Engineering the optical response of a-Se thin films by employing morphological disorder Rituraj Sharma,1 Deepak Kumar,1 Varadharajan Srinivasan,1,2 H. Jain,3 and K. V. Adarsh1,* 1 Department of Physics, Indian Institute of Science Education and Research, Bhopal 462023, India Department of Chemistry, Indian Institute of Science Education and Research, Bhopal 462066, India 3 Department of Materials Science and Engineering, Lehigh University, Bethlehem, Pennsylvania 18015, USA * [email protected] 2
Abstract: In this article, we experimentally demonstrate for the first time that photobleaching (PB) can be induced in morphologically disordered aSe thin film, an observation which is opposite of the previously well-known photodarkening (PD) effects in morphologically ordered films. Further, the optical response of the film shows many fold increase with increase in control beam intensity. To explain the observed extraordinary phenomenon, we have proposed a model based on the morphological disorder of a modified surface and its subsequent photo-annealing. Our results demonstrate an efficient and yet simple new method to engineer the optical response of photosensitive thin films. We envision that this process can open up many avenues in optical field-enhanced absorption-based technologies. ©2015 Optical Society of America OCIS codes: (310.6860) Thin films, optical properties; (160.2750) Glass and other amorphous materials; (160.5335) Photosensitive materials.
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#236069 - $15.00 USD Received 11 Mar 2015; revised 7 May 2015; accepted 11 May 2015; published 20 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014085 | OPTICS EXPRESS 14085
14. G. Chen, H. Jain, M. Vlcek, S. Khalid, J. Li, D. A. Drabold, and S. R. Elliott, “Observation of light polarizationdependent structural changes in chalcogenide glasses,” Appl. Phys. Lett. 82(5), 706–708 (2003). 15. X. Zhang and D. A. Drabold, “Direct molecular dynamic simulation of light-induced structural change in amorphous selenium,” Phys. Rev. Lett. 83(24), 5042–5045 (1999). 16. A. V. Kolobov, H. Oyanagi, K. Tanaka, and K. Tanaka, “Structural study of amorphous selenium by in situ EXAFS: Observation of photoinduced bond alternation,” Phys. Rev. B 55(2), 726–734 (1997). 17. P. Domachuk, H. C. Nguyen, B. J. Eggleton, M. Straub, and M. Gu, “Microfluidic tunable photonic band-gap device,” Appl. Phys. Lett. 84(11), 1838–1840 (2004). 18. D. Englund, D. Fattal, E. Waks, G. Solomon, B. Zhang, T. Nakaoka, Y. Arakawa, Y. Yamamoto, and J. Vucković, “Controlling the spontaneous emission rate of single quantum dots in a two-dimensional photonic crystal,” Phys. Rev. Lett. 95(1), 013904 (2005). 19. M. Y. Shen, C. H. Crouch, J. E. Carey, and E. Mazur, “Femtosecond laser-induced formation of submicrometer spikes on silicon in water,” Appl. Phys. Lett. 85(23), 5694–5696 (2004). 20. A. Saliminia, T. V. Galstian, and A. Villeneuve, “Optical field-induced mass transport in As2S3 chalcogenide glasses,” Phys. Rev. Lett. 85(19), 4112–4115 (2000). 21. R. Swanepoel, “Determination of the thickness and optical constants of amorphous silicon,” J. Phys. E Sci. Instrum. 16(12), 1214–1222 (1983). 22. T. H. Her, R. J. Finlay, C. Wu, S. Deliwala, and E. Mazur, “Microstructuring of Silicon with femtosecond laser pulses,” Appl. Phys. Lett. 73(12), 1673–1675 (1998). 23. P. Khan, A. R. Barik, E. M. Vinod, K. S. Sangunni, H. Jain, and K. V. Adarsh, “Coexistence of fast photodarkening and slow photobleaching in Ge19As21Se60 thin films,” Opt. Express 20(11), 12416–12421 (2012). 24. G. P. Lindberg, T. O’Loughlin, N. Gross, A. Reznik, S. Abbaszadeh, K. S. Karim, G. Belev, D. M. Hunter, and B. A. Weinstein, “Raman and AFM mapping studies of photo-induced crystallization in a-Se films: Substrate strain and thermal effects,” Can. J. Phys. 92(7/8), 728–731 (2014). 25. S. Abbaszadeh, K. Rom, O. Bubon, B. A. Weinstein, K. S. Karim, J. A. Rowlands, and A. Reznik, “The effect of the substrate on transient photodarkening in stabilized amorphous selenium,” J. Non-Cryst. Solids 358(17), 2389–2392 (2012). 26. S. R. Elliott, “A unified model for reversible photostructural effects in chalcogenide glasses,” J. Non-Cryst. Solids 81(1–2), 71–98 (1986). 27. A. Reznik, S. D. Baranovskii, M. Klebanov, V. Lyubin, O. Rubel, and J. A. Rowlands, “Reversible vs irreversible photodarkening in a-Se: The kinetics study,” J. Mater. Sci. Mater. Electron. 20(S1), S111–S115 (2009). 28. H. Fritzsche, “Photo-induced fluidity of chalcogenide glasses,” Solid State Commun. 99(3), 153–155 (1996). 29. P. Karageorgiev, D. Neher, B. Schulz, B. Stiller, U. Pietsch, M. Giersig, and L. Brehmer, “From anisotropic photo-fluidity towards nanomanipulation in the optical near-field,” Nat. Mater. 4(9), 699–703 (2005).
1. Introduction Recently, amorphous chalcogenides (ACs) have garnered much surge of research interest due to their unique photosensitivity depicted in the shift of bandgap either to longer or shorter wavelengths accompanied by a parallel change in refractive index [1–4]. These light-induced effects are important to fundamental science and as well as have many potential applications in photonic devices such as waveguides for routing the optical beams, passive ultrafast optical switches, dense holographic recording for data storage etc [5–9]. Among photosensitive ACs, amorphous selenium (a-Se) is of particular interest, being the simplest of them and exhibiting both semiconducting and photoconducting properties [10,11]. Due to its high sensitivity to xrays, a-Se is used as high gain avalanche rushing photoconductor in harpicon tube to capture images at extremely low intensities and employed heavily in medical radiographic imaging [12,13]. One of the most striking light-induced effects in a-Se is PD, the red shift of the optical absorption edge when illuminated with bandgap light. This effect is attributed to the interaction of photoexcited electron-hole pairs with lattice and the concomitant formation of coordination defects that provide a path through which the amorphous network relaxes to form a more stable state [14–16]. This change modifies the initial structure and consequently, the optical absorption shifts towards longer wavelengths. More optical functionalities of a-Se can be effectively harnessed by manipulating its optical response. For example, PD and the corresponding increase in refractive index (via Kramers–Kronig relations) is used to tune the cavity resonant mode and post-tune (after fabrication) the dispersion of a two-dimensional photonic crystal waveguide [17]. Typically, the tuning of optical properties is required for single quantum dot devices based on cavity quantum electrodynamics, where the emission wavelength of the quantum dot must match the cavity resonance [18]. Therefore, it is highly desirable to post tune the optical response of a-Se in quantum dot − cavity devices, which #236069 - $15.00 USD Received 11 Mar 2015; revised 7 May 2015; accepted 11 May 2015; published 20 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014085 | OPTICS EXPRESS 14086
eventually will relax the fabrication tolerances. Thus far, all the previous studies have reported PD in a-Se, with no indication of PB i.e. shift of optical absorption edge towards shorter wavelength. In this letter, we demonstrate for the first time that PB can be induced in a-Se by engineering the surface morphology. Such an effect can be a breakthrough from the viewpoint of applications, since the optical response of a-Se, perhaps many other ACs, can be engineered by introducing morphological disorder. To explain the observed extraordinary phenomenon, we have proposed a model based on the morphological disorder of a modified surface and its subsequent photo-annealing. 2. Methodology to engineer the optical response of patterned films To engineer the optical response of a-Se thin films, we have prepared two sets of samples: one with uniform thickness (morphologically ordered) called A1 and the other with random thickness variations (morphologically disordered) called A2. ~1μm thick a-Se thin films were deposited on a quartz substrate by conventional thermal evaporation technique at a vacuum of 1 × 10−6 Torr using 99.99% pure Selenium pellets. Figure 1 shows the AFM image (Agilent Tech., Model 5500) and transmission spectrum of A1. The images were recorded in contact mode using DPE-18 cantilever (tip diameter 10 nm with 75 Hz frequency and 3.5 N/m force constant). The surface roughness of A1 is found to be less than 3 nm which confirms that the sample is uniformly thick or morphologically ordered. It can also be seen from the Fig. 1(b) that morphologically ordered a-Se films show good transparency in the below bandgap region.
Fig. 1. (a) AFM image and (b) optical transmission spectrum of morphologically ordered sample (A1). It shows good transparency over the wavelength range from 630 to 780 nm. We also see interference fringes originating from the uniform thickness.
To prepare morphologically disordered sample (A2), we introduced disorder on a separate A1 sample by photostructuring using 5 ns Nd-YAG laser pulses centered at 532 nm (second harmonics). The sample was placed in a quartz cuvette filled with triple distilled water and was exposed to the intense laser pulses at a repetition rate of 10 Hz. The best results were obtained when the sample was exposed to more than 300 pulses. Photostructuring was more efficient in liquid than in air [19]. Apparently, the surrounding water helped the molten selenium layer on the surface to solidify before the next laser pulse arrived. The surface modification and photostructuring of a-Se is known to occur by photoinduced selective mass transport [20]. AFM images of A2 show different morphologies corresponding to different intensity regions of the Gaussian pulse as shown in Fig. 2. Spherical nanostructures with average diameter ~150-nm are formed in the low intensity regions of the Gaussian pulse. Interestingly, these spherical protrusions merge in the higher intensity regions. Near the pulse maxima, the structures stretch and coagulate into long rods.
#236069 - $15.00 USD Received 11 Mar 2015; revised 7 May 2015; accepted 11 May 2015; published 20 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014085 | OPTICS EXPRESS 14087
Fig. 2. AFM images of morphologically disordered sample (A2) showing different morphologies of spheres, interconnected ellipsoids, and nano-rods corresponding to different intensity regions of the Gaussian pulse.
Figure 3(a) shows a larger area AFM image of A2. Next, we have recorded the optical transmission spectrum of A2 (Fig. 3(b)). As we can see, the optical transmission decreases by at least ~42% at longer wavelengths. For example, the transmission value of A1/A2 is 20.2/8.7 at 600 nm. The decrease in optical transmission indicates that the photostructured aSe has enhanced defects and free volume, which induce a continuous weak absorption background throughout the visible and IR range. Further, the interference fringes disappear due to random thickness variations [21].
Fig. 3. (a) AFM image and (b) optical transmission spectrum of A2.
To account for scattering, we have measured the scattering profile of the A1 and A2 samples using 671 nm laser. From Figs. 4(a) and 4(b), it can be seen that there was no appreciable scattering from either of the samples. However, we could see a decrease in transmission in A2 as compared to A1. This result confirms that what we see in A2 is enhanced absorption. Similar observations are also reported in nanopatterned Si [22]. Additionally, we have measured the reflection at normal incidence for the samples A1 and A2. As shown in Fig. 4(c), we observe that the reflection from A2 is much lower than that of A1 (R(A2)/R(A1) ≈0.37, where R is reflectivity at 560 nm). This decrease in reflection can be attributed to the increase in absorption due to surface texturing [22] and thereby enhanced structural and morphological defects. The intrinsic absorbance caused by these defects is even
#236069 - $15.00 USD Received 11 Mar 2015; revised 7 May 2015; accepted 11 May 2015; published 20 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014085 | OPTICS EXPRESS 14088
more augmented due to multiple reflections that reduce the reflectance significantly for the specified wavelength. The XRD patterns obtained before and after photostructuring (Fig. 4(d)) clearly indicate the amorphous nature of the samples. XRD pattern was obtained with 0.3 mm mono-capillary point focus incident optics (PANalytical-Emperean), such that the size of the X-ray beam was much lower than the illuminated spot (spot diameter ̴ 1 cm) and care was exercised that only illuminated region was scanned. Also, the diffraction was measured by Pixel3D with anti scatter solar slit (0.04 Radians) diffracting optics so that entire diffracted beam was collected at the detector. The XRD pattern of sample A2 shows that the amorphous structure of Selenium is maintained even after photostructuring.
Fig. 4. (a) Scattering profile of (a) A1 and (b) A2 (c) Reflection measurements for A1 and A2. The reflection from A2 is very much lower than from A1. (b) XRD pattern of the samples before (A1) and after (A2) photostructuring.
After engineering the surface morphology, we now proceed to study the optical response of A1 and A2. In our experimental setup shown in Fig. 5(a), a cw diode pumped solid state laser (DPSSL) of wavelength 532 nm (intensity is varied using a variable neutral density filter) was used as the pump or control beam and the probe beam was a low intensity white light. Changes in the transmission of the probe beam were measured using a high resolution Ocean Optics HR 4000 spectrometer. As can be seen in the Fig. 5(b), transmission spectrum of A1 shifts towards longer wavelength region with illumination, an observation that is consistent with PD. On the other hand, for A2, transmission spectrum shifts towards shorter wavelength region indicating PB (Fig. 5(c)). The results are reproducible within the error limit of ≈5%.
#236069 - $15.00 USD Received 11 Mar 2015; revised 7 May 2015; accepted 11 May 2015; published 20 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014085 | OPTICS EXPRESS 14089
Fig. 5. (a) Experimental setup to study the transition from PD to PB. (b) Full spectra showing PD in A1 and in the inset zoomed part of bandgap region (550-600 nm). (c) PB in A2.
This change offers opportunity to engineer the optical response of a-Se films by introducing morphological disorder. To assess the optical response of the samples, first we recorded the transmission spectrum in the initial dark condition and denoted it as Ti. Next, we turned on the control beam and recorded the transmission spectrum continuously as a function of time (Tt) until the overall effect plateaued. Figure 6 shows the temporal evolution of transmission ratio (Tt/Ti) for A1 and A2, at probe wavelengths for which the transmission was 20% of the value in the dark condition (close to the bandgap). During control beam illumination, for A1 Tt/Ti decreases in a manner that is consistent with PD (Fig. 6(a)), which nearly saturates within 50 seconds. On the other hand, for sample A2, Tt/Ti increases with illumination, a clear manifestation of PB (Fig. 6(b)). Interestingly, Tt/Ti curve for A2 exhibits weak PD at first (almost immediately after turning on the pump beam) which is taken over soon by PB within a few tens of seconds. After the completion of PD, PB becomes evident and grows, showing a remarkable shift in the Tt/Ti curve. Finally, it saturates at a value which is well above the initial value, producing an overall bleached film. Similar results are observed with band gap illumination (Figs. 6(c) and 6(d)). However, the kinetics appears to be slow. To determine directly the contribution of photooxidation, if any, in causing PB, we nanostructured a sample in vacuum (10−6 Torr) thus avoiding oxidation. Then we performed pump-probe experiment again in vacuum. The results for the two conditions are compared in Figs. 7(a) and 7(b). Although the magnitude of PB is less in comparison to the sample photostructured in water, the transmission is well above the initial transmission before excitation. This clearly indicates that although there may be some degree of photooxidation in our samples, it is not the primary cause of PB. As our next step, we model the reaction kinetics using stretched exponential functions to get detailed information on the magnitudes and time constants of PD and PB. For sample A1, the change in transmission can be modeled by a stretched exponential function [23] used for photodarkening βd ΔT = C exp − t + ΔTd τ d
(1)
#236069 - $15.00 USD Received 11 Mar 2015; revised 7 May 2015; accepted 11 May 2015; published 20 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014085 | OPTICS EXPRESS 14090
Fig. 6. Temporal evolution of Tf/Ti for (a) A1 and (b) A2 with 532 nm illumination and with 671 nm illumination for (c) A1 and (d) A2.
where ΔTd, τd, β, t and C are PD, effective time constant, dispersion parameter, illumination time and temperature dependent constant which is equal to maximum transient changes, respectively. The experimental data fit very well to this function and the fitting parameters are listed in Table 1. The behavior of PD and PB in A2 can be modeled by an extended stretched exponential function [23] of the form βd βb ΔT = C exp − t + ΔTd + ΔTb 1 − exp − t τ τ d b
(2)
where the subscripts ‘d’ and ‘b’ correspond to PD and PB components, respectively, and ΔTb is the PB part. The experimental data fit very well to this stretched exponential function and the fitting parameters are listed in Table 1. It can be seen that the effective reaction time for PD in A1 is relatively short, a few seconds. By contrast, PB in A2 is a slower process compared to PD, with much longer reaction time.
Fig. 7. (a) Full spectra showing PB in samples photostructured and illuminated in vacuum. Inset shows zoomed part of bandgap region (550-600 nm). Blue and red colors represent the spectra before and after illumination respectively. (b) Temporal evolution of PB in sample exposed in vacuum.
#236069 - $15.00 USD Received 11 Mar 2015; revised 7 May 2015; accepted 11 May 2015; published 20 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014085 | OPTICS EXPRESS 14091
It is now interesting to examine the effect of control beam intensity on the optical response of A1 and A2. In this context, we have measured the change in the absorption coefficient T Δα = 1 ln 0 , where d is the film thickness and T0 and T are the transmittance before d T and after control beam illumination of A1 and A2 for the wavelength at 600 nm at different control beam intensities. For sample A1, increasing the control beam intensity from 0 to 320 mW/cm2 results in an increase in Δα from 0 to 700 cm−1 (see Fig. 8). By comparison, Δα for Table 1. Kinetic parameters for PD and PB in a-Se thin films Sample A1
Illumination (nm) 532 671
A2
532 671
C
βb
ΔTb
0.061 ( ± 0.003) 0.03 ( ± 0.008) 0.25 ( ± 0.08) 0.85 ( ± 0.06)
-
βd
ΔTd
-
τb (sec) -
1
-
-
-
1
0.62 ( ± 0.01) 0.42 ( ± 0.02)
0.65 ( ± 0.07) 0.9 ( ± 0.08)
440 ( ± 13) 8904 ( ± 190)
0.58 ( ± 0.10) 0.35 ( ± 0.02)
0.9 ( ± 0.01) 0.92 ( ± 0.07) 0.74 ( ± 0.06) 0.49 ( ± 0.05)
τd (sec) 9 ( ± 1) 113 ( ± 21) 30 ( ± 7) 80 ( ± 9)
A2 decreases from 0 to −1895 cm−1 for the same increase in intensity. These results demonstrate that introducing morphological disorder markedly changes the optical response of a-Se thin films, thereby providing direct evidence that it can be used as an effective tool to control the light sensitivity of these materials. Here it is important to note that Δα values of A2 are always greater than that of A1 for any control beam intensity. Thus we have demonstrated that control beam induced optical response of a-Se thin films can be engineered by introducing morphological disorder. Our results are similar to that of S. Abbaszadeh et al, where they have shown that the magnitude of PD and photocrystallization in a-Se can be effectively controlled by utilizing the strain at the interface of the sample and the substrate [24, 25]. However, they were not able to observe a cross over from PD to PB in their studies.
Fig. 8. Change in absorption coefficient (Δα) as a function of intensity for A1 and A2. For A1, increasing the intensity of the pump beam from 0 to 320 mW/cm2 results in an increase in Δα and for A2, Δα decreases with an increase in intensity. The solid black line is a guide to the eye.
Now we explain the observed control beam induced response of A1 and A2. The increase in absorption coefficient observed in morphologically ordered, smooth a-Se films (sample A1) is a well-studied phenomenon of PD that has been attributed to the increase in defects resulting from photo-structural transformations [15,16,26,27]. The top of the valence band in A1 is formed by nonbonding orbitals of Se atoms (lone pair electrons) [16]. During control beam illumination, one of the Se lone pair electrons from the valence band gets excited into the conduction band, leaving behind the other electron in the lone pair orbital. This electron unpairing favors the formation of additional dynamic inter-chain bonds between the Se atoms, #236069 - $15.00 USD Received 11 Mar 2015; revised 7 May 2015; accepted 11 May 2015; published 20 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014085 | OPTICS EXPRESS 14092
introducing three- and single-coordinated defects [15], thereby increasing the absorption coefficient. The control beam intensity dependence of absorption in A2, on the other hand, may be attributed to light-induced restoration of a dynamical equilibrium state produced by photostructuring. This dynamical state can be reverted to the initial state via efficient ordering of the morphological disorder by light. We can understand the restoration process using the intuitive idea of photo-induced (athermal) fluidity [18,28], which has the effect of athermal melting and coalescing of nearby nanostructures with continued illumination [29]. This ultimately leads to a morphologically ordered film. We obtained direct evidence of this morphological ordering from AFM images taken immediately after illuminating A2. Indeed the nanostructures deform or completely disappear as seen in Fig. 9(a). To understand the morphological ordering, we compare the transmission spectra of A1, A2, and control beam illuminated A2 in Fig. 9(b). Strikingly, the control beam illumination increases the transmission of A2, and the interference fringes try to reappear, i.e., the illuminated state of A2 approaches that of A1.
Fig. 9. (a) AFM images of A1 before (left) and after (right) illuminating with pump beam. (b) Optical transmission spectra of A1 (blue), A2 (red), and pump beam illuminated A2 (green). Control beam illumination increases the transmission of A2 and the interference fringes try to reappear, i.e., the illuminated state of A2 is similar to A1.
3. Conclusion In conclusion, we have demonstrated that the presence of morphological disorder in a-Se thin films can drastically alter its control beam induced optical response. A striking consequence of creating such kind of disorder is observed in the change in sign of the intensity dependence of absorption coefficient as well as a significant enhancement in the photosensitivity at a particular intensity. PD in A1 is due to the control beam induced electron unpairing, which favors the formation of additional dynamic inter-chain bonds between the Se atoms, introducing three- and single-coordinated defects, thereby increasing the absorption
#236069 - $15.00 USD Received 11 Mar 2015; revised 7 May 2015; accepted 11 May 2015; published 20 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014085 | OPTICS EXPRESS 14093
coefficient. On the other hand, PB in A2 has different origin and is due to the morphological ordering by photo-induced fluidity, confirmed by our AFM and optical absorption spectroscopies. We foresee that tuning the optical parameters by introducing defects can be used to significantly improve efficiency of solar cells, single quantum dot devices based on cavity quantum electrodynamics and of two-photon and transient absorption responses. We believe that other materials such as conjugated polymers and amorphous semiconductors like a-Si:H, may exhibit similar effects. Acknowledgments 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. Our international collaboration is supported by NSF’s International Materials Institute for New Functionality in Glass (DMR-0844014)
#236069 - $15.00 USD Received 11 Mar 2015; revised 7 May 2015; accepted 11 May 2015; published 20 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014085 | OPTICS EXPRESS 14094
Abstract: In this article, we experimentally demonstrate for the first time that photobleaching (PB) can be induced in morphologically disordered aSe thin film, an observation which is opposite of the previously well-known photodarkening (PD) effects in morphologically ordered films. Further, the optical response of the film shows many fold increase with increase in control beam intensity. To explain the observed extraordinary phenomenon, we have proposed a model based on the morphological disorder of a modified surface and its subsequent photo-annealing. Our results demonstrate an efficient and yet simple new method to engineer the optical response of photosensitive thin films. We envision that this process can open up many avenues in optical field-enhanced absorption-based technologies. ©2015 Optical Society of America OCIS codes: (310.6860) Thin films, optical properties; (160.2750) Glass and other amorphous materials; (160.5335) Photosensitive materials.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
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#236069 - $15.00 USD Received 11 Mar 2015; revised 7 May 2015; accepted 11 May 2015; published 20 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014085 | OPTICS EXPRESS 14085
14. G. Chen, H. Jain, M. Vlcek, S. Khalid, J. Li, D. A. Drabold, and S. R. Elliott, “Observation of light polarizationdependent structural changes in chalcogenide glasses,” Appl. Phys. Lett. 82(5), 706–708 (2003). 15. X. Zhang and D. A. Drabold, “Direct molecular dynamic simulation of light-induced structural change in amorphous selenium,” Phys. Rev. Lett. 83(24), 5042–5045 (1999). 16. A. V. Kolobov, H. Oyanagi, K. Tanaka, and K. Tanaka, “Structural study of amorphous selenium by in situ EXAFS: Observation of photoinduced bond alternation,” Phys. Rev. B 55(2), 726–734 (1997). 17. P. Domachuk, H. C. Nguyen, B. J. Eggleton, M. Straub, and M. Gu, “Microfluidic tunable photonic band-gap device,” Appl. Phys. Lett. 84(11), 1838–1840 (2004). 18. D. Englund, D. Fattal, E. Waks, G. Solomon, B. Zhang, T. Nakaoka, Y. Arakawa, Y. Yamamoto, and J. Vucković, “Controlling the spontaneous emission rate of single quantum dots in a two-dimensional photonic crystal,” Phys. Rev. Lett. 95(1), 013904 (2005). 19. M. Y. Shen, C. H. Crouch, J. E. Carey, and E. Mazur, “Femtosecond laser-induced formation of submicrometer spikes on silicon in water,” Appl. Phys. Lett. 85(23), 5694–5696 (2004). 20. A. Saliminia, T. V. Galstian, and A. Villeneuve, “Optical field-induced mass transport in As2S3 chalcogenide glasses,” Phys. Rev. Lett. 85(19), 4112–4115 (2000). 21. R. Swanepoel, “Determination of the thickness and optical constants of amorphous silicon,” J. Phys. E Sci. Instrum. 16(12), 1214–1222 (1983). 22. T. H. Her, R. J. Finlay, C. Wu, S. Deliwala, and E. Mazur, “Microstructuring of Silicon with femtosecond laser pulses,” Appl. Phys. Lett. 73(12), 1673–1675 (1998). 23. P. Khan, A. R. Barik, E. M. Vinod, K. S. Sangunni, H. Jain, and K. V. Adarsh, “Coexistence of fast photodarkening and slow photobleaching in Ge19As21Se60 thin films,” Opt. Express 20(11), 12416–12421 (2012). 24. G. P. Lindberg, T. O’Loughlin, N. Gross, A. Reznik, S. Abbaszadeh, K. S. Karim, G. Belev, D. M. Hunter, and B. A. Weinstein, “Raman and AFM mapping studies of photo-induced crystallization in a-Se films: Substrate strain and thermal effects,” Can. J. Phys. 92(7/8), 728–731 (2014). 25. S. Abbaszadeh, K. Rom, O. Bubon, B. A. Weinstein, K. S. Karim, J. A. Rowlands, and A. Reznik, “The effect of the substrate on transient photodarkening in stabilized amorphous selenium,” J. Non-Cryst. Solids 358(17), 2389–2392 (2012). 26. S. R. Elliott, “A unified model for reversible photostructural effects in chalcogenide glasses,” J. Non-Cryst. Solids 81(1–2), 71–98 (1986). 27. A. Reznik, S. D. Baranovskii, M. Klebanov, V. Lyubin, O. Rubel, and J. A. Rowlands, “Reversible vs irreversible photodarkening in a-Se: The kinetics study,” J. Mater. Sci. Mater. Electron. 20(S1), S111–S115 (2009). 28. H. Fritzsche, “Photo-induced fluidity of chalcogenide glasses,” Solid State Commun. 99(3), 153–155 (1996). 29. P. Karageorgiev, D. Neher, B. Schulz, B. Stiller, U. Pietsch, M. Giersig, and L. Brehmer, “From anisotropic photo-fluidity towards nanomanipulation in the optical near-field,” Nat. Mater. 4(9), 699–703 (2005).
1. Introduction Recently, amorphous chalcogenides (ACs) have garnered much surge of research interest due to their unique photosensitivity depicted in the shift of bandgap either to longer or shorter wavelengths accompanied by a parallel change in refractive index [1–4]. These light-induced effects are important to fundamental science and as well as have many potential applications in photonic devices such as waveguides for routing the optical beams, passive ultrafast optical switches, dense holographic recording for data storage etc [5–9]. Among photosensitive ACs, amorphous selenium (a-Se) is of particular interest, being the simplest of them and exhibiting both semiconducting and photoconducting properties [10,11]. Due to its high sensitivity to xrays, a-Se is used as high gain avalanche rushing photoconductor in harpicon tube to capture images at extremely low intensities and employed heavily in medical radiographic imaging [12,13]. One of the most striking light-induced effects in a-Se is PD, the red shift of the optical absorption edge when illuminated with bandgap light. This effect is attributed to the interaction of photoexcited electron-hole pairs with lattice and the concomitant formation of coordination defects that provide a path through which the amorphous network relaxes to form a more stable state [14–16]. This change modifies the initial structure and consequently, the optical absorption shifts towards longer wavelengths. More optical functionalities of a-Se can be effectively harnessed by manipulating its optical response. For example, PD and the corresponding increase in refractive index (via Kramers–Kronig relations) is used to tune the cavity resonant mode and post-tune (after fabrication) the dispersion of a two-dimensional photonic crystal waveguide [17]. Typically, the tuning of optical properties is required for single quantum dot devices based on cavity quantum electrodynamics, where the emission wavelength of the quantum dot must match the cavity resonance [18]. Therefore, it is highly desirable to post tune the optical response of a-Se in quantum dot − cavity devices, which #236069 - $15.00 USD Received 11 Mar 2015; revised 7 May 2015; accepted 11 May 2015; published 20 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014085 | OPTICS EXPRESS 14086
eventually will relax the fabrication tolerances. Thus far, all the previous studies have reported PD in a-Se, with no indication of PB i.e. shift of optical absorption edge towards shorter wavelength. In this letter, we demonstrate for the first time that PB can be induced in a-Se by engineering the surface morphology. Such an effect can be a breakthrough from the viewpoint of applications, since the optical response of a-Se, perhaps many other ACs, can be engineered by introducing morphological disorder. To explain the observed extraordinary phenomenon, we have proposed a model based on the morphological disorder of a modified surface and its subsequent photo-annealing. 2. Methodology to engineer the optical response of patterned films To engineer the optical response of a-Se thin films, we have prepared two sets of samples: one with uniform thickness (morphologically ordered) called A1 and the other with random thickness variations (morphologically disordered) called A2. ~1μm thick a-Se thin films were deposited on a quartz substrate by conventional thermal evaporation technique at a vacuum of 1 × 10−6 Torr using 99.99% pure Selenium pellets. Figure 1 shows the AFM image (Agilent Tech., Model 5500) and transmission spectrum of A1. The images were recorded in contact mode using DPE-18 cantilever (tip diameter 10 nm with 75 Hz frequency and 3.5 N/m force constant). The surface roughness of A1 is found to be less than 3 nm which confirms that the sample is uniformly thick or morphologically ordered. It can also be seen from the Fig. 1(b) that morphologically ordered a-Se films show good transparency in the below bandgap region.
Fig. 1. (a) AFM image and (b) optical transmission spectrum of morphologically ordered sample (A1). It shows good transparency over the wavelength range from 630 to 780 nm. We also see interference fringes originating from the uniform thickness.
To prepare morphologically disordered sample (A2), we introduced disorder on a separate A1 sample by photostructuring using 5 ns Nd-YAG laser pulses centered at 532 nm (second harmonics). The sample was placed in a quartz cuvette filled with triple distilled water and was exposed to the intense laser pulses at a repetition rate of 10 Hz. The best results were obtained when the sample was exposed to more than 300 pulses. Photostructuring was more efficient in liquid than in air [19]. Apparently, the surrounding water helped the molten selenium layer on the surface to solidify before the next laser pulse arrived. The surface modification and photostructuring of a-Se is known to occur by photoinduced selective mass transport [20]. AFM images of A2 show different morphologies corresponding to different intensity regions of the Gaussian pulse as shown in Fig. 2. Spherical nanostructures with average diameter ~150-nm are formed in the low intensity regions of the Gaussian pulse. Interestingly, these spherical protrusions merge in the higher intensity regions. Near the pulse maxima, the structures stretch and coagulate into long rods.
#236069 - $15.00 USD Received 11 Mar 2015; revised 7 May 2015; accepted 11 May 2015; published 20 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014085 | OPTICS EXPRESS 14087
Fig. 2. AFM images of morphologically disordered sample (A2) showing different morphologies of spheres, interconnected ellipsoids, and nano-rods corresponding to different intensity regions of the Gaussian pulse.
Figure 3(a) shows a larger area AFM image of A2. Next, we have recorded the optical transmission spectrum of A2 (Fig. 3(b)). As we can see, the optical transmission decreases by at least ~42% at longer wavelengths. For example, the transmission value of A1/A2 is 20.2/8.7 at 600 nm. The decrease in optical transmission indicates that the photostructured aSe has enhanced defects and free volume, which induce a continuous weak absorption background throughout the visible and IR range. Further, the interference fringes disappear due to random thickness variations [21].
Fig. 3. (a) AFM image and (b) optical transmission spectrum of A2.
To account for scattering, we have measured the scattering profile of the A1 and A2 samples using 671 nm laser. From Figs. 4(a) and 4(b), it can be seen that there was no appreciable scattering from either of the samples. However, we could see a decrease in transmission in A2 as compared to A1. This result confirms that what we see in A2 is enhanced absorption. Similar observations are also reported in nanopatterned Si [22]. Additionally, we have measured the reflection at normal incidence for the samples A1 and A2. As shown in Fig. 4(c), we observe that the reflection from A2 is much lower than that of A1 (R(A2)/R(A1) ≈0.37, where R is reflectivity at 560 nm). This decrease in reflection can be attributed to the increase in absorption due to surface texturing [22] and thereby enhanced structural and morphological defects. The intrinsic absorbance caused by these defects is even
#236069 - $15.00 USD Received 11 Mar 2015; revised 7 May 2015; accepted 11 May 2015; published 20 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014085 | OPTICS EXPRESS 14088
more augmented due to multiple reflections that reduce the reflectance significantly for the specified wavelength. The XRD patterns obtained before and after photostructuring (Fig. 4(d)) clearly indicate the amorphous nature of the samples. XRD pattern was obtained with 0.3 mm mono-capillary point focus incident optics (PANalytical-Emperean), such that the size of the X-ray beam was much lower than the illuminated spot (spot diameter ̴ 1 cm) and care was exercised that only illuminated region was scanned. Also, the diffraction was measured by Pixel3D with anti scatter solar slit (0.04 Radians) diffracting optics so that entire diffracted beam was collected at the detector. The XRD pattern of sample A2 shows that the amorphous structure of Selenium is maintained even after photostructuring.
Fig. 4. (a) Scattering profile of (a) A1 and (b) A2 (c) Reflection measurements for A1 and A2. The reflection from A2 is very much lower than from A1. (b) XRD pattern of the samples before (A1) and after (A2) photostructuring.
After engineering the surface morphology, we now proceed to study the optical response of A1 and A2. In our experimental setup shown in Fig. 5(a), a cw diode pumped solid state laser (DPSSL) of wavelength 532 nm (intensity is varied using a variable neutral density filter) was used as the pump or control beam and the probe beam was a low intensity white light. Changes in the transmission of the probe beam were measured using a high resolution Ocean Optics HR 4000 spectrometer. As can be seen in the Fig. 5(b), transmission spectrum of A1 shifts towards longer wavelength region with illumination, an observation that is consistent with PD. On the other hand, for A2, transmission spectrum shifts towards shorter wavelength region indicating PB (Fig. 5(c)). The results are reproducible within the error limit of ≈5%.
#236069 - $15.00 USD Received 11 Mar 2015; revised 7 May 2015; accepted 11 May 2015; published 20 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014085 | OPTICS EXPRESS 14089
Fig. 5. (a) Experimental setup to study the transition from PD to PB. (b) Full spectra showing PD in A1 and in the inset zoomed part of bandgap region (550-600 nm). (c) PB in A2.
This change offers opportunity to engineer the optical response of a-Se films by introducing morphological disorder. To assess the optical response of the samples, first we recorded the transmission spectrum in the initial dark condition and denoted it as Ti. Next, we turned on the control beam and recorded the transmission spectrum continuously as a function of time (Tt) until the overall effect plateaued. Figure 6 shows the temporal evolution of transmission ratio (Tt/Ti) for A1 and A2, at probe wavelengths for which the transmission was 20% of the value in the dark condition (close to the bandgap). During control beam illumination, for A1 Tt/Ti decreases in a manner that is consistent with PD (Fig. 6(a)), which nearly saturates within 50 seconds. On the other hand, for sample A2, Tt/Ti increases with illumination, a clear manifestation of PB (Fig. 6(b)). Interestingly, Tt/Ti curve for A2 exhibits weak PD at first (almost immediately after turning on the pump beam) which is taken over soon by PB within a few tens of seconds. After the completion of PD, PB becomes evident and grows, showing a remarkable shift in the Tt/Ti curve. Finally, it saturates at a value which is well above the initial value, producing an overall bleached film. Similar results are observed with band gap illumination (Figs. 6(c) and 6(d)). However, the kinetics appears to be slow. To determine directly the contribution of photooxidation, if any, in causing PB, we nanostructured a sample in vacuum (10−6 Torr) thus avoiding oxidation. Then we performed pump-probe experiment again in vacuum. The results for the two conditions are compared in Figs. 7(a) and 7(b). Although the magnitude of PB is less in comparison to the sample photostructured in water, the transmission is well above the initial transmission before excitation. This clearly indicates that although there may be some degree of photooxidation in our samples, it is not the primary cause of PB. As our next step, we model the reaction kinetics using stretched exponential functions to get detailed information on the magnitudes and time constants of PD and PB. For sample A1, the change in transmission can be modeled by a stretched exponential function [23] used for photodarkening βd ΔT = C exp − t + ΔTd τ d
(1)
#236069 - $15.00 USD Received 11 Mar 2015; revised 7 May 2015; accepted 11 May 2015; published 20 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014085 | OPTICS EXPRESS 14090
Fig. 6. Temporal evolution of Tf/Ti for (a) A1 and (b) A2 with 532 nm illumination and with 671 nm illumination for (c) A1 and (d) A2.
where ΔTd, τd, β, t and C are PD, effective time constant, dispersion parameter, illumination time and temperature dependent constant which is equal to maximum transient changes, respectively. The experimental data fit very well to this function and the fitting parameters are listed in Table 1. The behavior of PD and PB in A2 can be modeled by an extended stretched exponential function [23] of the form βd βb ΔT = C exp − t + ΔTd + ΔTb 1 − exp − t τ τ d b
(2)
where the subscripts ‘d’ and ‘b’ correspond to PD and PB components, respectively, and ΔTb is the PB part. The experimental data fit very well to this stretched exponential function and the fitting parameters are listed in Table 1. It can be seen that the effective reaction time for PD in A1 is relatively short, a few seconds. By contrast, PB in A2 is a slower process compared to PD, with much longer reaction time.
Fig. 7. (a) Full spectra showing PB in samples photostructured and illuminated in vacuum. Inset shows zoomed part of bandgap region (550-600 nm). Blue and red colors represent the spectra before and after illumination respectively. (b) Temporal evolution of PB in sample exposed in vacuum.
#236069 - $15.00 USD Received 11 Mar 2015; revised 7 May 2015; accepted 11 May 2015; published 20 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014085 | OPTICS EXPRESS 14091
It is now interesting to examine the effect of control beam intensity on the optical response of A1 and A2. In this context, we have measured the change in the absorption coefficient T Δα = 1 ln 0 , where d is the film thickness and T0 and T are the transmittance before d T and after control beam illumination of A1 and A2 for the wavelength at 600 nm at different control beam intensities. For sample A1, increasing the control beam intensity from 0 to 320 mW/cm2 results in an increase in Δα from 0 to 700 cm−1 (see Fig. 8). By comparison, Δα for Table 1. Kinetic parameters for PD and PB in a-Se thin films Sample A1
Illumination (nm) 532 671
A2
532 671
C
βb
ΔTb
0.061 ( ± 0.003) 0.03 ( ± 0.008) 0.25 ( ± 0.08) 0.85 ( ± 0.06)
-
βd
ΔTd
-
τb (sec) -
1
-
-
-
1
0.62 ( ± 0.01) 0.42 ( ± 0.02)
0.65 ( ± 0.07) 0.9 ( ± 0.08)
440 ( ± 13) 8904 ( ± 190)
0.58 ( ± 0.10) 0.35 ( ± 0.02)
0.9 ( ± 0.01) 0.92 ( ± 0.07) 0.74 ( ± 0.06) 0.49 ( ± 0.05)
τd (sec) 9 ( ± 1) 113 ( ± 21) 30 ( ± 7) 80 ( ± 9)
A2 decreases from 0 to −1895 cm−1 for the same increase in intensity. These results demonstrate that introducing morphological disorder markedly changes the optical response of a-Se thin films, thereby providing direct evidence that it can be used as an effective tool to control the light sensitivity of these materials. Here it is important to note that Δα values of A2 are always greater than that of A1 for any control beam intensity. Thus we have demonstrated that control beam induced optical response of a-Se thin films can be engineered by introducing morphological disorder. Our results are similar to that of S. Abbaszadeh et al, where they have shown that the magnitude of PD and photocrystallization in a-Se can be effectively controlled by utilizing the strain at the interface of the sample and the substrate [24, 25]. However, they were not able to observe a cross over from PD to PB in their studies.
Fig. 8. Change in absorption coefficient (Δα) as a function of intensity for A1 and A2. For A1, increasing the intensity of the pump beam from 0 to 320 mW/cm2 results in an increase in Δα and for A2, Δα decreases with an increase in intensity. The solid black line is a guide to the eye.
Now we explain the observed control beam induced response of A1 and A2. The increase in absorption coefficient observed in morphologically ordered, smooth a-Se films (sample A1) is a well-studied phenomenon of PD that has been attributed to the increase in defects resulting from photo-structural transformations [15,16,26,27]. The top of the valence band in A1 is formed by nonbonding orbitals of Se atoms (lone pair electrons) [16]. During control beam illumination, one of the Se lone pair electrons from the valence band gets excited into the conduction band, leaving behind the other electron in the lone pair orbital. This electron unpairing favors the formation of additional dynamic inter-chain bonds between the Se atoms, #236069 - $15.00 USD Received 11 Mar 2015; revised 7 May 2015; accepted 11 May 2015; published 20 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014085 | OPTICS EXPRESS 14092
introducing three- and single-coordinated defects [15], thereby increasing the absorption coefficient. The control beam intensity dependence of absorption in A2, on the other hand, may be attributed to light-induced restoration of a dynamical equilibrium state produced by photostructuring. This dynamical state can be reverted to the initial state via efficient ordering of the morphological disorder by light. We can understand the restoration process using the intuitive idea of photo-induced (athermal) fluidity [18,28], which has the effect of athermal melting and coalescing of nearby nanostructures with continued illumination [29]. This ultimately leads to a morphologically ordered film. We obtained direct evidence of this morphological ordering from AFM images taken immediately after illuminating A2. Indeed the nanostructures deform or completely disappear as seen in Fig. 9(a). To understand the morphological ordering, we compare the transmission spectra of A1, A2, and control beam illuminated A2 in Fig. 9(b). Strikingly, the control beam illumination increases the transmission of A2, and the interference fringes try to reappear, i.e., the illuminated state of A2 approaches that of A1.
Fig. 9. (a) AFM images of A1 before (left) and after (right) illuminating with pump beam. (b) Optical transmission spectra of A1 (blue), A2 (red), and pump beam illuminated A2 (green). Control beam illumination increases the transmission of A2 and the interference fringes try to reappear, i.e., the illuminated state of A2 is similar to A1.
3. Conclusion In conclusion, we have demonstrated that the presence of morphological disorder in a-Se thin films can drastically alter its control beam induced optical response. A striking consequence of creating such kind of disorder is observed in the change in sign of the intensity dependence of absorption coefficient as well as a significant enhancement in the photosensitivity at a particular intensity. PD in A1 is due to the control beam induced electron unpairing, which favors the formation of additional dynamic inter-chain bonds between the Se atoms, introducing three- and single-coordinated defects, thereby increasing the absorption
#236069 - $15.00 USD Received 11 Mar 2015; revised 7 May 2015; accepted 11 May 2015; published 20 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014085 | OPTICS EXPRESS 14093
coefficient. On the other hand, PB in A2 has different origin and is due to the morphological ordering by photo-induced fluidity, confirmed by our AFM and optical absorption spectroscopies. We foresee that tuning the optical parameters by introducing defects can be used to significantly improve efficiency of solar cells, single quantum dot devices based on cavity quantum electrodynamics and of two-photon and transient absorption responses. We believe that other materials such as conjugated polymers and amorphous semiconductors like a-Si:H, may exhibit similar effects. Acknowledgments 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. Our international collaboration is supported by NSF’s International Materials Institute for New Functionality in Glass (DMR-0844014)
#236069 - $15.00 USD Received 11 Mar 2015; revised 7 May 2015; accepted 11 May 2015; published 20 May 2015 (C) 2015 OSA 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.014085 | OPTICS EXPRESS 14094
