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Defect formation by excited-state mechanisms in rare gas solids
LUMINESCENCE
Journal of Luminescence 53(1992)517—518
JOURNAL OF
Defect formation by excited-state mechanisms in rare gas solids I.Ya. Fugol’, O.N. Grigorashchenko, A.N. Ogurtsov and E.V. Savchenko Institute for Low Temperature Physics and Engineering, Ukranian Academy of Sciences, Kharkor, Ukraine
The electronically-induced defect formation in rare gas solids is observed by means of luminescent VUV-spectroscopy. Defect formation in the excited states during self-trapping in Ne and Xe is established. The microscopic excited-state mechanisms (on-center in Ne and off-center in Xe) are proposed for defect formation during trapping of electronic excitation.
1. Introduction
excluded classical (knock-on and thermo-induced) mechanisms of defect formation.
The creation of defects in solids induced by trapping of excitation is presently a topic of active interest [1,21.The work of refs. [1,21deals mainly with semiconductors and ionic crystals. Rare gas solids (RGS) offer the best opportunity to study some of the electronically-induced defect formation mechanisms because of their simple structure, small binding energy and strong electron— phonon interaction. Such mechanisms were predicted [31and found [41in RGS. This paper presents the spectroscopic observation of defect formation in the excited states of solid Ne and Xe.
3. Results and discussion The essence of electronically-induced defect formation is the conversion of electronic “potential” energy into kinetic energy of lattice atoms. The necessary conditions for this transformation are: I) the presence of trapping of the electronic excitation and II) the energy release during trapping must exceed the threshold energy of the defect formation. The most effective channel of trapping is the exciton self-trapping into atomictype (A-STE) and molecular-type (M-STE) states. R*
R*(A)
+
L~E*(A~STE) hr’(A) -‘
2. Experiment
R (1)
Samples were grown from the vapour phase in a special cryogenic cell [51.The vapour pressure in the cell was about 10 mbar. Subthreshold irradiation by low-energy electrons was used for excitation. Defect formation was detected by means of luminescent VUV-spectroscopy. The measurements were performed at 5—10 K for Ne and 5—60 K for Xe. Conditions of the experiments
Correspondence to: Dr. E.V. Savchenko, Institute for Low Temperature Physics and Engineering, Ukranian Academy of Sciences, Kharkov, Ukraine. 0022-2313/92/$05.OO © 1992
+
—
R~(M)+ L~E*(M~STE)
R + R*
hv(M) + R + R + ~E (D). (2) There are two different stages in which the defect formation processes occur: i) during self-trapping and ii) after annihilation of STE-states. The case i) was studied experimentally in Ne and Xe. In Ne, the self-trapping into A-STE is most effective; in Xe, only the channel of M-STE self-trapping exists. —*
Figure 1 shows the emission spectra of solid
Xe at low temperature. The excimer Xe~’band
contains two components M1 and M2. The analy-
Elsevier Science Publishers B.V. All rights reserved
I
I. a. I ugal et a!.
Dc/eel formation by excited-state meehu,uons in rare gas soOth,
tion between the intensities of the M1 and M hands. The rate of defect accumulation S is found
Xe
S
Bmir 2mm
~
2
7 2
/ 4
1
H
~
rmm~rqy, I V Fig. I. Evolution of the Xe M-band luminescence with time. Inset: temperature dependence of the defect accumulation rate S.
sis of the spectra taken from the samples of different qualities permits one to relate M1 to a defect center and M2 to Xe~’centers in a regular lattice. The component M1 increases with the time of irradiation. A mechanism of defect formation in the excited state is proposed which consists of the offcenter displacement of the excimer (from the on-center state D25 to the off-center one C2~). The process is found to proceed in two stages: a reversible stage (the symmetry persist) and an irreversible one (a defect is formed). The offcenter transition is followed by overcoming the potential harrier. In the experiment this mechanism is displayed by a temperature anticorrela-
excited state lifetime. At temperatures T> 30 K the stable defect accumulation slows down due to a drastic decrease efficiency of this mechanism in the excitation is governed lifetime by the [61. to Besides, be temperature we have detected dependent defect (seeformation the inset).inThe Xc connected with dissociation after radiative decay of the excited state (case ii)). The analogous process of defect formation was observed for Kr. The evaluation of the data is in progress. In solid Ne and Ar it was found that local deformations appear around an excited center in a regular lattice under the action of the electron component during exciton trapping and lead to structural defects [4]. The analysis of the relaxation kinetics of in the energy levels reveals that the defect formation probability is proportional to the population time of the states. The correlation between the defect formation efficiency and the lifetime of the excited state is a direct cvidence of the defect formation in the excited state.
References [I] N. Itoh. Adv. Phys. .31 (1982)491. [21 R.T. Williams, KS. Song, W.L. Faust and C.H. Leung, Phys. Rev. B 33 (1986) 7232 [3] F.V. Kusrnartsev and El. Rashba. Czech. J. Phys. B 32 (1982) 54. [4] E.V. Savchenko. Yu.l. Ryhalko and l.Ya. Fugol’. Soy. J. Low Temp. Phys. 14 (1988) 220. [5] l.Ya. Fugol’. A.N. Ogurtsos, ON. Grigorashchenko and LV. Savchenko. Soy J Low. Temp. Phys. 18 (1992) 44. [6] (3. Zimmerer, in: Excited state speetroscopy in solids (North-Holland. Amsterdam. 1987) p. 37.
Journal of Luminescence 53(1992)517—518
JOURNAL OF
Defect formation by excited-state mechanisms in rare gas solids I.Ya. Fugol’, O.N. Grigorashchenko, A.N. Ogurtsov and E.V. Savchenko Institute for Low Temperature Physics and Engineering, Ukranian Academy of Sciences, Kharkor, Ukraine
The electronically-induced defect formation in rare gas solids is observed by means of luminescent VUV-spectroscopy. Defect formation in the excited states during self-trapping in Ne and Xe is established. The microscopic excited-state mechanisms (on-center in Ne and off-center in Xe) are proposed for defect formation during trapping of electronic excitation.
1. Introduction
excluded classical (knock-on and thermo-induced) mechanisms of defect formation.
The creation of defects in solids induced by trapping of excitation is presently a topic of active interest [1,21.The work of refs. [1,21deals mainly with semiconductors and ionic crystals. Rare gas solids (RGS) offer the best opportunity to study some of the electronically-induced defect formation mechanisms because of their simple structure, small binding energy and strong electron— phonon interaction. Such mechanisms were predicted [31and found [41in RGS. This paper presents the spectroscopic observation of defect formation in the excited states of solid Ne and Xe.
3. Results and discussion The essence of electronically-induced defect formation is the conversion of electronic “potential” energy into kinetic energy of lattice atoms. The necessary conditions for this transformation are: I) the presence of trapping of the electronic excitation and II) the energy release during trapping must exceed the threshold energy of the defect formation. The most effective channel of trapping is the exciton self-trapping into atomictype (A-STE) and molecular-type (M-STE) states. R*
R*(A)
+
L~E*(A~STE) hr’(A) -‘
2. Experiment
R (1)
Samples were grown from the vapour phase in a special cryogenic cell [51.The vapour pressure in the cell was about 10 mbar. Subthreshold irradiation by low-energy electrons was used for excitation. Defect formation was detected by means of luminescent VUV-spectroscopy. The measurements were performed at 5—10 K for Ne and 5—60 K for Xe. Conditions of the experiments
Correspondence to: Dr. E.V. Savchenko, Institute for Low Temperature Physics and Engineering, Ukranian Academy of Sciences, Kharkov, Ukraine. 0022-2313/92/$05.OO © 1992
+
—
R~(M)+ L~E*(M~STE)
R + R*
hv(M) + R + R + ~E (D). (2) There are two different stages in which the defect formation processes occur: i) during self-trapping and ii) after annihilation of STE-states. The case i) was studied experimentally in Ne and Xe. In Ne, the self-trapping into A-STE is most effective; in Xe, only the channel of M-STE self-trapping exists. —*
Figure 1 shows the emission spectra of solid
Xe at low temperature. The excimer Xe~’band
contains two components M1 and M2. The analy-
Elsevier Science Publishers B.V. All rights reserved
I
I. a. I ugal et a!.
Dc/eel formation by excited-state meehu,uons in rare gas soOth,
tion between the intensities of the M1 and M hands. The rate of defect accumulation S is found
Xe
S
Bmir 2mm
~
2
7 2
/ 4
1
H
~
rmm~rqy, I V Fig. I. Evolution of the Xe M-band luminescence with time. Inset: temperature dependence of the defect accumulation rate S.
sis of the spectra taken from the samples of different qualities permits one to relate M1 to a defect center and M2 to Xe~’centers in a regular lattice. The component M1 increases with the time of irradiation. A mechanism of defect formation in the excited state is proposed which consists of the offcenter displacement of the excimer (from the on-center state D25 to the off-center one C2~). The process is found to proceed in two stages: a reversible stage (the symmetry persist) and an irreversible one (a defect is formed). The offcenter transition is followed by overcoming the potential harrier. In the experiment this mechanism is displayed by a temperature anticorrela-
excited state lifetime. At temperatures T> 30 K the stable defect accumulation slows down due to a drastic decrease efficiency of this mechanism in the excitation is governed lifetime by the [61. to Besides, be temperature we have detected dependent defect (seeformation the inset).inThe Xc connected with dissociation after radiative decay of the excited state (case ii)). The analogous process of defect formation was observed for Kr. The evaluation of the data is in progress. In solid Ne and Ar it was found that local deformations appear around an excited center in a regular lattice under the action of the electron component during exciton trapping and lead to structural defects [4]. The analysis of the relaxation kinetics of in the energy levels reveals that the defect formation probability is proportional to the population time of the states. The correlation between the defect formation efficiency and the lifetime of the excited state is a direct cvidence of the defect formation in the excited state.
References [I] N. Itoh. Adv. Phys. .31 (1982)491. [21 R.T. Williams, KS. Song, W.L. Faust and C.H. Leung, Phys. Rev. B 33 (1986) 7232 [3] F.V. Kusrnartsev and El. Rashba. Czech. J. Phys. B 32 (1982) 54. [4] E.V. Savchenko. Yu.l. Ryhalko and l.Ya. Fugol’. Soy. J. Low Temp. Phys. 14 (1988) 220. [5] l.Ya. Fugol’. A.N. Ogurtsos, ON. Grigorashchenko and LV. Savchenko. Soy J Low. Temp. Phys. 18 (1992) 44. [6] (3. Zimmerer, in: Excited state speetroscopy in solids (North-Holland. Amsterdam. 1987) p. 37.