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DYNAMICS OF EXCITONS IN GaSe AS A
FUNCTION OF TEMPERATURE
V. Capozzi, M. Montagna
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C7, supplément au n°10, Tome 46, octobre 1985 page C7-191
DYNAMICS OF EXCITONS IN GaSe AS A FUNCTION OF TEMPERATURE
V. Capozzi . and M. Montagna
Centvo Studi del Consiglio Nazionale delle Ricerche, Trento, Italy Résumé - Nous avons étudié la dynamique des excitons directs et indirects à la fois libres et localisés par le désordre d'empilementdes couches, dans le semiconducteur GaSe aux températures de 2 à 150 K et en fonction de l'éner-gie de photoexcitation.
Abstract - The dynamics of direct and indirect excitons, either free or loca-lized by the layer stacking disorder is studied in GaSe at temperatures from 2 to 150 K and as a function of the excitation energy.
I - INTRODUCTION
The conduction band of the layered semiconductor GaSe has an indirect minimum at the M point of the Brillouin zone which is only 25 meV lower than the direct minimum /l/. In the luminescence spectra (LS), contributions from both direct and indirect excito-nic recombinations are observed /2,3/. Direct and indirect excitons can be localized by the lattice defects, as shallow and deep centres (acceptors or donors), mainly due to the stacking faults disorder, which easily form in layer compounds /4/. As a conse-quence, the luminescence spectra are very structured: besides the free exciton lines; many features due to bound exciton recombinations are present in the LS /2,5/. At re-latively high temperature (above 60 K) the emission spectra show only four lines due to free - as well as bound - direct and indirect excitonic emissions and the excito-nic dynamics is relatively simple, because free and bound exciton states are in ther-mal equilibrium /3/. On the contrary, at low temperatures the LS are much more struc-tured and sample dependent. In fact, excitons can be localized either by shallow or deep centres, being their hopping times larger or comparable to the radiative life-time /4/. In this work we studied the excitonic dynamics by investigating the depen-dence on the temperature and on excitation energy of the LS, for some GaSe samples with different structural disorder.
II - EXPERIMENTAL DETAILS
The GaSe crystals (0.1 mm thick and£-type /6/) were grown by the Bridgman method. The samples were put in a liquid He cryostat whose temperature could vary from 1.8 to 300 X. The samples were excited by the line 5145 A of an Ar-ion laser for the LS measured vs T, while for the selective luminescence spectra (SLS), a cw-dye laser (band-width = 0.5 cm and fed with Rhodamine 6G) was used. The illuminated spot had a diameter of about 0.1 mm. The luminescence was analysed by a double spectrometer having a weak stray-light, in order to detect the signal at frequencies very close (a few cm ) to the excitation line. The signal was recorded by using photon counting techniques.
also Dipart. di Fisica, University di Bari, 70126 Bari, Italy also Dipart. di Fisica, University di Trento, 38050 Povo, Italy
C7-192 JOURNAL
DE
PHYSIQUEI11 - EXPERIMENTAL RESULTS AND DISCUSSION The low temperature LS of GaSe shows three main contributions as reported in fig. 1. The first group of lines
-1
(above 16900 cm ) are attributed to the free or weakly localized direct excitons /4/, the second one (from
-1
about 15500 to 16900 cm ) are due to the recombinations of direct bound excitons /2,3/, the remaining lines
-1
at energies below 16500 cm were assl gned to the recombination of free (IFX) and bound (IBE) indirect excitons /2,3,5/. The three samples whose LS are reported in fig. 1 were chosen among all the investigated crystals because of the different relative im- portance of the above three groups of lines. The spectrum (c) in which the
triplet state of the direct free C . ) , . . .
.
exciton line is well visible at 17016 I " ' " ' I I I I t '
-1 17000 16500 16000
cm /1,2,4/ is typical of the high
quality samples cleaved without strains
PHOTON ENERGY
(cm-1)(crystals of type A). The spectrum (a) ?ig. 1
-
Luminescence s ectra of <-GaSe is typical of samples containing many at 4.2 K and at 2 W-cm-5
from three sam- strains after the cleavage (crystals ples whoose strain density decreases from of type C). The spectrum (b) comes from(a) to (.)., crystals of intermediate quality
(type B). We observe that the lumine-
scence of the first two groups of lines increases relatively to the indirect exciton emissions, going from the spectrum (a) to (c) of fig. 1. A more complete coinparison among the different samples is got by studing the temperature dependence of their LS. Fig. 2, in particular shows a series of spectra from 1.9 to 100 X relative to a cry? - tal of type B (see also fig. 1 b). By raising the temperature, the first group of lines merge into the band DFE, which persists .up to 300 K. As reported in ref. / 4 / ,
I
this is due to the thermal depopulation of the shallow localized excitons. In fact the excitons localized within a few layers can thermally get moving freedom also along the c-axis /4/.
Fig. 2
-
Temperature dependence of lumine- -2 scence spectra of6 -
GaSe at 2 Wscm.
(a) 1.9 K , (b) 13 K, (c) 21 K , (d) 27 K , (e) 45 K , (f) 61 K and (g) 100 K.
also indirect excitons are localized by the stacking disorder which prevents the motion along the c-axis.
For understing the dynamics between the direct and indirect exciton states, we meg sured the intensities of the different coy! tributes to the luminescence. In fig. 3 we report the ratio of the intensities of the first two groups of lines (direct emis sions) over the third group (indirect emis sion) vs. 1/T. Two different behaviour are evident: at high temperatures R follows the law R-exp (AE/kT), with A E = 32 meV in good agreement with the energy separa- tion between the direct and indirect free exciton ground states /I/. By lowering T, R does not indefinitely decreases, but below 45 K it rises again up to a constant value. This behaviour indicates that the electrons pumped in the conduction band
17000 16500 16000 can relaxe forming both direct and indi-
PHOTON ENERGY
(=rn-l) rect excitons, which are not in thermalequilibrium below 50 K. A better insight can be obtained comparing the analogous results for the 3 types of samples. The sam- ples of quality A and B {see the spectrum (c) and respectively (b) in fig. 1) have a temperature behaviour very similar to that of fig. 2. For these crystals, the ratio R vs.l/T is always the same above nearly 50 K and has a minimum at 45 K. But in the low temperature range, the asymptotic value of R varies within a factor 2 or 3, sample by sample. The highly strained samples (see fig.la) behave differently: i.e. the indi rect luminescence is dominant even at low temperature and the direct free exciton li- nes are not detectable. By increasing T, the direct bound excitonic lines decrease as in the other samples, but DFE continues to be absent. Moreover, above nearly 60 K also the indirect luminescence tends to disappear in connection with the onset of the termalization between direct and indirect exciton states. These results seem to indi- cate that by increasing the lattice disorder, non radiative recombination channels become more important for direct than for indirect excitons.
The model we are discussing points out that direct and indirect excitons are formed indipendently at the two minima with comparable probabilities. At low temperatures, direct excitons do not relaxe to the indirect states; oniy at T = 50 K this transfer time becomes comparable to the direct exciton life-time (about 1 ns, sample depen- dent). The existance of an activation energy between direct and iridirect states indi- cates that the one-phonon transfer process is not active, at least at low temperatu- res, where excitons are localized within a few layers. A confirmation of this idea comes from the selective luminescence spectra.
C7-194 JOURNAL DE PHYSIQUE
Fig. 3
-
Plot of the ratio ( R ) of the direct excitonic luminescence intensity over -2the indirect emission intensity vs.l/T, at 2 Wcm
.
Fig. 4
-
Luminescen e spectra of c - ~ a ~ e at-5
4.2 K and at 1 Wcm with excitation: (a) at 2.103 eV, (b) at 2.106 eV, (c) at 2.110 eV
corresponding to the peak of the excitonic
-
absorption line. The arrow and blanks indicate-
the laser excitation energy. >
b
-
0: z LL cerning the direct excitons bound to shallpw
+
donors. Line narrowing effects of some lines Z
LL are also observed. Our SLS confirm that the c
z hopping time among localized exciton states w
C:
are longer or comparable to their life-time. (I]
W
-
ACKNOWLEDGMENTSWe express our thanks to F. LBvy for the cry2 tals and to A . Hinafra for helpful and sti- mulating discussions.
PHOTON ENERGY feV) 2.1 0 2.0 5 2.00
REFERENCES
/l/ Xercier A., Mooser
E.
and Voitchovsky J.P., Phys. Rev.-
B12 (1975) 4307./2/ Belenkii G.L., Godzhaev M.O., Nani R., Salaev E. and Sulimanov R.A., Sov. Phys. Semicond.
11
(1977) 506./3/ Capozzi V., Caneppele S. and Montagna M., Phys. Status Solidi ( b )
129
(1985) 176 and Capozzi V., Phys.Rev.B23
(1981: 836./4/ Sasaki Y. and Nishina Y., Physica (1981) 45. /5/ Sasaki Y. and Nishina Y., Phys. Rev.
