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Long lifetime photoconductivity in semi-insulating bulk GaAs
0038-1098/85 $3.00 + .00 Pergamon Press Ltd.
Solid State Communications, Vol. 55, No. 5, pp. 459--462, 1985. Printed in Great Britain.
LONG LIFETIME PHOTOCONDUCTIVITY IN SEMI-INSULATING BULK Gabs J. Jim~nez, P. Hern~ndez and J.A. de Saja Lab. de Ffsica del Estado S61ido, Facultad de Ciencias, 47011 Valladolid, Spain and J. Bonnaf~ CEM (Centre d'Electronique de Montpellier), USTL, 34060 Montpellier, France
(Received 18 February 1985 by E.F. Bertaut) We report a new photomemory effect in semi.insulating bulk Gabs. Persistent illumination with 1.15 eV photons induces a photosensitivity state in Gabs, which is characterized by a slow decay of the photoconductivity, after the light is switched-off. The most relevant phenomenological aspects of this effect are summarized in this communication.
INTRODUCTION SEMI-INSULATING Gabs is a very promising material in order to be used in FET and IC technologies. The properties of the substrate are controlled by the deep levels, which compensate the background shallow levels. This fact has generated numerous investigations devoted to the study of the mid-gap levels in this material. These levels have a great influence on the performance of the devices. On the other hand they present an undoubtful interest, because of their striking physical properties. Thus, it is well-known that optical excitation with impurity light induces strong transformations in the photoelectronic properties of the samples, i.e., an optical quenching of the photoresponse [ I - 5 ] and a photogeneration of photosensitive traps [6-9] have been reported to occur in this material. In this paper, we report experimental evidence for a persistent photoconductivity effect (PPC) in semiinsulating Gabs. Such a phenomenon has been observed in some other II-VI [10-12] and III-V [13-17] single or alloyed compounds by several authors. Basically, the persistent photoconductivity consists of an anomalously long decay of photogenerated free carrier density after the switching off of the illumination [18]. The origin of the slow relaxation time of the nonequilibrium free carriers is the key to understanding this phenomenon in Gabs. Thus, several mechanisms have been proposed for explaining the microscopic nature of the persistent photoconductivity in the different materials showing this phenomenon. To our knowledge such a slow relaxation has never been observed in semiinsulating bulk Gabs. It has only been observed in nonhomogeneous configurations, i.e. n-n +, p - n or p-p+ structures [19, 20]. In these configurations it is assumed
that the long relaxation time of the free carriers is due to the existence of a macroscopic potential barrier between the layer and the substrate, which avoids the capture of the free electrons, and hence produces a slow decay of the electron density when the sample is kept in the dark after the end of the illumination. Our aim in this paper is to describe the most important experimental features of the persistent photoconductivity effect (PPC) in semi-insulating bulk Gabs. To give a detailed description of the origin of this phenomenon in our samples is beyond the purpose of this paper. In fact this paper deals with the most relevant phenomenological aspects of the long decay time of the photoconductivity. EXPERIMENTAL We studied the photocurrent of high resistivity Gabs samples (I07-109-cm at room temperature) cut from undoped LEC (Liquid Encapsulated Czochralski) ingots made by Thomson CSF. Several of them exhibited persistent photoconductivity, whereas the others showed either an optical quenching of the photoresponse or a photogeneration of traps. Electrical contacts were made either with silver paste or alloyed indium, annealed in vacuum at 200C for several minutes. The electrical contacts were masked in order to avoid photovoltage at the junction regions. Electrical current measurements were always performed in the linear part of the I - V characteristics in order to avoid non-linear effects. Optical excitation was provided by light from a halogen lamp passing through a high luminosity grating monochromator (Bausch-Lomb) and a filter system. The samples were mounted in the cold finger of a small liquid nitrogen cryostat, characterized by a weak thermal
459
460
LONG LIFETIME PHOTOCONDUCTIVITY IN SEMI-INSULATING BULK GaAs
1
0
-
1
Vol. 55, No. 5
~
10-'
tO-"
2
'~
II E
m-*~
, 1~.
10-1~
--
1 0 -1
1 0 -tl
I
4
I
6
I
I
8
10 t
I
12
I
14
I
16
(mln)
Fig. 1. Extrinsic photocurrent (1.13 eV) as a function of time. A strong change in photosensitivity is observed. inertia. The typical dimensions of the samples were 5 x 2 x 0.3mm 3. From optical absorption tests we can conclude that the excitation was homogeneous across the bulk of the sample in the full spectral range scanned in photoconductivity measurements. RESULTS AND DISCUSSION The experimental characterization of the persistent photoconductivity effect in semi-insulating bulk Gabs includes measurements of photoconductivity rising time, photoconductivity decay and thermal evolution of the photocurrent. The rising of the photoconductivity is very slow. The evolution time of the 1.13 eV extrinsic photocurrent is shown in Fig. 1. The steady-state is only reached after several minutes of continuous light excitation. The time needed to reach this steady-state was a function of both the photon-flux and the energy of the photons. In the curve of Fig. 1 one observes several stages of the photocurrent rising, each of them corresponds to different photosensitivity states of the samples. These states are characterized by different decay curves when the light is switched off. In Fig. 2 are shown the decays of the 1.13eV extrinsic photocurrent after several different times of excitation. It is clearly seen that the longer the excitation time, the slower is the decay of the conductivity. It can be seen that the persistency of the photoconductivity is observed after long excitation time, once the steady state is reached.
I 1
1 2
I 3
I 4
I 5
I 6
I ?
t (mln)
Fig. 2. Extrinsic photocurrent decays for different excitation periods, t, with 1.13eV photons, ( 1 ) t = 13 min, (2) t = 10.5 min, (3) t = 9.5 min, (4) t = 8 min.
The rising of the photocurrent is very similar to the building up of the photomemory effects in Gabs [4-9] i.e., the optical quenching of the photorespo,ase and the photoregeneration of traps. This fact suggests that the persistent photoconductivity in bulk Gabs could be associated to a photosensitivity state, which is generated during the excitation with photons of energies ranging from 1 to 1.25 eV at low temperatures [5, 9]. Once the new photosensitivity state is created, the sample exhibits persistent photoconductivity. We have also studied the thermal evolution of the photocurrent. The steady-state extrinsic photocurrent (1.13eV) is shown as a function of the temperature in Fig. 3. A strong thermal quenching is observed above 135 K. A decay over two orders of magnitude occurs between 120 and 135 K. When this heating is achieved the persistency of the photoconductivity is not observed any more, and hence the normal photosensitivity of the sample is restored. Then, the slow relaxation time of the free carriers is not observed without a previous persistent optical excitation at low temperature with the same photons. We can summarize the most important features of the persistent photoconductivity effect in semi-insulating bulk Gabs, as observed by us: (i) Creation of the PPC effect: long time excitation with 1-1.25eV photons at low temperature (below 130K).
Vol. 55, No. 5
LONG LIFETIME PHOTOCONDUCTIVITY IN SEMI-INSULATING BULK GaAs
10 - i
2
E
-- 1 0 - *
10-'o
I 90
I
I
I
100
110
120
I 130
I
I
140
1SO
T ('k)
Fig. 3. Thermal evolution of the extrinsic photocurrent, showing thermal quenching between 120 and 135 K. (ii) This effect is associated to a special photosensitivity state of the sample. (iii) The persistent photoconductivity is thermally destroyed. Above 135K the normal photosensitivity is restored. These results show strong similarities with the photomemory effects typically observed in GaAs, which are usually ascribed to the EL2 level [ 1 - 5 ] . Thus, the same photons produce both effects. Furthermore, the photomemory effects are thermally erased at the same temperature that the persistent photoconductivity is destroyed. Therefore, the persistent photoconductivity in semi-insulating bulk GaAs, can be considered to be a new photomemory effect of semiinsulating GaAs when it is subject to optical excitation with photons ranging from 1-1.25 eV. It is usually assumed that persistent photoconductivity is due to the existence of a repulsive potential barrier, which avoids the capture of free carriers by traps. These free carriers remain then for a long time in the free bands. The nature of such a barrier permits an understanding of the characteristic slow relaxation of the persistent photoconductivity. In the case of the high resistivity GaAs, the observed results suggest the creation of metastable states, during the optical excitation with photons of energies ranging from 1-1.25eV, having small capture cross-section for free carriers, which have then very long lifetimes. The kinetics of both the formation and the destruction of these states rules out the hypothesis of a single trap accounting for the observed behaviour. Indeed, we can assume that optical excitation
461
produces rearrangements in the neighbourhood of a photosensitive complex centre (deep level) as a consequence of the change in its charge state during the photoionization process [4, 8, 9, 21]. The metastable states created by this complex mechanism provide the physical support of the repulsive barrier, a consequence of which is the long lifetime of the free carriers. In our opinion such a kind of lattice transformations must be more easily accomplished in centres of rather low symmetry, such as aggregates [22-24]. Recently Taniguchi has proposed As-rich amorphous-like aggregates for describing the EL2 centre [25]. Obviously this kind of defect should be made more available for achieving transformations with the photons used (low energy) and on the other hand thermal annealing is achieved at very low temperature ( " 135 K), which rules out the possibility of a motion of the isolated stoichiometric defects [26,271. These results are in agreement with our previous papers [5-9] about other photomemory effects, showing the importance that optically induced defect reactions have on the properties of deep levels in semi-insulating bulk GaAs. In a further experiment currently in progress, a more complete study of these features is being made. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
G. Vincent, D. Bois & A. Chantre, J. Appl. Phys. 53, 3643 (1982). G.P. Peka, V.A. Brodovoi, I.I. Mishova & L.Z. Mirets, Soy. Phys. Semicond. 12, 540 (1978). P. Leyral, G. Vincent, A. Nouailhat & G. Guillot, Solid State Commun. 42, 67 (1982). J.P. Fillard, J. Bonnaf6 & M. Castagne, J. Appl. Phys. 56, 3020 (1984). J. Jim6nez, P. Hem~ndez, J.A. de Saja & J. Bonnaf6, J. Appl. Phys. (to be published). J. Jim6nez, M.A. Gonzalez, J.A. de Saja & J. Bonnaf6,J. Mater ScL 19, 1207 (1984). J. Bonnaf6, J. Jim6nez, M.A. Gonzalez & M. Castagn~, Phys. Scr. 30, 198 (1984). J. Jim6nez, M.A. Gonz~lez & L.F. Sanz, Solid State Commun. 49,917 (1984). J. Jim6nez, M.A. Gonzhlez, P. Hemhndez, J.A. de Saja &J. Bonnaf~, J. AppL Phys. 57, 1152 (1985). M.R. Lorenz, B. Segall & H.H. Woodbury, Phys. Rev. 134, A75 (1964). H.C. Wright, R.J. Downey & J.R. Canning, J. Phys. DI, 1593 (1968). G.W. Iseler, J.A. Kafalas A.J. Strauss, H.F. Mac Millan & R.H. Bube, Solid State Commun. 10, 619 (1972). Y.A. VUl, L.V. Golubev, L.V. Sharanova & Y. Chmartsev, Sov. Phys. Semicond. 4, 2017 (1971). M.G. Crawford, G.E. Stillman, J.A. Rossi & N. Holonyak, Phys. Rev. 168, 867 (1968). R.J. Nelson,Appl. Phys. Lett. 31,351 (1977). D.V. Lang & R.A. Logan, Phys. Rev. Lett. 39, 635 (1979).
462
LONG LIFETIME PHOTOCONDUCTIVITY IN SEMI.INSULATINGBULK GaAs Vol. 55, No. 5
D.V. Lang & R.A. Logan, Phys. Rev. BI9, 1015 (1979). 18. M. Sheikman & A.Y. Shik, Soy. Phys. Semicond. 10, 128 (1976). 19. H.J. Queisser & D.E. Theodorcu, Phys. Rev. Lett. 43,401 (1979). 20. D.E. Theodorou, H.J. Queisser & E. Bausch, AppL Phys. Lett. 41,628 (1982). 21. M. Levinson,Phys. Rev. B28, 3660 (1983). 22. D.E. Holmes, R.T. Chen, K.R. Elliot & C.E. Kirpatrick, Appl. Phys. Lett. 40, 46 (1982). 17.
23. 24. 25. 26. 27.
L.B. Ta, H.M. Hobgood, A. Rohatgi & R.N. Thomas,J. Appl. Phys. 53, 5771 (1982). J. Lagowski, H.C. Gatos, J.M. Parsey, W. Wada, M. Kaminska & W. Walukiewicz,Appl. Phys. Lett. 40, 342 (1982). M. Taniguchi & T. Ikoma, Appi. Phys. Lett. 45, 69 (1984). D.V. Lan8, R.A. Logan & L.C. Kimmerling, Phys. Rev. B15, 4874 (1977). K. Thommen,Radiat. Elf 2, 201 (1970).
Solid State Communications, Vol. 55, No. 5, pp. 459--462, 1985. Printed in Great Britain.
LONG LIFETIME PHOTOCONDUCTIVITY IN SEMI-INSULATING BULK Gabs J. Jim~nez, P. Hern~ndez and J.A. de Saja Lab. de Ffsica del Estado S61ido, Facultad de Ciencias, 47011 Valladolid, Spain and J. Bonnaf~ CEM (Centre d'Electronique de Montpellier), USTL, 34060 Montpellier, France
(Received 18 February 1985 by E.F. Bertaut) We report a new photomemory effect in semi.insulating bulk Gabs. Persistent illumination with 1.15 eV photons induces a photosensitivity state in Gabs, which is characterized by a slow decay of the photoconductivity, after the light is switched-off. The most relevant phenomenological aspects of this effect are summarized in this communication.
INTRODUCTION SEMI-INSULATING Gabs is a very promising material in order to be used in FET and IC technologies. The properties of the substrate are controlled by the deep levels, which compensate the background shallow levels. This fact has generated numerous investigations devoted to the study of the mid-gap levels in this material. These levels have a great influence on the performance of the devices. On the other hand they present an undoubtful interest, because of their striking physical properties. Thus, it is well-known that optical excitation with impurity light induces strong transformations in the photoelectronic properties of the samples, i.e., an optical quenching of the photoresponse [ I - 5 ] and a photogeneration of photosensitive traps [6-9] have been reported to occur in this material. In this paper, we report experimental evidence for a persistent photoconductivity effect (PPC) in semiinsulating Gabs. Such a phenomenon has been observed in some other II-VI [10-12] and III-V [13-17] single or alloyed compounds by several authors. Basically, the persistent photoconductivity consists of an anomalously long decay of photogenerated free carrier density after the switching off of the illumination [18]. The origin of the slow relaxation time of the nonequilibrium free carriers is the key to understanding this phenomenon in Gabs. Thus, several mechanisms have been proposed for explaining the microscopic nature of the persistent photoconductivity in the different materials showing this phenomenon. To our knowledge such a slow relaxation has never been observed in semiinsulating bulk Gabs. It has only been observed in nonhomogeneous configurations, i.e. n-n +, p - n or p-p+ structures [19, 20]. In these configurations it is assumed
that the long relaxation time of the free carriers is due to the existence of a macroscopic potential barrier between the layer and the substrate, which avoids the capture of the free electrons, and hence produces a slow decay of the electron density when the sample is kept in the dark after the end of the illumination. Our aim in this paper is to describe the most important experimental features of the persistent photoconductivity effect (PPC) in semi-insulating bulk Gabs. To give a detailed description of the origin of this phenomenon in our samples is beyond the purpose of this paper. In fact this paper deals with the most relevant phenomenological aspects of the long decay time of the photoconductivity. EXPERIMENTAL We studied the photocurrent of high resistivity Gabs samples (I07-109-cm at room temperature) cut from undoped LEC (Liquid Encapsulated Czochralski) ingots made by Thomson CSF. Several of them exhibited persistent photoconductivity, whereas the others showed either an optical quenching of the photoresponse or a photogeneration of traps. Electrical contacts were made either with silver paste or alloyed indium, annealed in vacuum at 200C for several minutes. The electrical contacts were masked in order to avoid photovoltage at the junction regions. Electrical current measurements were always performed in the linear part of the I - V characteristics in order to avoid non-linear effects. Optical excitation was provided by light from a halogen lamp passing through a high luminosity grating monochromator (Bausch-Lomb) and a filter system. The samples were mounted in the cold finger of a small liquid nitrogen cryostat, characterized by a weak thermal
459
460
LONG LIFETIME PHOTOCONDUCTIVITY IN SEMI-INSULATING BULK GaAs
1
0
-
1
Vol. 55, No. 5
~
10-'
tO-"
2
'~
II E
m-*~
, 1~.
10-1~
--
1 0 -1
1 0 -tl
I
4
I
6
I
I
8
10 t
I
12
I
14
I
16
(mln)
Fig. 1. Extrinsic photocurrent (1.13 eV) as a function of time. A strong change in photosensitivity is observed. inertia. The typical dimensions of the samples were 5 x 2 x 0.3mm 3. From optical absorption tests we can conclude that the excitation was homogeneous across the bulk of the sample in the full spectral range scanned in photoconductivity measurements. RESULTS AND DISCUSSION The experimental characterization of the persistent photoconductivity effect in semi-insulating bulk Gabs includes measurements of photoconductivity rising time, photoconductivity decay and thermal evolution of the photocurrent. The rising of the photoconductivity is very slow. The evolution time of the 1.13 eV extrinsic photocurrent is shown in Fig. 1. The steady-state is only reached after several minutes of continuous light excitation. The time needed to reach this steady-state was a function of both the photon-flux and the energy of the photons. In the curve of Fig. 1 one observes several stages of the photocurrent rising, each of them corresponds to different photosensitivity states of the samples. These states are characterized by different decay curves when the light is switched off. In Fig. 2 are shown the decays of the 1.13eV extrinsic photocurrent after several different times of excitation. It is clearly seen that the longer the excitation time, the slower is the decay of the conductivity. It can be seen that the persistency of the photoconductivity is observed after long excitation time, once the steady state is reached.
I 1
1 2
I 3
I 4
I 5
I 6
I ?
t (mln)
Fig. 2. Extrinsic photocurrent decays for different excitation periods, t, with 1.13eV photons, ( 1 ) t = 13 min, (2) t = 10.5 min, (3) t = 9.5 min, (4) t = 8 min.
The rising of the photocurrent is very similar to the building up of the photomemory effects in Gabs [4-9] i.e., the optical quenching of the photorespo,ase and the photoregeneration of traps. This fact suggests that the persistent photoconductivity in bulk Gabs could be associated to a photosensitivity state, which is generated during the excitation with photons of energies ranging from 1 to 1.25 eV at low temperatures [5, 9]. Once the new photosensitivity state is created, the sample exhibits persistent photoconductivity. We have also studied the thermal evolution of the photocurrent. The steady-state extrinsic photocurrent (1.13eV) is shown as a function of the temperature in Fig. 3. A strong thermal quenching is observed above 135 K. A decay over two orders of magnitude occurs between 120 and 135 K. When this heating is achieved the persistency of the photoconductivity is not observed any more, and hence the normal photosensitivity of the sample is restored. Then, the slow relaxation time of the free carriers is not observed without a previous persistent optical excitation at low temperature with the same photons. We can summarize the most important features of the persistent photoconductivity effect in semi-insulating bulk Gabs, as observed by us: (i) Creation of the PPC effect: long time excitation with 1-1.25eV photons at low temperature (below 130K).
Vol. 55, No. 5
LONG LIFETIME PHOTOCONDUCTIVITY IN SEMI-INSULATING BULK GaAs
10 - i
2
E
-- 1 0 - *
10-'o
I 90
I
I
I
100
110
120
I 130
I
I
140
1SO
T ('k)
Fig. 3. Thermal evolution of the extrinsic photocurrent, showing thermal quenching between 120 and 135 K. (ii) This effect is associated to a special photosensitivity state of the sample. (iii) The persistent photoconductivity is thermally destroyed. Above 135K the normal photosensitivity is restored. These results show strong similarities with the photomemory effects typically observed in GaAs, which are usually ascribed to the EL2 level [ 1 - 5 ] . Thus, the same photons produce both effects. Furthermore, the photomemory effects are thermally erased at the same temperature that the persistent photoconductivity is destroyed. Therefore, the persistent photoconductivity in semi-insulating bulk GaAs, can be considered to be a new photomemory effect of semiinsulating GaAs when it is subject to optical excitation with photons ranging from 1-1.25 eV. It is usually assumed that persistent photoconductivity is due to the existence of a repulsive potential barrier, which avoids the capture of free carriers by traps. These free carriers remain then for a long time in the free bands. The nature of such a barrier permits an understanding of the characteristic slow relaxation of the persistent photoconductivity. In the case of the high resistivity GaAs, the observed results suggest the creation of metastable states, during the optical excitation with photons of energies ranging from 1-1.25eV, having small capture cross-section for free carriers, which have then very long lifetimes. The kinetics of both the formation and the destruction of these states rules out the hypothesis of a single trap accounting for the observed behaviour. Indeed, we can assume that optical excitation
461
produces rearrangements in the neighbourhood of a photosensitive complex centre (deep level) as a consequence of the change in its charge state during the photoionization process [4, 8, 9, 21]. The metastable states created by this complex mechanism provide the physical support of the repulsive barrier, a consequence of which is the long lifetime of the free carriers. In our opinion such a kind of lattice transformations must be more easily accomplished in centres of rather low symmetry, such as aggregates [22-24]. Recently Taniguchi has proposed As-rich amorphous-like aggregates for describing the EL2 centre [25]. Obviously this kind of defect should be made more available for achieving transformations with the photons used (low energy) and on the other hand thermal annealing is achieved at very low temperature ( " 135 K), which rules out the possibility of a motion of the isolated stoichiometric defects [26,271. These results are in agreement with our previous papers [5-9] about other photomemory effects, showing the importance that optically induced defect reactions have on the properties of deep levels in semi-insulating bulk GaAs. In a further experiment currently in progress, a more complete study of these features is being made. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
G. Vincent, D. Bois & A. Chantre, J. Appl. Phys. 53, 3643 (1982). G.P. Peka, V.A. Brodovoi, I.I. Mishova & L.Z. Mirets, Soy. Phys. Semicond. 12, 540 (1978). P. Leyral, G. Vincent, A. Nouailhat & G. Guillot, Solid State Commun. 42, 67 (1982). J.P. Fillard, J. Bonnaf6 & M. Castagne, J. Appl. Phys. 56, 3020 (1984). J. Jim6nez, P. Hem~ndez, J.A. de Saja & J. Bonnaf6, J. Appl. Phys. (to be published). J. Jim6nez, M.A. Gonzalez, J.A. de Saja & J. Bonnaf6,J. Mater ScL 19, 1207 (1984). J. Bonnaf6, J. Jim6nez, M.A. Gonzalez & M. Castagn~, Phys. Scr. 30, 198 (1984). J. Jim6nez, M.A. Gonz~lez & L.F. Sanz, Solid State Commun. 49,917 (1984). J. Jim6nez, M.A. Gonzhlez, P. Hemhndez, J.A. de Saja &J. Bonnaf~, J. AppL Phys. 57, 1152 (1985). M.R. Lorenz, B. Segall & H.H. Woodbury, Phys. Rev. 134, A75 (1964). H.C. Wright, R.J. Downey & J.R. Canning, J. Phys. DI, 1593 (1968). G.W. Iseler, J.A. Kafalas A.J. Strauss, H.F. Mac Millan & R.H. Bube, Solid State Commun. 10, 619 (1972). Y.A. VUl, L.V. Golubev, L.V. Sharanova & Y. Chmartsev, Sov. Phys. Semicond. 4, 2017 (1971). M.G. Crawford, G.E. Stillman, J.A. Rossi & N. Holonyak, Phys. Rev. 168, 867 (1968). R.J. Nelson,Appl. Phys. Lett. 31,351 (1977). D.V. Lang & R.A. Logan, Phys. Rev. Lett. 39, 635 (1979).
462
LONG LIFETIME PHOTOCONDUCTIVITY IN SEMI.INSULATINGBULK GaAs Vol. 55, No. 5
D.V. Lang & R.A. Logan, Phys. Rev. BI9, 1015 (1979). 18. M. Sheikman & A.Y. Shik, Soy. Phys. Semicond. 10, 128 (1976). 19. H.J. Queisser & D.E. Theodorcu, Phys. Rev. Lett. 43,401 (1979). 20. D.E. Theodorou, H.J. Queisser & E. Bausch, AppL Phys. Lett. 41,628 (1982). 21. M. Levinson,Phys. Rev. B28, 3660 (1983). 22. D.E. Holmes, R.T. Chen, K.R. Elliot & C.E. Kirpatrick, Appl. Phys. Lett. 40, 46 (1982). 17.
23. 24. 25. 26. 27.
L.B. Ta, H.M. Hobgood, A. Rohatgi & R.N. Thomas,J. Appl. Phys. 53, 5771 (1982). J. Lagowski, H.C. Gatos, J.M. Parsey, W. Wada, M. Kaminska & W. Walukiewicz,Appl. Phys. Lett. 40, 342 (1982). M. Taniguchi & T. Ikoma, Appi. Phys. Lett. 45, 69 (1984). D.V. Lan8, R.A. Logan & L.C. Kimmerling, Phys. Rev. B15, 4874 (1977). K. Thommen,Radiat. Elf 2, 201 (1970).