Bidadi, H

Kalafi, M

Kouhi, M

Salmanov, V M

Tajalli, H

IACS Institutional Repository

IndianJ. Phys. 72A (1). 43-48 (1998)I J P  A— an international journalNonlinear light absorption in GaSei_*Sx solid solutions under high excitation levelsH Tajalli, M Kalafi, H Bidadi, M Kouhi and V M Salmanov1Center for Applied Physics Research, Tabriz University, Tabriz, Iran Received 8 January 1997, atcepted 19 November 1997Abstract : Transmission, phoioluminescence and photoconductivity spectra of GoSe] solid solutions have been investigated experimental!) in the exciton resonance region at high optical excitations The absorption edge of GaSe|_rSK is caused by exciton transitions and lineal shifts towards short wavelengths by raising the value of \  in the solution The exciton peak disappears and nonlinear absorption appears in GaSe|_*St crystals by increasing the excitation intensity The new luminescence band appears at about 20 meV below the free exciton line at high excitation levels These pccularities are interpreted by means of the excilon-exciton scattering processKeywords : Exciton, GaSe|. xSl solid solutions, nonlinear light absorption PACS Nos. : 71 3SCc.7H55.CiI. IntroductionGallium Selenidc (GaSc), Gallium Sulfide (GaS) and Gallium Selenium Suitidc (GaSC|_,St) arc III—VI semiconductors which crystallize with a lamellar structure. The bonding between two adjacent layers is of the Van dcr Waals type, while within the layer the bonding is predominantly covalent. Therefore, the bulk material obtains a strong- mechanical anisotropy which allows easily to prepare thin samples using a simple peeling procedure. The optical c-axes of crystals are orthogonal to the layers having thicknesses of ~0.8 nm 11], The exciton binding energy is equal to 20 rneV, which is dose to the room temperature thermal energy [2]. Therefore, one can observe the exciton in GaSc^S* at room temperature from optical transmission experiments. Exciton absorption and luminescence in GaSe^S* have been investigated by a number of investigators |3-7|. In these crystals (with the exception of GaSe [8-18]), the optical absorption has not practically been considered at high excitation levels.Permanent address : Baku Stale University, Baku-37014S, Z Khalilov Av. 23© 1998 I ACSIn the present work, the nonlinear light absorption has been investigated experimentally in the exciton resonance region at high optical excitation levels in GaSe^jS* layered crystals.2. Experimental methodGaSej.jS* crystals (x = 0, 0.05, 0.1, 0.2 and 0.25) were grown by Bridgman technique. Thicknesses of samples were about 30-100 pm. Ohmic contacts were obtained by deposition of high-purity indium on the surface of samples. As an excitation source, a dye laser (PR A, LN-107) pumped by the output of a N2-laser (PRA, LN-1000) were used. The dye laser gave possibility of selecting different wavelengths (473-547) nm, (568-605) nm and (594-643) nm with the resolution 0.04 nm. The pulse power was 120 kW at the repetition frequency of 10 Hz and at pulse width 1 ns. The laser light was focussed opto the sample with the focus diameter of about 0.5 mm. Laser beam intensity was vaijied by inserting calibrated neutral density filters. Luminescence was excited by dye laser photons with energy more than the band gap width ER of the GaSe^jS* crystals ( /b c = 500', nm). Luminescence was detected under a small angle with respect to the c-optical axis <\f the crystal. Photoluminescence spectra were analysed by means of a diffraction grating monochromator (JOBIN-YVON) with the reciprocal dispersion 2.4 nm/mm. The output signal was detected by a photomultiplier and then was sent to a recorder (HP-7475A) through a storage oscilloscope (Lc Croy 9400).3. Results and discussionTransmission spectra of GaSe|_(S( crystals are shown in Figure I near the fundamental absorption edge at low excitation intensities. These spectra show positions of the IVcc-exciton peaks for different jt-values |4J. Nonlinear light absorption is observed in these44 H Tajalli, M Kalafi, H Bidadi, M Kouhi andVM SalmanovFigure I. Transmission spectra of GaSej.jS^ (300 K) for various values of -x 1- 0 . 2 -0  05.3-0 1,4-0.2(arb. units)Nonlinear light absorption in GaSe]^ J(Sx solid solutions etc 45crystals at the high pumping levels. Figures 2 and 3 show transmission spectra of GaSe and GaSe095Soo5 crystals at different excitation levels, respectively. Bleaching of the samples takes place in the region of the exciton resonance by increasing the excitation intensity.. /V605 550 ST- Mnm)Figure 2. Transmission spectra of GaSe (80 K) at two pumping intensities (inMW/cm2) 1-17.2-12Figure 3. Transmission spectra of GaSeo 95S0 05 (300 K) at different pumping intensities (in MW/cm2) 1-0. J 3, 2-2  01,3-12.Figure 4. Exciton absorption peaks (300 K) versus pumping intensities for (1) GaSe and (2) GaSeo.gSo j.Figure 5. Luminescence spectra (80 K) of GaSeo.^So 05 f°r various pumping intensities (in MW/cm2) • 1-0.12, 2-1.01,3-4.02,4-6.03, 5-12,Figure 4 illustrates the dependence of absorption on the excitation intensity at the wavelength where the exciton absorption is maximum. According to Figure 4, the absorption is constant up to the intensity Iq = (0.1 -0.2) MW/cm2, and then decreases more than three times in the region 0.15 <I0<\2  MW/cm2.46 H Tajalli, M Kalafi, H Bidadi, M Kouhi and V M SalmanovFigure 5 shows luminescence spectra of GaSe095Soo5 at various pumping levels. These spectra include the low-energy band ( L band) besides the free-exciton peak (A = 589 nm). The cxciton peak takes place also at low excitation intensities. On the other hand, the L band appears when /q > 0.5 MW/cm2 at A = 595 nm (i.e. 20 meV below energy of the free exc:ton) The peak of the L band exhibits a red shift by increasing the pumping excitation. L emission strongly predominates at highest pumping levels. Dependence of the L-cmission on excitation levels is a square-law, while the free exciton dependency is a linear one.The photoconductivity spectra of GaSeogSg i at various pumping levels are shown in Figure 6. One can see from this figure, that both exciton (A) and impurity (B) photoconductivities are observed in the spectrum (curve 1). The exciton peak first increasesFigure 6 . Photoconductivity spectra (300 K) of GaScQ gS0 | for various pumping intensities (in MW/cm2) 1-1.01. 2-402, 3-6.03. 4-12.hy increasing the laser intensity (curve 2), then begins to diminish (curve 3), and almost disappears at higher intensities (curve 4). The dependence of the exciton photoconductivity on the pumping intensity is shown in Figure 7. It is clearly seen that the exciton photoconductivity Ao, first increases linearly with growth of the incident intensity up to /„ = I MW/cm2, then varies according to Ao -  l '0/2 , and at last decreases at /„ >' 4 MW/cm2 (curve 1). In the case of the impurity excitation, the photoconductivity changes first linearly, and then approaches with a farther trend to the saturation(curve 3). The concentration of impurities determined from the region of the saturationIS equal to 1.0 x 10,4cnr3.It IS known that photoconductivity of nonequilibrium earners in semiconductors is of the form Ao ~ al„, where a  is the optical absorption coefficient and 10 is the excitation intensity (19J. Dependences of a/„ on /„ are shown in Figure 7 (curve 2), where values of or correspond to Figure 4. It is seen that, dependences of al0 and 4<ron /0 are similar. Thus, the disappearance of the exciton peak in the photoconductivity spectrum is caused by the same mechanism, as in the transmission spectrum.Nonlinear light absorption in GaSe^jSx solid solutions etc 47Exciton-exciton collision process is one of the possible mechanisms of the nonlinear light absorption in GaSe^S* solid solutions [20,21], Disappearance of the exciton absorption and appearance of the new band of the luminescence (at 20 meV below the freeFigure 7. Photoconductivity of GaSeo 9S01 versus incident intensity . 1, 3-exciton and impurity photoconductivity, 2-oIq.cxciton) at high pumping levels indicate such possibility. The density of the absorbed photons, averaged over the sample thickness, reached 3 x 1019 cnr-1 which exceeds the exciton density necessary for the Mott transition in GaSc|_xSx [12,22].4. ConclusionTransmission, luminescence and photoconductivity spectia of GaSe|_xSx solid solution (upto 25% S) contain only lines corresponding to free-excitons at low pumping intensities. Thus, GaSel_rSr present materials which are especially convenient for the study of the interactions between excitons. Such interaction leads to disappearance of the exciton lines and gives rise to new radiative transitions.References(11 Z S Bazinski, C B Dove and A Moose Helv Phys Ada 34 373 (1961)12] Landolt-Bftrnstein Numerical Data and Fundamental Relationships in Science and Technology eds K H Hellwege and O Madelung Vol 2 p 525 (Berlin : Springer) (1983)131 N Kuroda and Y Nishina Phys State Sol. (b) 72 81 (1975)14] S G Abdullaeva, G L Belenkii, P Ch Nani, E Yu Salaev and R A Suleimanov Sov. Phys. Semicond 9 161 (1975)15] M Schluter, J Gamassel, S Kohn, J P Voitchovsky, Y R Shen and M L Cohen Phys Rev. 13 3534 (1976) 161 A Mercier, E Mooscr, J R Voitchovsky and A Baldsresehi J. 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Nonlinear Light Absorption in GaSe1-xSx Solid Solutions Under High Excitation Levels

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Nonlinear Light Absorption in GaSe1-xSx Solid Solutions Under High Excitation Levels

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