Photostructural changes in amorphous chalcogenide thin films and chalcogenide glasses have been extensively studied for many decades [1]. The chemical bond rearrangement induced by light excitation having energy near the bandgap of these materials leads to the modifications of the amorphous matrix, resulting in changes of optical properties [2]. These are referred to as photodarkening (PD) and photobleaching (PB), depending on the sign of the change [3]. The magnitude of PD and PB is related to the composition and structure of chalcogenide materials. While in binary arsenic-based chalcogenides only PD occurs, both germanium-based binaries [4,5] and ternaries show rather complex behavior with the sign of the change being a product of the competitive processes of PD and PB [6,7].

The PD of exposed annealed films of ${\mathrm{As}}_2{\mathrm{S}}_3$ and ${\mathrm{As}}_2{\mathrm{Se}}_3$ that may be partially recovered by the consequent annealing was reported as early as the 1970s by De Neufville et al. [1]. It is considered that as-deposited amorphous chalcogenide thin films possess irreversible photoinduced changes, while annealed films and melt-quenched glasses undergo reversible ones [8]. Recently, a waveguide inscription using femtosecond lasers [ultrafast laser inscription (ULI)] attracted the attention of some researchers [9,10]. This process includes the nonlinear absorption of sub-bandgap light, resulting in changes in the refractive index. Results on ULI in Ge-Sb-S glasses by means of structural studies suggest that the driving forces of PD during ULI in chalcogenide glasses are of the same origin as photostructural changes due to the near-bandgap continuous-wave laser irradiation [10]. A better comprehension of the processes and variables involved in photoinduced phenomena may play an important role for potential applications, optimization, and tailoring properties of these materials in the fields of photonic integrated circuits, holographic gratings, and waveguide inscription [11].

The aim of this paper is to provide a study of photoinduced changes in Ge-Sb-Se chalcogenide thin films, which are of particular interest in the field of photonic integrated circuits for their potential application in nonlinear optics [12] and optical sensors in the mid-infrared domain [13,14]. The nominal composition of ${\mathrm{Ge}}_{25}{\mathrm{Sb}}_{10}{\mathrm{Se}}_{65}$ was chosen based on our previous work related to a photosensitivity study on this system [15]. It combines favorable optical properties and expected photosensitivity. Relatively high optical bandgap energy causes the orange color of thin films within the range of thicknesses under study, which in combination with low surface reflectivity allows easy detection of photoinduced changes. The insight into the photoinduced changes provided in this paper summarizes up to date available references and tries to recognize the challenges in understanding the general principles of these phenomena. The influence of thickness and the laser optical intensity on the kinetics of these processes is discussed.

A sputtering target ($ø=50\mathrm{mm}$) of ${\mathrm{Ge}}_{25}{\mathrm{Sb}}_{10}{\mathrm{Se}}_{65}$ chalcogenide glass was synthesized by a melt-quenching technique described elsewhere [16]. Thin films with thicknesses $d$ of 350, 500, and 650 nm were deposited by rf-sputtering (13.56 MHz) at $5\times {10}^{-3}\mathrm{mbar}$ working pressure of argon onto well-cleaned microscope slides (soda-lime float glass, substrate length of 76 mm). The variance in the thickness was estimated to be $<6\%$ within the substrate length. Selected thicknesses were found to be optimal for our experiments, as the penetration depths of the pump laser described below are $\sim 700$ and $\sim 1800\mathrm{nm}$ for as-deposited and annealed films, respectively.

The composition of thin films was verified by energy-dispersive X-ray spectroscopy (EDS) with a scanning electron microscope (JEOL IT-300, JEOL Ltd., Japan). X-ray diffraction (XRD) patterns were collected within the range of $2\theta$ from 10° to 80° with a step of 0.02° (Cu $\mathrm{K}\mathrm{\alpha }$, Rigaku, Japan). Thicknesses of the thin films and their initial optical properties, specifically, refractive index $n$ and optical bandgap energy $E_g$, were determined from variable angle spectroscopic ellipsometry (VASE, J. A. Woollam, USA).

Annealing of samples was performed at a temperature of 250°C (i.e., glass transition temperature of bulk $T_g-50°\mathrm{C}$) for 2 h (heating rate of $2°\mathrm{C}·{\min }^{-1}$, cooling rate of $1°\mathrm{C}·{\min }^{-1}$) under an argon atmosphere.

The kinetics of photoinduced changes of both as-deposited and annealed samples at room temperature was studied by means of in situ transmittance measurements captured by a fiber-coupled high-resolution spectrometer (OceanInsight, USA). Samples were placed in a hermetically sealed cell equipped with quartz windows filled with pure argon gas and irradiated by near-bandgap light coming from a 589 nm (2.1 eV) continuous-wave diode-pumped solid state (DPSS) laser source (Shanghai Laser & Optics Century Co., Ltd., China) for 180 min. The experimental setup for in situ kinetics measurements is depicted in Fig. 1(a). To retain the athermal nature of photoinduced changes, low to moderate laser optical intensities, specifically, $3.0\pm 0.5$, $19.0\pm 2.5$, $50.0\pm 3.0$, and $125\pm 5.0\mathrm{mW}·{\mathrm{cm}}^{-2}$, were used.

The changes were expressed as the time evolution of the absorption coefficient $\mathrm{\Delta }\alpha (t)$ calculated as $\mathrm{\Delta }\alpha (t)=\ln [T_0·T{(t)}^{-1}]·d^{-1}$, where $T_0$ is the initial transmittance, and $T(t)$ is the transmittance at elapsed time $t$. The selected value of the initial transmittance at irradiation wavelength was $\approx 18\%$, corresponding to the region of exponential absorption edge (Urbach edge, at initial value of absorption coefficient $\alpha$ of order of $\sim {10}^4{\mathrm{cm}}^{-1}$). It is worth to note that the small changes in thickness are expected during irradiation (such as photoinduced volume change in ${\mathrm{As}}_2{\mathrm{S}}_3$ chalcogenide glass [17]); however, this does not affect the value of $\mathrm{\Delta }\alpha$ significantly. Changes in reflectance during irradiation were also neglected.

The magnitude of the absorption edge shift by means of a change of $E_g$ succeeding irradiation was determined directly from transmittance measurements using WVASE32 software (J. A. Woollam, USA). It should be emphasized that the evaluation of the absolute values of $E_g$ and its changes was not the target of this work, and so the precision of the estimation using this approach is rather coarse with expected accuracy of about $\pm 0.02\mathrm{eV}$. Potential changes in local structure upon thermal annealing and irradiation were examined by a μ-Raman spectrometer (LabRam HR800, Horiba Jobin-Yvon, USA) coupled with a 100× microscope (Olympus, Japan).

The chemical composition of the vitreous target was found to be Ge₂₄Sb₁₁Se₆₆ ($\pm 1\%$, atomic fraction). In agreement with a previous work on this system [16], slight changes in stoichiometry during the sputtering process were observed, as the composition of thin films was ${\mathrm{Ge}}_{29}{\mathrm{Sb}}_8{\mathrm{Se}}_{63}$ ($\pm 1\%$, atomic fraction). The optical bandgap energy $E_g$ of as-deposited thin films was $1.87\pm 0.02\mathrm{eV}$, and the refractive index $n$ at 1550 nm was $2.55\pm 0.01$ as determined by VASE using a Cody–Lorentz oscillator model. An annealing treatment caused bleaching of thin films, resulting in an optical bandgap energy increase to $1.96\pm 0.02\mathrm{eV}$ accompanied by a slight refractive index decrease to $2.54\pm 0.01$. Both as-deposited and annealed ones were found to be amorphous according to XRD measurements.

As shown in Fig. 1(b), the prolonged irradiation caused the change of an opposite sign, i.e., PB in as-deposited films and PD in annealed ones, reasonably of the highest magnitude when the laser optical intensity of $125\mathrm{mW}·{\mathrm{cm}}^{-2}$ was used. It should be noted that even after 3 h of irradiation, the photoinduced change was not fully saturated, and so for measurements at lower laser optical intensities, the irradiation time was conserved.

The highest magnitude of PB by means of an increase in $E_g$ of about $0.04\pm 0.02\mathrm{eV}$ was found in the sample with a thickness of $\sim 350\mathrm{nm}$. On the other hand, the highest measure of PD, specifically, the decrease in $E_g$ by $0.05\pm 0.02\mathrm{eV}$, was found in the sample with a thickness of $\sim 650\mathrm{nm}$ under the abovementioned conditions.

The study of the time evolution of the absorption coefficient $\mathrm{\Delta }\alpha (t)$ revealed several types of photoinduced effects. First, as-deposited films exhibit fast PD, which is entirely dominated by PB in the long term. Such behavior has already been observed in some as-deposited selenide films, e.g., in evaporated ${\mathrm{GeSe}}_2$ [18] and Ge-As-Se [7], pulsed laser deposited Ge-Sb-Se [19], or in cosputtered Ga-Sb-Se [20]. Moreover, only PD takes place during the irradiation of annealed films. Finally, a transitory change, specifically, transient PD (TPD), was observed in both as-deposited and annealed thin films.

As seen in Fig. 2 (top panel), PB in as-deposited thin films is strongly thickness dependent, having the magnitude of a process indirectly proportional to the thickness. It is worth to note that the PB effect in the film with a thickness of $\sim 350\mathrm{nm}$ (violet curve) is closest to being saturated when irradiated by $125\mathrm{mW}·{\mathrm{cm}}^{-2}$ laser optical intensity. Presumably, the observed PB effect upon prolonged near-bandgap irradiation is of a similar origin to the bleaching upon thermal annealing. Such a change comes from a mutual polymerization of defective entities resulting in ordered structures via homo- to heteropolar bond conversion [1]. The excitation generates the charged radiation defects followed by recombination-induced bond rearrangements due to the diffusion of atoms [21]. Although the photoexcited states have not yet been proven experimentally [22], their existence leading to an increased short- and medium-range order [23] is a generally accepted explanation of irreversible changes in different properties including an absorption edge shift. Other stabilization mechanisms, such as void collapsing [22], evaporation of volatile entities, and photo-enhanced oxidation [24], may contribute to these changes. Because the experiments performed in this work were carried out in a pure argon atmosphere, we do not expect that the latter has an important contribution to the observed effects.

In contrast with PB observed in as-deposited thin films, the PD in annealed thin films as seen in Fig. 2 (bottom panel) is proportionally dependent on the thickness and is a universal process observed in annealed chalcogenide glasses and thin films, i.e., reversible PD effect, which may be recovered by thermal annealing. The mechanism of reversible changes is not yet known, as no subtle structural changes have yet been experimentally determined. They are likely the result of a similar elemental process to the irreversible ones, i.e., recombination-induced bond rearrangements and atom motions [3]. Presumably, the reversible PD comes from valence band broadening, resulting in a redshift [22].

The origin of fast PD in as-deposited films is not yet fully understood. Sati and Jain [6] proposed that it originates in homo- to heteropolar bond conversion at deformed sites such as Se–Se dimers at the [${\mathrm{GeSe}}_{4/2}$] or Sb–Sb bonds caused by Sb–Sb–Se in defective [${\mathrm{SbSe}}_{3/2}$] pyramidal units, present in evaporated Ge-Sb-Se thin films, and is simply a competitive process to slow PB, which becomes dominant in the long term. Relative to time, the magnitude of PB is rather small compared to PD.

Besides being thickness dependent, fast PD and slow PB in as-deposited and reversible PD in annealed Ge-Sb-Se thin films are strongly dependent on laser optical intensity as depicted in Figs. 3(a) and 3(b), respectively. The time evolution of fast PD shows a saturation at $125\mathrm{mW}·{\mathrm{cm}}^{-2}$ within $\sim 245\mathrm{s}$ and clearly delays with decreasing intensity. A similar observation was reported for evaporated Ge-As-Se films by Khan et al. [25]. The process of fast PD becomes completely dominated by the slow PB process.

Transitory changes by means of TPD were easily noticed by the on/off experiments in Fig. 3(c). TPD has already been observed by various authors [7,19,26–29] and may be related to a phenomenon termed as an “optical stopping effect” by Matsuda et al. in the 1970s [30]. Ganjoo et al. [27] observed TPD in annealed evaporated amorphous selenium a-Se and ${\mathrm{As}}_2{\mathrm{Se}}_3$ of $\sim 500\mathrm{nm}$ thickness when the film was irradiated by an Ar ion laser ($\lambda =514\mathrm{nm}$, optical intensity of $80\mathrm{mW}·{\mathrm{cm}}^{-2}$). An apparent rise in the transmittance after switching the laser off ($\lambda =488$ and 660 nm, $150\mathrm{mW}·{\mathrm{cm}}^{-2}$) was also reported for 1 μm thick as-deposited ${\mathrm{GeSe}}_2$ and ${\mathrm{Ge}}_2{\mathrm{Se}}_3$ evaporated films by Yan et al. [28]. A similar behavior was found in sputtered as-deposited Ge-As-Se thin films at different intensities when the films were exposed to a 655 nm laser diode [29]. Additionally, the magnitude of TPD among the references herein seems to be affected by the optical intensity and metastable PD or PD, depending on composition. The structural origin of TPD is yet to be understood. In situ extended X-ray absorption fine structure (EXAFS) experiments on amorphous selenium indicate that it may be associated with a chalcogen coordination number increase upon irradiation [22,31].

Finally, the results on millisecond kinetics of photoinduced changes in as-deposited and annealed Ge-Sb-Se thin films suggest that they are of the very same origin as in the Ge-As-Se system [29]. Even though the sign and the magnitude of a change may differ with composition and structure [5,7,15], the TPD and reversible PD in annealed thin films seem to be a universal feature of photoinduced changes in amorphous chalcogenides.

Changes in local structure upon thermal annealing and irradiation were investigated using Raman scattering spectroscopy measurements. As depicted in Fig. 4, the Raman spectra of Ge-Sb-Se thin films are prima facie dominated by the band peaking at $\sim 199{\mathrm{cm}}^{-1}$, which corresponds to ${\mathrm{A}}_1$ symmetric stretching vibrations of heteropolar Ge–Se bonds in a corner-sharing [${\mathrm{GeSe}}_{4/2}$] tetrahedron. The companion mode ${\mathrm{A}}_1^{\mathrm{C}}$ assigned to edge-sharing tetrahedral units is located at $\sim 214{\mathrm{cm}}^{-1}$ [32]. The overlapping ${\mathrm{E}}_1$ mode of [${\mathrm{SbSe}}_{3/2}$] pyramidal units around $\sim 191{\mathrm{cm}}^{-1}$ may contribute to the Raman intensity of the ${\mathrm{A}}_1$ mode [33]. The peak around $\sim 174{\mathrm{cm}}^{-1}$ is considered to come from Ge–Ge homopolar bonds in ethane-like [${\mathrm{Ge}}_2{\mathrm{Se}}_{6/2}$] units [34]. Vibrational modes of Sb–Sb in [${\mathrm{Se}}_2\mathrm{Sb}\text{−}{\mathrm{SbSe}}_2$] found at $\sim 160{\mathrm{cm}}^{-1}$ probably contribute to this peak [35].

To justify changes in structure, Raman spectra were deconvoluted using eight Gaussian peaks. Besides the five peaks mentioned above, the rotational modes in selenium polymeric chains ($\sim 140{\mathrm{cm}}^{-1}$ [35]) and bending vibrational modes of Se–Se at the outrigger ($135–145{\mathrm{cm}}^{-1}$ [36]) are altogether represented by the Gaussian peak centered at $\sim 134{\mathrm{cm}}^{-1}$. The ${\mathrm{A}}_1$ vibrational mode of ${\mathrm{Se}}_n$ chains in amorphous selenium found at $\sim 235{\mathrm{cm}}^{-1}$, stretching vibrational modes of Se–Se bonds at the outrigger ($\sim 245{\mathrm{cm}}^{-1}$), ${\mathrm{A}}_1$ vibrational modes of Se–Se dimers and short ${\mathrm{Se}}_n$ chains in corner-linked [${\mathrm{GeSe}}_{4/2}$], ($\sim 260–265{\mathrm{cm}}^{-1}$ [35]), and Ge–Ge homopolar bonds’ vibrations coming from [$\mathrm{Ge}-{\mathrm{Ge}}_m{\mathrm{Se}}_{4-m}$ ($m=1$, 2, 3, 4)], found at $\sim 270{\mathrm{cm}}^{-1}$ [34] are collectively represented by Gaussian at $\sim 262{\mathrm{cm}}^{-1}$. Finally, ${\mathrm{F}}_1$ asymmetric vibrational modes of ${\mathrm{GeSe}}_4$ tetrahedrons are found at $\sim 300{\mathrm{cm}}^{-1}$ [32].

Upon thermal annealing, an apparent change in the ratio between ${\mathrm{A}}_1$ (green) and ${\mathrm{A}}_1^{\mathrm{C}}$ (blue) peaks occurred. A simultaneous decrease in the intensity of a peak centered at $\sim 134{\mathrm{cm}}^{-1}$ (red) and $\sim 262{\mathrm{cm}}^{-1}$ (orange) indicates the consumption of selenium atoms as a consequence of increased local order. Other noticeable changes occurred around the $\sim 174{\mathrm{cm}}^{-1}$ peak. These changes may be due to a decrease in Ge–Ge homopolar bonds in ethane-like [${\mathrm{Ge}}_2{\mathrm{Se}}_{6/2}$] units. Changes in intensity of surrounding peaks upon thermal annealing are low and difficult to justify.

Photostructural changes by means of Raman scattering spectroscopy have already been described by several authors. The bond breaking mechanism upon irradiation followed by bond rearrangement and/or crystallization of structural sites is usually connected to changes in optical properties (i.e., shift of the absorption edge and refractive index changes upon irradiation) [4,7,37]. Being noteworthy, even in the highly photosensitive composition of ${\mathrm{As}}_2{\mathrm{S}}_3$, the fully saturated PD process results in only up to 1% of nearest neighbor bond alterations at available atomic sites [2]. As only low to moderate laser optical intensities were used in the present study, negligible changes in Raman spectra were observed and do not allow a fine analysis of the mechanisms involved.

It can be assumed that the abovementioned structural changes upon thermal annealing are responsible for a transition from the PB observed in as-deposited and reversible PD in annealed Ge-Sb-Se thin films. In addition, although the experiments were carried out under a pure argon atmosphere, the photo-enhanced oxidation should not be completely ruled out from the contribution to the photoinduced changes, as the concentration of germanium is $\sim 29\%$ (atomic fraction) [38].

A comprehensive study of photosensitivity in amorphous ${\mathrm{Ge}}_{29}{\mathrm{Sb}}_8{\mathrm{Se}}_{63}$ thin films by means of time evolution of the absorption coefficient $\mathrm{\Delta }\alpha (t)$ revealed several types of photoinduced effects in these materials. Fast PD followed by a slow dominative PB process is present in as-deposited films, while only PD takes place during prolonged irradiation of annealed films. The structural changes upon thermal annealing seem to be responsible for a transition from the PB observed in as-deposited and reversible PD in annealed thin films. TPD was observed in both as-deposited and annealed thin films and, together with a PD in annealed films, seems to be a universal feature of photoinduced changes in amorphous chalcogenides. The processes behind metastable PD and PB in the as-deposited films as well as TPD, which seem to be associated with transitory photoexcited states, remain to be better understood. Furthermore, a thorough study of surface and thickness changes upon irradiation and annealing as well as long-term stability at normal room conditions in similar systems is also of interest. Such investigations may lead to the implementation of these materials and photonic processes in them in the design and engineering of photonic integrated circuits by means of tailoring and optimization of optical properties and inscription of three-dimensional structures (e.g., waveguides, holographic gratings, and diffractive optics elements).