Journal of Non-Crystalline Solids 97&98 (1987) 1147-1150 North-Holland, Amsterdam
HOT PHOTOLUMINESCENCE IN CHALCOGENIDE GLASSES
Kazuro MURAYAMA Dept. of Physics, Nihon University, 3 Sakurajosui, Setagaya-ku, Tokyo 156 Hot photoluminescence and i t s p o l a r i z a t i o n memory were observed at room temperature in a-As2S 3 and a-GeSe2. The hot photoluminescence was i n t e r p r e t e d with the Franck-Condon p r i n c i p l e as well as the steady-state photoluminescence. I.
INTRODUCTION Amorphous (a-) As2S3 and a-GeSe2 e x h i b i t s a steady-state photoluminescence
(PL) spectrum of wide band width centered at h a l f of the o p t i c a l gap energy at low temperature [ 1 , 2 ] .
The r a d i a t i v e quantum e f f i c i e n c y is several tens percent.
This PL has been associated with the r a d i a t i v e recombination at l o c a l i z e d states of band t a i l s .
The large Stokes s h i f t has been i n t e r p r e t e d as due to electron-
phonon i n t e r a c t i o n and the PL has been understood with the Franck-Condon p r i n c i ple.
At room temperature, the steady-state PL disappears
in a-As2S 3 and instead
a broad hot photoluminescence (hot-PL) band with the low quantum e f f i c i e n c y of 5xlO -5 is observed in the energy region higher than the steady-state PL [ 3 ] . In t h i s paper, we report (PM) in a-As2S 3 and a-GeSe2.
the hot-PL spectrum and i t s p o l a r i z a t i o n memory I t is shown that the hot-PL can be understood with
the Franck-Condon p r i n c i p l e as well as the steady-state PL. 2.
EXPERIMENTAL The samples of a-As2S 3 with the thickness of 2n~n and a-GeSe2 with the t h i c k -
ness of 1 mm were prepared.
The hot-PL measurements were carried out on the
f l a t surfaces of the samples which had been polished with diamond paste. samples were excited with an Argon ion laser.
The hot-PL was analyzed by a
double-grating monochromator (Spex 1680) and detected with a GaAs p h o t o m u l t i p l i e r . The weak signal of the hot-PL was processed by a computer-based photoncounting system. 3.
In the measurements of the PM, p o l a r i z e r s (Polaroid HN-38) were used.
RESULTS (a) Hot-PL in a-As2S 3 The hot-PL spectrum in a-As2S 3 observed with the e x c i t a t i o n energy of 2.54 eV
(488.0 nm) at room temperature is shown in Fig.l(a). The quantum efficiency of the hot-PL was 10-4 to lO"5. The intensity of the hot-PL was measured in the unit of W/nm cm2. The intensity of the hot-PL decreased with decreasing PL energy from 2.45 to 2.10eV, increased a l i t t l e b i t at 2.0 eV and decreased again.
0022-3093]87/$03.50 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
K. Murayama / Hot photoluminescence
Fig.l Hot-PL spectrum and i t s PM observed with the excitation energy of 2.54 eV at room temperature in a-As2S3.
(a) ............. 3
In the other word, the spectrum 2
has a hole in the structure at 2.1 eV.
Whena position on
the sample surface where the
hot-PL •." "'". . . . . . . .
hot-PL was measured moved from 0 1.0
the excitation position on the surface, the hole was growing.
This property of the h61e can be understood as due to the ~-- O.5
reabsorption of the hot-PL into the sample.
The hot-PL i l °m
emitted from a site in the
sample is detected in the two
channels: in the f i r s t channel
i t is detected without the ref-
lection on the sample surface
and in the second channel i t is detected after the multi-reflections on the sample surface.
The coefficient of
the band-to-band absorption in a-As2S3 is about 5 cm-l at 2.1 eV  as shown in Fig.l(a).
Considering the thickness of 2 mm, this shows that the PL component
with the energy higher than 2.1 eV is never detected in the second channel for the effect of the reabsorption.
This suggests that the hole observed in the
hot-PL spectrum arises from the reabsorption of the hot-PL into the sample. The hot-PL excited with the polarized l i g h t was observed by the monochromatordetector system with a polarizer. at room temperature. P
The PM was measured for the hot-PL in a-As2S3
The PM ( or depolarization ) is defined by the equation  I#
I// + I z where I//
is the hot-PL intensity when the axis of the polarizer used for the hot-
PL measurement is parallel to the polarization of the exciting l i g h t and I~_ is the intensity when the axis is perpendicular to the exciting l i g h t . The PM of the hot-PL observed with the exciting l i g h t of 2.54 eV is shown in Fig.l(b).
The high PM of 0.95 observed near the exciting l i g h t corresponds to
the depolarization of the resonant Ramanscattering appearing near 2.5 eV.
PM kept 0.42 from 2.45 to 2.10 eV. In the region lower than 2.10 eV, the PM decreased with decreasing PL energy. We had the similar results through
K. Murayama / Hot photoluminescence
"" "',"..,......... "'"'hot- PL
exc.'. ~OQ I l
,,o • • ",•'°•
L~oround J State
I State ,
1.6 1.8 2.0 2.2 2.~. 2.6 ZB Energy (eV)
Fig.2 Hot-PL spectrum and i t s PM observed with the excitation energy of 2.54 eV at room temperature in a-GeSe2. The optical absorption spectrum observed by Tronc et al.  is also shown.
I Fig.3 The Franck-Condon model of PL. Q shows the l a t t i c e distortion around the PL center and E is the total energy of electron and l a t t i c e .
different wavelengths of the Argon ion laser. (b) Hot-PL in a-GeSe2 The hot-PL with the quantum efficiency of lO-4- lO"5 was also observed at room temperature in a-GeSe2.
The spectrum and the PM of the hot-PL observed
with the excitation energy of 2.54 eV are shown in Fig.2(a) and (b), respectively. The intensity decreased with decreasing PL energy from 2.45 eV and the spectrum had a minimumat 1.8 eV.
The minimum can be regarded as due to the reabsorption
of the hot-PL into the sample as well as in a-As2S3 because the absorption coefficient at 1.8 eV in a-GeSe2 is about 5 cm- l as shown in Fig.2(a). The high PM of 0.7 was observed near 2.SeV where the resonant Ramanscattering line appeared.
The PM kept 0.45-0,35 from 2.45 eV to 1.9 eV and decreased with
decreasing PL energy. 4.
DISCUSSIONS The PM of the hot-PL was 0.35-0.45 near the exciting l i g h t in both samples.
In amorphous semiconductors where the optical transition axis is randomly oriented,
K. Murayama / Hot photoluminescence
the maximum of the PM is 0.5 [ 7 ] .
The high PM close to the maximum observed
is only achieved by the d i r e c t e x c i t a t i o n to PL centers at band t a i l s .
d i r e c t e x c i t a t i o n and the r e s u l t a n t PL and hot-PL are c l e a r l y described by the Franck-Condon model shown in Fig.3.
Here, the horizontal and v e r t i c a l axes show
the l a t t i c e d i s t o r t i o n around the PL center and the t o t a l energy of electron and lattice.
The o p t i c a l e x c i t a t i o n of the PL center is indicated by the v e r t i c a l
t r a n s i t i o n from the minimum of the ground state to the excited state.
excited electron or hole d i s s i p a t e the energy by inducing the l a t t i c e d i s t o r t i o n around the PL center.
The steady-state PL is indicated by the v e r t i c a l t r a n s i -
tion from the minimum of the excited state to the ground state.
In t h i s Franck-
Condon model, the hot-PL is the PL obtained when a part of electrons or holes on the way to the minimum of the excited state f a l l s r a d i a t i v e l y to the ground state.
Therefore, the r a d i a t i v e quantum e f f i c i e n c y of the hot-PL :
is given by (2)
where Tl is the l a t t i c e r e l a x a t i o n time and Tr is the r a d i a t i v e t r a n s i t i o n time from the excited state to the ground state. Assuming Zl=lO -12 sec and z~:lO -8 sec, the value of n
Since the minimum r a d i a t i v e t r a n s i t i o n
time is 10-8 sec, the quantum e f f i c i e n c y of the hot-PL is always smaller than 10-4 The reason why the hot-PL remembers the p o l a r i z a t i o n of the e x c i t i n g l i g h t is that the hot-PL is induced by the o p t i c a l t r a n s i t i o n between the same states as those in the e x c i t a t i o n .
The decreasing of the PM at the low hot-PL energy
cannot be explained with the Franck-Condon model shown in Fig.3.
As the reason
of the decreasing of the PM, the hopping of electron or hole to another PL centers and the level crossing of the PL center are l i s t e d . ACKNOWLEDGEMENT The author thanks Mr. M. Kaneko and Mr. K. Sugiyama f o r the help with the hot-PL measurements. REFERENCES [ I ] R.A. Street, Adv. Phys. 2_55, 397 (1976).  V.A. Vassilyev, M. Koos and I.K. Somogyi, P h i l . Mag. B39, 333 (1979).  K. Murayama and M.A. Bosch, Phys. Rev. B25, 6542 (1982).  F. Kosek and J. Tauc, Czech. J. Phys. B20, 94 (1970).  K. Murayama, H. Suzuki and T. Ninomiya, J. Non-crystalline Solids 35/36, 917 (1980).  P. Tronc, M. Bensoussan and A. Brenac, Phys. Rev. B8, 5947 (1973).  K. Murayama, "Disordered Semiconductors" (Plenum Publishing Corp. 1987) p-185.