Observation of nanosecond light induced thermally tunable transient dual absorption bands in a-ge 5 As 30 Se 65 thin film

Transcription

1 Observation of nanosecond light induced thermally tunable transient dual absorption bands in a-ge 5 As 30 Se 65 thin film Pritam Khan, Tarun Saxena and K.V. Adarsh Department of Physics, Indian Institute of Science Education and Research, Bhopal , India. In this communication, we report the first observation of nanosecond laser induced transient dual absorption bands, one in the bandgap (TA 1 ) and the other in the sub-bandgap (TA 2 ) regions of a-ge 5 As 30 Se 65 thin films. Strikingly, these TA dual bands are thermally tunable (redshift of the bands as the temperature was raised from 15 to 405 K), however exhibit a unique contrasting characteristic, i.e. the magnitude of TA 1 decreases while that of TA 2 increases with increase in temperature. Further, the decay kinetics of these bands are strongly influenced by temperature that signifies strong temperature dependent exciton recombination mechanism. Moreover, magnitude of the induced absorption scales quadratic and the decay time scales linear with the fluence of the inducing beam. Author to whom correspondence should be addressed; electronic mail: adarsh@iiserb.ac.in 1

2 Chalcogenide glasses (ChG) consist of group six elements except oxygen as major constituent, are an important class of amorphous semiconductors distinguished itself from other inorganic amorphous semiconductors by their unique photosensitivity depicted in terms of chemical, physical and structural changes upon bandgap/sub-bandgap illumination [1]. The mechanisms of these effects are instigated by the formation of electron-hole pairs in localized band tail states [2-5] having long excitation lifetime and strong electron-lattice coupling [6]. Apart from the fundamental interest, photosensitivity in amorphous ChGs is of great interest to various applications ranging from holographic recording [7], designing waveguides [8], diffraction elements [9] and fabricating nano-antenna [10] etc. Light induced shift in optical absorption of ChG is a well studied phenomenon over many decades. Examples of such effects are photodarkening (PD) and photobleaching (PB) in As and Ge based chalcogenides respectively [11, 12]. PD/PB in ChG are thought to originate form the strong coupling of photogenerated carriers with lattice through phonons followed by structural changes. Recent, continuous wave (cw) optical pump probe studies on PD/PB revealed that such effect consists of two parts: one is metastable PD/PB and the other is transient PD (TPD) which persists in the medium only during illumination [13-15]. Interestingly, it has been shown that for short illumination with a cw light, transient part is more apparent than the slow and cumulative metastable part. However, most studies on transient effects were done with cw light with very low time resolution and consequently, an exact assessment of these effects (magnitude, kinetics and spectral width) remains rather unknown. Recently, G. Rosenblum et al [16] and A. Regmi et al [17] using transient grating experiments have demonstrated that ns illumination in As 50 Se 50 thin film leads to two time scales (fast and slow) in PD: Fast process in ns is attributed to carrier diffusion and thermalization, and the slow process in μs is due to the structural changes. However, since in all these experiments, the transient effects are probed at a single wavelength and hence 2

3 information on the induced absorption spectrum by ns excitation is still missing. Likewise, the role of temperature and fluence of illumination on the induced absorption and its kinetics is yet to be established, although it is apparent for cw illumination. In this communication, we report the 532 nm, 5ns short pulse induced TA in a- Ge 5 As 30 Se 65 thin films at various temperatures and fluence of illumination. In a stark contradict to previous predictions of a broad featureless absorption based on small polaron model [18], we demonstrate that the TA spectrum consists of thermally tunable and distinguishable dual absorption bands TA 1 and TA 2. Interestingly, magnitudes of TA 1 andta 2 show contrasting characteristics with temperature of illumination i.e. TA 1 decreases while that of TA 2 increases with rise in temperature. Nevertheless, kinetics of such effects becomes faster at high temperatures signifying strong temperature dependent exciton recombination. Moreover, magnitude of TA varies quadratically, however, on the otherhand decay time shows linear dependence on excitation fluence. The bulk sample of Ge 5 As 30 Se 65 glass was prepared by the melt quenching method starting with 5N pure Ge, As and Se powders. The cast sample was used as the source material for depositing thin films on a microscopic glass substrate by thermal evaporation in a vacuum of Torr. The deposition rate was kept below 10A o /s, which ensured that the composition of the thin films was close to that of the bulk sample and later confirmed with the EDAX measurements. For pump probe TA measurements, the pump beam was the second harmonics of the Nd:YAG laser (5ns pulses centred at 532 nm with an average fluence of 37mJ/cm 2 and having a repetition rate of 10 Hz) used in single shot mode and probed with desired wavelengths selected from a Xenon Arc lamp (120 W) using a holographic grating with 1200 grooves/mm. The delay between the pump and the probe beam was created using a digital delay generator. For all measurement, the sample was kept in vacuum inside an optical cryostat (scan range from 10K to 450K) and the experiments were 3

4 performed after stabilizing the desired temperature. To study the effect of temperature, we have performed TA at four temperatures (15, 165, 305 and 405 K). Shown in fig. 1(a) is the TA spectra of a-ge 5 As 30 Se 65 thin film measured at four different temperatures 15, 165, 305 and 405 K, after 0.5μs following the pump beam excitation (average fluence 37mJ/cm 2 ). It can be seen from the figure that, TA spectra of the sample consist of two discrete absorption bands TA 1 and TA 2, at all temperatures of illumination. Notably, TA spectra show a red shift (to longer wavelength) with increase in temperature which can be understood by looking into the ground state transmission spectra of the sample at different temperatures as shown in fig. 1b. It is quite evident from the figure that, similar to TA, ground state transmission spectra also shift to longer wavelength with increasing temperature. The observed redshift in ground state transmission and the concomitant decrease in optical bandgap (please see fig S1 in supplementary information file) can be explained as a consequence of the change in degree of deviation from the periodicity of the lattice vibrations with temperature [19]. Quantifying the optical bandgap as the energy of photons where the value of transmittance is 0.1, the dotted vertical lines in fig. 1a indicate the ground state bandgap of the sample at each temperature. Now looking into the fig. 1a & b, we can readily assign the different features of TA. The maxima of TA 1 lie in the bandgap region of the sample and therefore, TA 1 is ascribed to the interband transition. Since this is in the bandgap region, the origin of such an effect is assumed to occur from the light induced structural changes in the sample which is characteristic of ChG. On the otherhand, maxima of TA 2 occur at much longer wavelengths, corresponding to the sub-bandgap region and ascribed to the localized defect states formed by laser illumination. These defects are identified as the valence alternation pair (VAP) [2] and their formation will be discussed in the following section. At this point, we envision that the evolution of TA spectra with temperature portrays the distribution of localized state energies within the bandgap and change in their occupancy 4

5 at the measuring temperatures. To understand qualitatively how the magnitude of TA bands changes with temperature, we have shown in fig. 1c the variation of absorption maxima of TA 1 and TA 2 as a function of temperature. It can be seen from the figure that TA 1 and TA 2 exhibit contrasting behaviour with respect to temperature, i.e. TA 1 decreases whereas, TA 2 increases with temperature. Interestingly, at low and room temperatures TA 1 dominates over TA 2, however, at high temperature (405 K) TA 2 takes over TA 1. Nonetheless, the overall magnitude of TA (contributions from both TA 1 and TA 2 ) is found to be largest at low temperature. Having discussed about the magnitude and spectral profile of TA on temperature, it is of great interest to see the detailed kinetics of TA. In this regard, fig. 2a shows the time evolution of TA spectrum of the sample at 15 K (all other temperatures are shown in the supplementary information file) for four different probe delays. As shown in the figure, following pump beam excitation, the rise of TA is instantaneous and decays within 10 μs, however the recovery was not complete within our experimental time window. To get new insights on the decay kinetics at different temperatures, we have plotted in fig. 2b the temporal evolution of TA at 580 nm at the four temperatures and in the inset, variation of decay time constant (time required to decay 1/e of the TA max) at respective temperatures. Clearly, slope of the TA decay curve becomes steeper as the temperature is raised from 15 to 405 K, manifesting that the relaxation process becomes more rapid at higher temperatures. Such an observation portrays that the relaxation of TA is a thermally activated process i.e. the transient defect states associated with it relaxes immediately at high temperature with the aid of thermal vibration. From the figure, it is also clear that TA saturates at a value which is well above the ground state at 15 K compared to higher temperatures which indicates that metastable state is more prominent at lower temperatures. 5

6 After demonstrating the temperature dependence of TA, we have studied the effect of pump beam fluence on the TA. In this context, at a fixed temperature (in this case room temperature), we have measured the TA at five different fluence and is shown in fig. 3a. Although, the whole effect is reproducible, magnitudes of TA show many fold increase with increase in fluence. In addition to this, non-reversible component of the TA also increase which indicates that the sample undergo permanent changes (later confirmed with optical absorption measurements). For better understanding, TA maximum at different pump beam fluence are shown in fig. 3b. It is apparent from the figure that a second order polynomial equation fit very well to the experimental data and indicates that the observed effect is nonlinear in origin. Thereafter, to estimate the effect of pump beam fluence on the kinetics of TA, we have plotted in fig. 3c the variation of decay time constants at different fluence of the pump beam. From the figure, it can be seen that decay time constants scales linear with a positive slope, i.e. TA is found to decay slower at higher fluence. This can be understood by considering the fact that permanent structural changes become prominent, which makes the relaxation process drastically slower. Such structural changes are associated with photodarkening which is shown in supplementary information file (Fig. S3). It can be seen from the figure that red shifts of the transmission spectra increases with increasing fluence which indicates that permanent changes become prominent at higher fluence that can be associated with photodarkening. Thus our experimental result reveals that excitation fluence plays a crucial role in controlling the magnitude as well as the kinetics of TA in a- Ge 5 As 30 Se 65 thin film. Explaining and understanding the observed effects are of major importance. We assume that the dependence of magnitude and kinetics of TA on temperature is the result of the temperature dependent non-radiative recombination of electron-hole pairs through transient self trapped excitons [20]. Naturally, such non-radiative recombination mechanism is in 6

7 principle, leave the material in a different state than from the as-prepared state in most cases induce structural changes since the excess energy released in the non-radiative recombination was used to overcome the activation barriers to form the defects. In our films, the exciton (X) recombination can occur through two different paths: (1) directly to ground state (Z) through radiative path and (2) via a metastable state (Y) through defect creation. However, the latter is more probable in our samples than the former owing to the low photoluminescence efficiency [21]. The metastable state consists of oppositely charged defect pairs D + and D - commonly known as valence alternation pair (VAP), where D and the superscripts denote the chalcogen atom and charge state respectively. These mechanisms can be well understood from the configurational coordinate diagram [22] as shown in fig. 4. In chalcogenides, the non-bonding lone pair orbital posses strong electron-phonon coupling which strongly favors the formation of low energy defect pairs. The formation energy (E d ) of such defect pair is quite less than the bandgap energy (E g ) as well as the exciton energy (E x ). As a consequence, the formed excitons are highly unstable and interact with the lattice to form a D +, D - pair which exactly corresponds to self trapping of excitons. Now the amount of energy released in self trapping is quantified as (E x E d ). As this trapping energy is a large fraction of bandgap energy, rapid non radiative recombination occurs to the metastable state via self trapped exciton. The extra energy released when excitons get self trapped is used to modify the local structure, which give rise to the change in optical properties. Moreover, owing to the disorder, there will be distribution in energy of the D +, D - pair which results in a variation of thermal energy from one site to another site to reverse the reaction, i.e. the trapped state is metastable which requires some thermal energy to return to the ground state. This clearly explains the basis of large TA at low temperatures. Moreover, this was strongly supported by our experimental observation that the TA saturates at a value which is well above the ground state at 15 K compared to high temperatures as shown in fig. 2b. Such 7

8 observation clearly indicates that the sample freezes at a metastable state at low temperatures and requires some thermal energy to come to the ground state. The self trapped exciton model was further justified by the observation of faster relaxation dynamics at high temperatures with the aid of thermal vibration which was shown in the inset of fig. 2b. Thus our experimental results clearly demonstrate the temperature dependent exciton recombination mechanisms which well explains both the magnitude and kinetics of TA at all temperatures. In conclusion, we have demonstrated for the first time that nanosecond laser induced transient dual absorption bands near the bandgap and sub bandgap region of a-ge 5 As 30 Se 65 thin film which is in stark contradiction to the previous predictions of a broad featureless absorption based on small polaron model. Interestingly, TA spectra is thermally tunable which provides a novel way in controlling such effects and find potential applications in chalcogenide photo-resists and pulsed holography. Spectral and temporal behaviour of TA is well explained with temperature dependent self trapped exciton model. Moreover, magnitude of TA varies quadratically, however, on the otherhand decay time scales linearly with the fluence of the inducing beam. Finally our experimental result raises interesting open questions on the role of composition and network rigidity on the nanosecond light induced TA. Acknowledgements The authors thank Department of Science and Technology (Project no: SR/S2/LOP- 003/2010) and council of Scientific and Industrial Research, India, (grant No. 03(1250)/12/EMR-II) for financial support. 8

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11 Fig.1. (a) Transient absorption spectra of a-ge 5 As 30 Se 65 thin film that exhibit dual absorption bands at four different temperatures (b) Transmission spectra of as-prepared a-ge 5 As 30 Se 65 thin film at four different temperatures. Figure clearly indicates that absorption edge shifts towards longer wavelength with increasing temperature. (c) Shows the variation of magnitudes of TA 1 and TA 2 as a function of temperature. Solid lines in the figures are guide to the eye. 11

12 Fig.2. (a) Temporal evolution of TA spectra at 15K for different probe delays (b) Comparison of TA decay at different temperatures show larger magnitude and slower kinetics at low temperatures. Inset shows the variation of decay time constant as a function of illumination temperature. 12

13 Fig.3. (a) Temporal evolution of TA maxima as a function of excitation fluence (b) Change in TA maxima as a function of fluence at different probe delays. Clearly experimental data fit very well to a second order polynomial. (c) Decay time constant of TA at different excitation fluence which scales linear with the fluence of pump beam. Symbols and the solid line in the figure b and c, represents experimental data and theoretical fit respectively. 13

14 Fig.4. Configurational Coordinate diagram depicting two recombination paths of the excitons: (1) recombination directly to ground state and (2) to a metastable state via the creation of defect pair which requires thermal excitation to return to the ground state. 14

15 Supporting Information Observation of nanosecond light induced thermally tunable transient dual absorption bands in a-ge 5 As 30 Se 65 thin film Pritam Khan, Tarun Saxena and K.V. Adarsh * Department of Physics, Indian Institute of Science Education and Research, Bhopal , India. Supplementary figures Fig. S1. Change in bandgap of a-ge 5 As 30 Se 65 thin film as a function of temperature. Evidently, bandgap decreases as temperature increases.

16 Fig. S2. Temporal evolution of TA spectra at (a) 165 K, (b) 305 K and (c) 405 K for different probe delays.

17 Fig. S3. Transmission spectra of a-ge 5 As 30 Se 65 thin film when illuminated with (a) 25mJ/cm 2 and (b) 75mJ/cm 2. It can be seen from the figure that red shifts of the transmission spectra increases with increasing fluence which indicates that permanent changes become prominent at higher fluence that can be associated with photodarkening.