzmann factor; AS = entropy change due to the electron transition to the conduction band; o F = e l e c t r o n capture-cross section for 0 ~ ; 0 = temperature. The field-induced enhancement o f the electron emission seems to "be rather weak for field strengths below l 0 s V cm -1 . The temperature dependence of the electron capture cross section can be described b y
On(O) = (5 + 3)"
1 0 -16
exp ((-- 0.08 + O.02)eV/kO) cm 2 .
Comparing the emission and capture data we obtained for the actual enthalpy AH o f the transition A H = 0.40 -+ 0.04 eV and A S = 3 . 8 - 1 0 - 4 eVK -1. The value o f AS should be regarded as a rough approximation because o f the sensitivity o f the cross-overs of the Arrhenius plots with the ordinates to the uncertainties o f the measurements. Using optical pulses instead o f the conventional electrical ones (ODLTS) [1] we obtained ap >>On from the fact, that only for h v < E u thermal electron
* Present address: Isocommerz G.m.b.H, DDR-7050 Leipzig, Permoserstr. 15, GDR. 469
emission is observable after the pulse. We determined EN = 1.00-+0.03eV as threshold of the optical excitation at 237 K. As hv > Eu, we observed a pseudoemission signal with inverted sign (psuedo hole emission), which is typical for electron capturing at edge of the depletion layer, where we have the Debye electron tail. Additionally, we observed a strong increase of the signal for hv ~ Eg in the photoionization spectrum which was obtained by optical DLTS with electrical pulses. We also explain this fact b y Op >>On because the capture of photogenerated holes produces a DLTS signal with the same sign as the electron emission to the conduction band. We believe that this level is the same as already reported by Partin et al. [2]. They found that the level has controlled the minority carrier life time in their n-type samples (op ~ 1 0 -13 cm 2). They also observed an increased concentration after Ni diffusion. In Fig. 1 the spectral distribution of the photo cross sections o°n and o° for hv < Eg are shown, measured at 203 K. The experimental procedure is described elsewhere [3]. From o° we derive E N = l . 0 3 - + 0 . 0 3 e V , in accordance with the result above. The cross section o ° reveals a local maximum at hvmax = 0.62 -+ 0.02 eV. We suggest the involved centre has an excited state which causes the deviation from observed monotonous increase of a°n with increasing light energy [4] (in Fig. 1 indicated by the dashed line which is o f empirical nature). Comparing h v m ~ with the value of AH and A/-/-0AS, respectively, as derived from the measurements, it seems reasonable to assume that tiffs excited state lies well above the conduction band minimum. In this case the interaction o f the conduction band states is from the resonance type apparently [5, 6 ] . We reported the determination of o ° at 9 3 K , finding a low-energy edge shift o f more than 5 0 m e V to
ACCEPTORLIKE ELECTRON TRAP IN GaAs RELATED TO Ni
470 I0 C _
000000
°°°
0o
OS S
f
d r-, 5 d
.
~
10-I
@••
•
!
o cr~
10-2
I 0.4
I
0,6
I
08
I
1.0
,
1.2
[
1.4-
h.u [eV]
Fig. 1. S~ectral distribution o f the photo-cross sections o ° and o~ of the E e - - 0 . 4 0 eV level in GaAs at 203 K. The maximum at hvw_~ --0.62-+ 0.02 eV is caused by an excited state of the impurity centre. Note that the ratio o ° / o ° could be underestimated because of superposition effects from the hole trap H2 which produces a temperature dependent photoneutralization signal for h v > 1.05eV. The dashed line shows the assumed distribution of a°n without the resonance portion. higher energies. The absorption band centred at 0.62 eV contrasted more with the underground. This supports the assumption that the absorption band centred at 0.62eV is not related to a transition to the normal band states. Possibly, our E e - - 0 . 4 0 e V level is related to the transition Ni~a(3d 9 ) ~ Ni~*a(3d 8) + e- of the isolated Ni atom. NiGa is a double acceptor in GaAs [8]. From optical absorption measurements, the energy gap between the 2T 2 ground state and the 2E 2 excited state of the 3d 9 configuration is 0.57eV [ 7 , 8 ] . The internal luminescence of Ni+(3d 9) could not be detected, though the 2 7"2 -+ ZE transition is dominant in the i.r. absorption spectra of n-type GaAs:Ni [9]. The cause is probably that the 2T2 state lies higher than E e - - 0 . 5 7 eV. This agrees with our tentative interpretation of the DLTS level E e - 0.40 eV. From this interpretation we obtain the following picture: The ground state 2T 2 lies at E e --(0.40-+ 0.04)eV at 0 = 0 K. The position of the excited 2E state is E e + (0.17 + 0.04)eV. The phonon participation in the transition 2 7'2 ~ 2E is characterized by S ~ 5 0 m e V . We note a similar value Sht~ = 65 + 2 0 m e V has been reported for the same transition in GaP [10].
Vol. 59, No. 7
The position of the NlGa(3d .2÷ a ) ground state is in the lower half of the band gap [11]. The precise value of energy is not trustworthily known, but probably it is Ev + (0.2 . . . 0.22)eV [12]. Using ODLTS [1] (hv = 1.48eV) we observed two acceptorlike hole traps in the samples in which also the E e - - 0 . 4 0 e V level was detected. From an assessment of the peak position within the temperature scale we conclude E t = 0 . 2 0 . . . 0.25 eV (HI) and E t = 0.38 . . . 0.45 eV (H2), respectively. The level H1 could be related to the transition • 3+ ~ NIGa -2+ + h + . The level H2 is probably the often N1Ga reported Cu-related level at about E v + 0.45 eV [13, 14]. By a photoneutralization experiment we determined on < 3 " 1 0 - 2 ° c m 2 at 2 0 0 K for the level H2 in accordance with this assumption [13]. This photoneutralization signal is also the cause of the possible underestimation of the ratio o 0b / a 0n mentioned in Fig. 1 (similar difficulties have been reported from DLOS measurements [15]). H2 has been found also in Ni-doped GaAs [2, 14], but Kumar and Ledebo [18] showed that H2 is related to Cu and not to Ni. Dubecky e t al. [17] observed a significant field enhancement of the hole emission of H2. They reported the low-field value E t = 0.48 eV. For higher field strengths being typical for depletion layer measurements they found E t = 0.41 eV. At this point it is useful to compare our ODLTS results with the results of Fairman e t al. [18] who used the photoinduced current transient spectroscopy (PICTS) to study deep levels in GaAs. Using optical pulses with h v > Eg they found levels E t = 0.46 . . . 0.48 eV in all epilayers made by VPE. These levels cannot be identical to our level E c - - 0 . 4 0 eV which is not observable by pulses with h v > Eg. Also no traps in the range 0 . 2 0 . . . 0.25 eV were observed. At the very least we remark a part of the levels E t = 0.46 . . . 0.48eV should be identical to the Cu-related level labelled H2 by us since PICTS results are low-field results. Using optical pulses with h u < E u Fairman e t al. found a E t = 0.22 eV level of unknown chemical origin and also levels E t = 0.46 . . . 0.48 eV in all substrates. The latter ones can arise from a superposition of the PICTS signal from the Cu-related trap H2 and the Ni-related trap E e ~ 0.40 eV because PICTS cannot distinguish between electron and hole traps. In principle, a biexponential capture behavior for electrons can be expected for the level E e - - 0 . 4 0 eV, if it is related to Ni + as assumed and if the experimental conditions are chosen in a special way. (The electron occupation of the lower level N ~2+ ,Ga must be zero at the beginning of the electron refilling pulse, which means, all NiGa atoms must be in the neutral state.) In this way the Fe~a(3d 7) level could be identified [19]. If we -2+ assume that the emission level H1 is related to NIGa, the biexponential behavior should be not observable in our case because of the high electron capture rate of i l l . By
Vol. 59, No. 7
ACCEPTORLIKE ELECTRON TRAP IN GaAs RELATED TO Ni
combination of electrical and optical pulses [20] we have assessed on > 10 -16 cm 2 at 150K for this level. Of course we cannot exclude that the Ee -- 0.40 eV level is a Ni-related complex of point defects as proposed by Partin et al. [1]. It is well-known that Ni in GaAs tends to build up complexes, expecially Ni-donor-pairs [9]. These pairs revealed excited states with energy separations to the ground state in the range 0.54 . . . 0.58eV [9,21]. A good candidate is the Ni-Si pair (ZPL at hv= 0.58eV [9]). Si is known from Hall measurements to be present very probably in our samples [22]. However, the internal transition of the pairs could be observed also by luminescence. From this we believe the ground state of the related deep levels is lower than E e-O.4OeV. Furthermore, Ennen et al. [9] assessed the portion of Ni atoms bounded in pairs to be less than 10%. They found the 0.57 absorption line was always dominant in their GaAs:Ni samples [9]. Thus it seems to be more likely to fired a deep level in DLTS measurements which is related to the isolated NiGa than to find a complex. Further experiments have to be performed to clear this point.
Acknowledgements - We would like to thank K. Kreher for critical reading of the manuscript and D. Suisky and W. Ulrici for discussions. We are also grateful to W. Seifert for supplying the epitaxial material and G. Biehne for sample preparation.
REFERENCES 1.
A. Mitonneau, G.M. Martin & A. Mircea, Proc. GaAs and Related Compounds, Edinburgh 1976, in: Int. Phys. Conf. Set. 334 73, (1977).
2. 3. 4.
5. 6. 7. 8. 9. 10. 11.
12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
471
D.L. Partin, J.W. Chen, A.G. Milnes & L.F. Vassamillet, J. Appl. Phys. 50 6845 (1979). S. Brehme & R. Pickenhain, Phys. Status Solidi (a) 88, K 63 (1985); 90, K 119 (1985). S. Brehme, Dissertation A, Leipzig (1985) (unpublished). U. Fano, Phys. Rev. 124, 1866 (1961). A. Shibatani & Y. Toyozawa, Z Phys. Soc. Japan 25,355 (1968). H. Ennen, U. Kaufmarm & J. Schneider, Solid State Commun. 34, 603 (1980). W. Drozdzewicz, A.M. Hennel, Z. Wasilewski, B. Clerjaud, F. Gendron, C. Porte & R. Germer, Phys. Rev. B29, 2438 (1984). H. Ennen, U. Kaufmann & J. Schneider, Appl. Phys. Lett. 38,355 (1981). X.-Z. Yang, L. Samuelson & H.G. Grimmeiss, J. Phys. C17, 6521 (1984). N.I. Suchkova, D.G. Andrianov, E.M. Om61'yanovskii, E.P. Rashevskaya & N.N. Solovjev, Fiz. Tekh. Polu-prov. 9,718 (1975); English translation in: Soy. Phys. Semicond. 9, 469 (1975). B. Clerjaud, J. Phys. C18,3615(1985). A. Mitonneau, G.M. Martin & A. Mircea, Electronic Lett. 13,666 (1977). D.V. Lang & R.A. Logan, J. Electron. Mater. 4, 1053 (1975). A. Chantre, G. Vincent & D. Bois, Phys. Rev. B23, 5335 (1981). V. Kumar & L.-A. Ledebo, J. Appl. Phys. 52, 4866 (1981). F. Dubeck3~, J. N6vak & P. Kordos, IV. "Lund" Int. Conf. on Deep Levels in Semicond., Eger (1983). R.D. Fairman, F.J. Morin & J.R. Oliver, lnst. Phys. Conf. Ser. No. 45, 134 (1979). S. Brehme, J. Phys. C18, L319(1985). A. Mitonneau, A. Mircea, G.M. Martin & D. Pons Rev. Phys. Appl. FI4, 853 (1979). Fujiwara, A. Koshima, T. Nishino & Y. Hamakawa, Jap. J. Appl. Phys. 22, L 476 (1983). W. H/Srig, private communication.
0 0 3 8 - 1 0 9 8 / 8 6 $3.00 + .00 Pergamon Journals Ltd.
AN ACCEPTORLIKE ELECTRON TRAP IN GaAs RELATED TO Ni S. Brehme* and R. Pickenhain Karl Marx University Leipzig Section o f Physics, Department o f Semiconductors Physics, 7010 Leipzig, Linn6 str. 5, GDR
(Received 18 November 1985; in revised form l OMarch 1986 by M. Balkanski) We have investigated an emission level at E e -- 0.40 eV in GaAs b y DLTS. The photo cross section o ° reveals a resonance due to an excited state of the involved impurity centre. Tentatively we identify the level as related to the 3d 9 configuration of the isolated Nioa impurity. Two deep levels in the lower half of the band gap observed b y ODLTS are discussed as possible candidates for the single acceptor level of Nioa. As an alternative model a Ni-related complex is discussed. BY CAPACITIVE LOCK-IN DLTS we investigated unintentionally doped VPE GaAs which was from n-type with a net doping level of about 1 • 10 is cm -3 . We used semitransparent Schottky barriers made by Au evaporation, In the thermal DLTS spectra we often found an electron-emitting level with an effective activation energy E t = 0.48 -+ 0.02 eV. The concentration reached some 1014 cm -3 in several samples. F r o m the cross-over of the Arrhenius plot of the thermal emission data with the ordinate we obtained On exp (AS/k) = (4 -+ 2)" 10 -14 cm 2 with k = Boltzmann factor; AS = entropy change due to the electron transition to the conduction band; o F = e l e c t r o n capture-cross section for 0 ~ ; 0 = temperature. The field-induced enhancement o f the electron emission seems to "be rather weak for field strengths below l 0 s V cm -1 . The temperature dependence of the electron capture cross section can be described b y
On(O) = (5 + 3)"
1 0 -16
exp ((-- 0.08 + O.02)eV/kO) cm 2 .
Comparing the emission and capture data we obtained for the actual enthalpy AH o f the transition A H = 0.40 -+ 0.04 eV and A S = 3 . 8 - 1 0 - 4 eVK -1. The value o f AS should be regarded as a rough approximation because o f the sensitivity o f the cross-overs of the Arrhenius plots with the ordinates to the uncertainties o f the measurements. Using optical pulses instead o f the conventional electrical ones (ODLTS) [1] we obtained ap >>On from the fact, that only for h v < E u thermal electron
* Present address: Isocommerz G.m.b.H, DDR-7050 Leipzig, Permoserstr. 15, GDR. 469
emission is observable after the pulse. We determined EN = 1.00-+0.03eV as threshold of the optical excitation at 237 K. As hv > Eu, we observed a pseudoemission signal with inverted sign (psuedo hole emission), which is typical for electron capturing at edge of the depletion layer, where we have the Debye electron tail. Additionally, we observed a strong increase of the signal for hv ~ Eg in the photoionization spectrum which was obtained by optical DLTS with electrical pulses. We also explain this fact b y Op >>On because the capture of photogenerated holes produces a DLTS signal with the same sign as the electron emission to the conduction band. We believe that this level is the same as already reported by Partin et al. [2]. They found that the level has controlled the minority carrier life time in their n-type samples (op ~ 1 0 -13 cm 2). They also observed an increased concentration after Ni diffusion. In Fig. 1 the spectral distribution of the photo cross sections o°n and o° for hv < Eg are shown, measured at 203 K. The experimental procedure is described elsewhere [3]. From o° we derive E N = l . 0 3 - + 0 . 0 3 e V , in accordance with the result above. The cross section o ° reveals a local maximum at hvmax = 0.62 -+ 0.02 eV. We suggest the involved centre has an excited state which causes the deviation from observed monotonous increase of a°n with increasing light energy [4] (in Fig. 1 indicated by the dashed line which is o f empirical nature). Comparing h v m ~ with the value of AH and A/-/-0AS, respectively, as derived from the measurements, it seems reasonable to assume that tiffs excited state lies well above the conduction band minimum. In this case the interaction o f the conduction band states is from the resonance type apparently [5, 6 ] . We reported the determination of o ° at 9 3 K , finding a low-energy edge shift o f more than 5 0 m e V to
ACCEPTORLIKE ELECTRON TRAP IN GaAs RELATED TO Ni
470 I0 C _
000000
°°°
0o
OS S
f
d r-, 5 d
.
~
10-I
@••
•
!
o cr~
10-2
I 0.4
I
0,6
I
08
I
1.0
,
1.2
[
1.4-
h.u [eV]
Fig. 1. S~ectral distribution o f the photo-cross sections o ° and o~ of the E e - - 0 . 4 0 eV level in GaAs at 203 K. The maximum at hvw_~ --0.62-+ 0.02 eV is caused by an excited state of the impurity centre. Note that the ratio o ° / o ° could be underestimated because of superposition effects from the hole trap H2 which produces a temperature dependent photoneutralization signal for h v > 1.05eV. The dashed line shows the assumed distribution of a°n without the resonance portion. higher energies. The absorption band centred at 0.62 eV contrasted more with the underground. This supports the assumption that the absorption band centred at 0.62eV is not related to a transition to the normal band states. Possibly, our E e - - 0 . 4 0 e V level is related to the transition Ni~a(3d 9 ) ~ Ni~*a(3d 8) + e- of the isolated Ni atom. NiGa is a double acceptor in GaAs [8]. From optical absorption measurements, the energy gap between the 2T 2 ground state and the 2E 2 excited state of the 3d 9 configuration is 0.57eV [ 7 , 8 ] . The internal luminescence of Ni+(3d 9) could not be detected, though the 2 7"2 -+ ZE transition is dominant in the i.r. absorption spectra of n-type GaAs:Ni [9]. The cause is probably that the 2T2 state lies higher than E e - - 0 . 5 7 eV. This agrees with our tentative interpretation of the DLTS level E e - 0.40 eV. From this interpretation we obtain the following picture: The ground state 2T 2 lies at E e --(0.40-+ 0.04)eV at 0 = 0 K. The position of the excited 2E state is E e + (0.17 + 0.04)eV. The phonon participation in the transition 2 7'2 ~ 2E is characterized by S ~ 5 0 m e V . We note a similar value Sht~ = 65 + 2 0 m e V has been reported for the same transition in GaP [10].
Vol. 59, No. 7
The position of the NlGa(3d .2÷ a ) ground state is in the lower half of the band gap [11]. The precise value of energy is not trustworthily known, but probably it is Ev + (0.2 . . . 0.22)eV [12]. Using ODLTS [1] (hv = 1.48eV) we observed two acceptorlike hole traps in the samples in which also the E e - - 0 . 4 0 e V level was detected. From an assessment of the peak position within the temperature scale we conclude E t = 0 . 2 0 . . . 0.25 eV (HI) and E t = 0.38 . . . 0.45 eV (H2), respectively. The level H1 could be related to the transition • 3+ ~ NIGa -2+ + h + . The level H2 is probably the often N1Ga reported Cu-related level at about E v + 0.45 eV [13, 14]. By a photoneutralization experiment we determined on < 3 " 1 0 - 2 ° c m 2 at 2 0 0 K for the level H2 in accordance with this assumption [13]. This photoneutralization signal is also the cause of the possible underestimation of the ratio o 0b / a 0n mentioned in Fig. 1 (similar difficulties have been reported from DLOS measurements [15]). H2 has been found also in Ni-doped GaAs [2, 14], but Kumar and Ledebo [18] showed that H2 is related to Cu and not to Ni. Dubecky e t al. [17] observed a significant field enhancement of the hole emission of H2. They reported the low-field value E t = 0.48 eV. For higher field strengths being typical for depletion layer measurements they found E t = 0.41 eV. At this point it is useful to compare our ODLTS results with the results of Fairman e t al. [18] who used the photoinduced current transient spectroscopy (PICTS) to study deep levels in GaAs. Using optical pulses with h v > Eg they found levels E t = 0.46 . . . 0.48 eV in all epilayers made by VPE. These levels cannot be identical to our level E c - - 0 . 4 0 eV which is not observable by pulses with h v > Eg. Also no traps in the range 0 . 2 0 . . . 0.25 eV were observed. At the very least we remark a part of the levels E t = 0.46 . . . 0.48eV should be identical to the Cu-related level labelled H2 by us since PICTS results are low-field results. Using optical pulses with h u < E u Fairman e t al. found a E t = 0.22 eV level of unknown chemical origin and also levels E t = 0.46 . . . 0.48 eV in all substrates. The latter ones can arise from a superposition of the PICTS signal from the Cu-related trap H2 and the Ni-related trap E e ~ 0.40 eV because PICTS cannot distinguish between electron and hole traps. In principle, a biexponential capture behavior for electrons can be expected for the level E e - - 0 . 4 0 eV, if it is related to Ni + as assumed and if the experimental conditions are chosen in a special way. (The electron occupation of the lower level N ~2+ ,Ga must be zero at the beginning of the electron refilling pulse, which means, all NiGa atoms must be in the neutral state.) In this way the Fe~a(3d 7) level could be identified [19]. If we -2+ assume that the emission level H1 is related to NIGa, the biexponential behavior should be not observable in our case because of the high electron capture rate of i l l . By
Vol. 59, No. 7
ACCEPTORLIKE ELECTRON TRAP IN GaAs RELATED TO Ni
combination of electrical and optical pulses [20] we have assessed on > 10 -16 cm 2 at 150K for this level. Of course we cannot exclude that the Ee -- 0.40 eV level is a Ni-related complex of point defects as proposed by Partin et al. [1]. It is well-known that Ni in GaAs tends to build up complexes, expecially Ni-donor-pairs [9]. These pairs revealed excited states with energy separations to the ground state in the range 0.54 . . . 0.58eV [9,21]. A good candidate is the Ni-Si pair (ZPL at hv= 0.58eV [9]). Si is known from Hall measurements to be present very probably in our samples [22]. However, the internal transition of the pairs could be observed also by luminescence. From this we believe the ground state of the related deep levels is lower than E e-O.4OeV. Furthermore, Ennen et al. [9] assessed the portion of Ni atoms bounded in pairs to be less than 10%. They found the 0.57 absorption line was always dominant in their GaAs:Ni samples [9]. Thus it seems to be more likely to fired a deep level in DLTS measurements which is related to the isolated NiGa than to find a complex. Further experiments have to be performed to clear this point.
Acknowledgements - We would like to thank K. Kreher for critical reading of the manuscript and D. Suisky and W. Ulrici for discussions. We are also grateful to W. Seifert for supplying the epitaxial material and G. Biehne for sample preparation.
REFERENCES 1.
A. Mitonneau, G.M. Martin & A. Mircea, Proc. GaAs and Related Compounds, Edinburgh 1976, in: Int. Phys. Conf. Set. 334 73, (1977).
2. 3. 4.
5. 6. 7. 8. 9. 10. 11.
12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
471
D.L. Partin, J.W. Chen, A.G. Milnes & L.F. Vassamillet, J. Appl. Phys. 50 6845 (1979). S. Brehme & R. Pickenhain, Phys. Status Solidi (a) 88, K 63 (1985); 90, K 119 (1985). S. Brehme, Dissertation A, Leipzig (1985) (unpublished). U. Fano, Phys. Rev. 124, 1866 (1961). A. Shibatani & Y. Toyozawa, Z Phys. Soc. Japan 25,355 (1968). H. Ennen, U. Kaufmarm & J. Schneider, Solid State Commun. 34, 603 (1980). W. Drozdzewicz, A.M. Hennel, Z. Wasilewski, B. Clerjaud, F. Gendron, C. Porte & R. Germer, Phys. Rev. B29, 2438 (1984). H. Ennen, U. Kaufmann & J. Schneider, Appl. Phys. Lett. 38,355 (1981). X.-Z. Yang, L. Samuelson & H.G. Grimmeiss, J. Phys. C17, 6521 (1984). N.I. Suchkova, D.G. Andrianov, E.M. Om61'yanovskii, E.P. Rashevskaya & N.N. Solovjev, Fiz. Tekh. Polu-prov. 9,718 (1975); English translation in: Soy. Phys. Semicond. 9, 469 (1975). B. Clerjaud, J. Phys. C18,3615(1985). A. Mitonneau, G.M. Martin & A. Mircea, Electronic Lett. 13,666 (1977). D.V. Lang & R.A. Logan, J. Electron. Mater. 4, 1053 (1975). A. Chantre, G. Vincent & D. Bois, Phys. Rev. B23, 5335 (1981). V. Kumar & L.-A. Ledebo, J. Appl. Phys. 52, 4866 (1981). F. Dubeck3~, J. N6vak & P. Kordos, IV. "Lund" Int. Conf. on Deep Levels in Semicond., Eger (1983). R.D. Fairman, F.J. Morin & J.R. Oliver, lnst. Phys. Conf. Ser. No. 45, 134 (1979). S. Brehme, J. Phys. C18, L319(1985). A. Mitonneau, A. Mircea, G.M. Martin & D. Pons Rev. Phys. Appl. FI4, 853 (1979). Fujiwara, A. Koshima, T. Nishino & Y. Hamakawa, Jap. J. Appl. Phys. 22, L 476 (1983). W. H/Srig, private communication.