Photo-induced structural changes in Ge-Sb-Se films

Infrared Physics & Technology 81 (2017) 59–63

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Photo-induced structural changes in Ge-Sb-Se films Li Lin, Guoxiang Wang, Xiang Shen ⇑, Shixun Dai, Tiefeng Xu, Qiuhua Nie Laboratory of Infrared Material and Devices, Ningbo University, Ningbo, Zhejiang 315211, China Key Laboratory of Photoelectric Detection Materials and Devices of Zhejiang Province, Ningbo 315211, China

h i g h l i g h t s
Obtained amorphous Ge-Sb-Se thin films by RF magnetron co-sputtering deposition technique.
By contrasting a series of laser ‘on/off’ process and continued irradiation, the thermal effect can be negligible.
The photodarkening and photobleaching phenomena have been observed in Ge-Sb-Se films.

a r t i c l e

i n f o

Article history: Received 2 November 2016 Revised 10 December 2016 Accepted 20 December 2016 Available online 22 December 2016

a b s t r a c t Amorphous Ge-Sb-Se thin films have been prepared by the radio-frequency (RF) magnetron co-sputtering deposition technique, and their intrinsic photosensitivity and photo-induced structural changes have been investigated. The results show a crossover from photodarkening (PD) to photobleaching (PB) in the films when the film compositions change from Se-deficient to rich. Further Raman analysis on these as-prepared thin films irradiated with a laser of wavelength 655 nm in every five minutes provides direct evidence of photo-induced structure rearrangements. Ó 2016 Published by Elsevier B.V.

1. Introduction Chalcogenide (ChGs) glasses contain one or more S, Se or Te chalcogenide elements bonded with other network forming elements such as Ge, As and Sb. They exhibit attractive optical properties like high refractive index, low phonon energy, large optical nonlinearity and low transmission losses, and thus have been used for various non-linear applications [1,2]. Nevertheless, due to amorphous nature of the glasses, the structure relaxation usually occurs with a prolonged storage time. Moreover, such a structural relaxation can be accelerated via external energy input like thermal annealing and optical and ion irradiation [3,4]. Among these, the most notable observed effects are photodarkening (PD) and photobleaching (PB), which correspond to the decrease or the increase of the band gap via light shining, respectively, due to photo-induced change of glass structure [2,5]. The effects have been widely used in the applications such as optical writing, and photolithography [6,7]. While the photo-induced change of the glass structure is beneficial to these applications, obviously it is undesirable for other applications like optical lens for infrared optics. Therefore, it is potentially important to understand how to tune the photo-induced change of the glass structure via the ⇑ Corresponding author. Laboratory of Infrared Material and Devices, Ningbo University, Ningbo, Zhejiang 315211, China. E-mail address: [email protected] (X. Shen). http://dx.doi.org/10.1016/j.infrared.2016.12.016 1350-4495/Ó 2016 Published by Elsevier B.V.

design of material compositions in order to meet the different requirements for various applications. In recent years, PD and PB in Ge-As-Se and Ge-Se ChGs have been investigated, and the results indicate that photo-stable thin films exist in Ge-As-Se system. For examples, Yang et al. [8] reported a photo-stable glass with a composition of Ge10As35Se55, while Su et al. [9] showed that the films with a mean coordination number (MCN-the sum of the products of the valence times the abundance of the individual atomic constitutes) around 2.5 were most stable. Barik et al. [10] observed the kinetics of photodarkening in GexAs45xSe55 glasses when the network rigidity was increased by carrying x from 0 to 16. Kumar et al. [11] observed the transition from PD to PB when the composition of the chalcogenide glassy thin film changed from Ge-poor to rich in GexSe100x thin films. However, less attention was paid to Ge-Sb-Se system. The advantages of Ge-Sb-Se glasses are two-folded: the replacement of Arsenic in widely used Ge-As-Se system by less toxic antimony makes the glasses more environmentally friendly. Indeed, if laser power is beyond the damage threshold of the materials, the materials could be burnt or evaporated in any optical experiments. Certainly these evaporated toxic materials would be extremely harmful to the health, and thus material with less toxicity is highly desired [12,13]. On the other hand, Sb has a larger polarizability than As and this should lead to larger linear and nonlinear refractive indices [14]. Therefore, in this paper, we demonstrated the existence of photo-darkening and photo-bleaching in Ge-Sb-Se

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films fabricated using the magnetron co-sputtering deposition technique [9,15,16]. A comparison of the transmission of the film before and after irradiation was performed in order to study the photosensitivity under irradiation with band gap laser. The PD and PB behaviours were modelling with a stretched exponential dependence on time. Further Raman spectra analysis revealed that the intrinsic structural changes were responsible for PD/PB at room temperatures. The results confirmed the existence of the photostable composition in Ge-Sb-Se system that was suitable for optical applications. 2. Experiments Commercial GeSe2 and Sb targets were used for the deposition. The thin films were deposited onto microscope glass and Si substrates by RF magnetron co-sputtering deposition technique. The films with a thickness of 1 lm were in-situ controlled by a thickness monitor equipped in the chamber and further checked by Veeco Dektak 150 surface profiler. The chemical compositions of the films were determined by an Energy Dispersive X-ray Spectrometer (EDS) installed in a Tescan VEGA3 SB-Easyprobe Scanning Electron Microscope (SEM). For convenience, individual chemical compositions were shown in Table 1. PD/PB in these films were studied by a pump-probe optical absorption method using an experimental set up described previously [17]. We chose 655 nm light with a beam diameter 5 mm and an intensity of 0.2 W/cm2 (from a diode pumped solid state laser, DPSSL) as the pump beam. The probe beam was a low intensity white light with a beam diameter of 2 mm and a wavelength regime of 400–1000 nm. Pump and probe beams were directed into the surface of the film with an angle of 45° and 90°, respectively, where pump beam with a large beam size completely covers the probe beam with a smaller size. The change in transmission before, during, and after the pump beam illumination was recorded in an interval of 10 ms using a high-resolution optical absorption spectrometer (Ocean Optics HR2000). Consequently, we mainly concentrate the changes in the transmission at wavelengths near the band gap, where photo-induced effects are expected to be the largest. The structures of the films were analyzed using micro-Raman spectrometer. Raman spectra were recorded at room temperature by a 785 nm laser excitation with an InVia spectroscope (Renishaw) coupled to a Leica DM 2500 M microscope using an 50 magnification objective. 3. Results and discussion To estimate the effect of the pump beam illumination, first we recorded the transmission spectrum of the as-prepared sample in dark condition and denoted it as Ti. Next, we turned on the pump beam and simultaneously recorded the transmission spectrum as a function of time (Tf). The data showed temporal evolution of transmission ratio (Tf/Ti) for all samples at probe wavelengths for which transmission was 20% of the value in the dark and as-prepared condition. In the presence of the pump beam, however, an obvious change of Tf/Ti was observed for all the samples.

Fig. 1 shows Tf/Ti as a function of radiation time. It is clear that, in the initial stage without the pump beam, there is no change in Tf/Ti, which clearly confirms that the probe beam has no impact on the optical transmission. It should be also noted that, a small pump laser power density of 0.2 W/cm2 on the sample would result in a relatively insignificant change of the temperature during illumination. Fig. 1(a) shows a series of laser ‘on/off’ experiments, where the time of laser-off is about 400 s. It is believed that the off time can effectively reduce heat accumulation caused by the continuous irradiation. We can see that the final value of Tf/Ti in Fig. 1(a) is 1.15 for 8000 s. However, if we consider the-offtimes in series, the actual laser irradiation time is only 5200 s, which is almost the same value of Tf/Ti as that in Fig. 1(b) with 5200 s continuing irradiation, indicating the change of Tf/Ti observed is not due to the thermal effect but mainly due to the photo-induced effect. Fig. 2 presents the temporal evolution of Tf/Ti for S2, S4 and S5 samples under the 655 nm laser excitation. Dashed line indicates the time at which pump laser was turned on. It is clearly from Fig. 2 that Tf/Ti begins to decrease almost instantaneously after turning on the pump beam and saturate within a few tens of minutes, which is consistent with PD. Subsequently, PB starts to grow, showing a remarkable change in the transmission. For the composition of S2 (Fig. 2(a)), it saturates at a value which is well below the as-prepared state, i.e. the sample shows metastable PD. For the sample S4 (Fig. 2(b)), transmission completely recovers to the as-prepared state. In contrast to these result, sample S5 (Fig. 2 (c)) shows PD in the presence of light initially, but then switches to an overall PB state. Obviously, there exist two parallel mechanisms of PD and PB in these samples with light illumination. To model the reaction kinetics of the two opposite photo-induced effects, i.e., PD and PB, we used stretched exponential functions that describe PD and PB separately [15]. Rate equation for PD can be written as:

"

(
 )# b t d DT ¼ C exp  þ DTsd

ð1Þ

sd

and that for PB:

" (
 )# b t b DT ¼ DTsb 1  exp 

ð2Þ

sb

Here, the subscripts ‘d’ and ‘b’ correspond to PD and PB, respectively. DTS, s, b, t and C are the metastable part, the effective time constant, the dispersion parameter, the illumination time, and a temperature-dependent constant which is equal to the maximum transient changes, respectively. The net rate equation for the whole process is a summation of respective PD and PB components:

"

DT ¼ C exp

(
 )# b t d

sd

" þ DTsd þ DTsb

(
 )# b t b 1  exp 

sb

ð3Þ

The fitting results are presented in Fig. 2, we can see that the experimental data fit very well to the stretched exponential functional forms as described in Eq. (3), and the fitting parameters are also listed in Table 2.

Table 1 Chemical compositions and the mean coordination numbers of the films used in the paper. Element

S1

S2

S3

S4

S5

S6

Ge Sb Se MCN Change

34.9 11.1 54.0 2.809 PD

31.7 15.1 53.2 2.785 PD

30.9 16.2 52.9 2.78 PD

28.9 12.2 58.9 2.7 Close to stable

27.7 13.6 58.7 2.69 PB

27.1 12.6 60.3 2.668 PB

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Fig. 1. The plot of Tf/Ti as a function of time in Ge27.1Sb12.6Se60.3 (S6) thin film using a 655 nm laser irradiation with a power of intensity of 0.2 W/cm2. Dashed line indicates the time at which laser was turned on. (a) ‘on/off’ series and (b) continued irradiation.

Fig. 2. Temporal evolution of Tf/Ti for three Ge-Sb-Se thin films at probe wavelengths for which transmission is 20% of the value in the dark condition. Dashed line indicates the time at which laser was turned on. Dotted line shows the as-prepared state. The solid red lines represent the theoretical fit of the experimental data. (a) the sample of S2, (b) the sample of S4, (c) the sample of S5. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 2 Fitting parameters obtained from Eq. (3) that corresponds to PD and PB. The subscribe ‘b’ and ‘d’ refer to bleaching and darkening, respectively. Sample

sd

bd

4TSd

sb

bb

4TSb

S2 S4 S5

130 120 200

0.39 0.35 0.61

0.953 0.94 0.967

7900 7115 6700

2.3 2.01 1.7

0.041 0.0615 0.0708

After demonstrating the crossover form PD to PB with the change of the composition from Se-deficient to rich, it is of great

interest to obtain more detailed information on light-induced structure changes that are responsible for PD and PB, respectively.

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Raman scattering spectra of the as-prepared films illuminated with different time were measured as illustrated in Fig. 3. Structural diversity has been obviously observed in Raman spectra for these three samples. There are two main peaks located at 160 and 197 cm1 for the S2 and S4 samples, while the S5 sample exhibits only one dominant peak located at 197 cm1. It is commonly believed that the Raman band located at 160 cm1 is assigned to homopolar Sb-Sb bonds, also includes the partially vibrational contribution from Ge-Ge homopolar bonds, i.e. Ge2Se6/2 and Ge-GemSe4m (m = 1,2,3,4) entities, which is located at 170 cm1 [18,19]. Raman band with broad peak at 197 cm1, corresponds to Sb-Se bond stretching mode in SbSe3/2 pyramids and corner-sharing GeSe4 tetrahedra [20]. Another feature is an additional mode near 215 cm1 that belongs to edgesharing GeSe4 tetrahedra [19] as indicated in Fig. 3(c). A broad Raman active vibration with low intensity covered the 230– 320 cm1 range has been observed in S4 and S5 samples, which is mainly due to presence of homopolar Se-Se bonds from Se chains and rings [21]. In Fig. 3(a), the intensity ratio of the normalized Raman peaks between the 160 cm1 and 197 cm1 increase with increasing irradiation time. This indicates that more homopolar bonds (Sb-Sb and Ge-Ge bonds) will be formed with irradiation. The reason may be ascribed to the insufficient Se content in S2 sample since no obvious Raman peak in the 230–320 cm1 range is observed. The increase of homopolar bonds may result in the PD phenomenon. However, in Fig. 3(b), the change of the Raman spectra is very small before and after irradiation, and thus we believe the thin film is

close to stable. Additionally, the Raman peak in the 230– 320 cm1 range also has no significant change with increasing irradiation time. For the Fig. 3(c), in contrast with Fig. 3(a) and (b), the intensity of Raman peak at 197 cm1 is much stronger than that of the peak at 160 cm1, and a shoulder peak at 215 cm1 becomes obvious. With further irradiation, we can see that the intensity of peak at 160 cm1 decreases, while that at 215 cm1 increases. We believe that the decomposition of SbSb or Ge-Ge homopolar bonds and the new formed Ge-Se bonds 1 give rise to PB. The ratio of I1 160cm/I197cm for three samples is shown in Fig. 3(d), it is clear that S4 sample exhibits much better photostability with almost no structural changes, while the S2 sample increases slightly and S5 sample decreases dramatically upon laser irradiation, which corresponds to PD and PB, respectively. 4. Conclusion In this paper, Ge-Sb-Se amorphous chalcogenide thin films were deposited using RF magnetron co-sputtering deposition technique, and their photostability and structure changes under the laser irradiation were systematically investigated. We have presented the experimental results of photodarkening and photobleaching, showing a crossover from photodarkening to photobleaching when the composition varies from Se-deficient to rich. The Raman scattering results provide the direct evidence of photo-induced structural rearrangement in Ge-Sb-Se thin films. For the PD films, more Sb-Sb and Ge-Ge homopolar bonds are formed with irradiation. While for the PB films, the homopolar bonds are decomposed

Fig. 3. Raman scattering spectra of three different chemical compositions with different irradiation time: (a) the PD thin film of S2, (b) the thin film of S4, (c) the PB thin film 1 of S5, (d) Raman intensity ratio of I1 160/165cm/I197cm for three samples.

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and new edge-sharing GeSe4 tetrahedra are created. Additionally, this paper also provides a new insight in designing highly photosensitive and photostable ChG thin films by effectively controlling the right composition in Ge-Sb-Se glasses. Acknowledgments This work was financially supported by the Natural Science Foundation of China (Grant No. 61377061), the Natural Science Foundation of Zhejiang Province (Grant No. LQ15F040002), and sponsored by K. C. Wong Magna Fund in Ningbo University. References [1] R.P. Wang, Amorphous Chalcogenides: Advances and Applications, CRC Press, 2014. [2] B.J. Eggleton, B. Luther-Davies, K. Richardson, Chalcogenide photonics, Nat. Photonics 5 (3) (2011) 141–148. [3] D.A.P. Bulla et al., On the properties and stability of thermally evaporated GeAsSe thin films, Appl. Phys. A 96 (3) (2009) 615–625. [4] R.P. Wang et al., Structural relaxation and optical properties in amorphous Ge 33 As 12 Se 55 films, J. Non-Cryst. Solids 353 (8–10) (2007) 950–952. [5] A. Kolobov, Photoinduced effects and metastability in amorphous semiconductors and insulators, Adv. Phys. 44 (6) (1995) 475–588. [6] A. Zoubir et al., Direct femtosecond laser writing of waveguides in As2S3 thin films, Opt. Lett. 29 (7) (2004) 748–750. [7] M. Hughes, W. Yang, D. Hewak, Fabrication and characterization of femtosecond laser written waveguides in chalcogenide glass, Appl. Phys. Lett. 90 (13) (2007) 950–955.

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