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Abstract
In this work, GeSb and Ti-doped GeSb thin films were fabricated by the magnetron sputtering method and the optical phase change properties were systematically investigated through both experimental analyses and DFT calculation. High reflectivity contrast between amorphous and crystalline phase was observed in non-doped GeSb with a stable optical response at 480°C, leading to outstanding optical phase change capability. The red-shift of the optical absorption peak was studied, reflecting in the modulation of the electronic structure in GeSb during thermal-induced crystallization. With the introduction of Ti dopant, crystallization of GeSb was suppressed. A blue-shift trend of band-gap was clearly observed with the increase of Ti concentration, speculating a negative influence of phase change capability, thus resulting in the sharp decrease of optical reflectivity contrast by 10% at the visible wavelength.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Non-volatile memory devices are key elements of various electronics and portable systems such as digital cameras, solid state disks, smartphones, computers, e-books, tablets, etc., and their market has been increasing exponentially over the last decade [1]. Therefore, new emerging non-volatile memory concepts are under investigation and one of the leading candidates is phase-change memory (PCM). Up to now, novel applications have been utilized based on phase change behaviors in Ge-Sb-Te compounds through modulating the optical, electrical response between amorphous and crystalline state, such as electro-thermal color tuning of information display, all-optical phase change meta-switch [2,3]. Temperature distribution during crystallization in phase change memories has also been investigated for future development of high-integrated memory devices [4]. Generally, PCMs based on Germanium, Antimony and Tellurium (Ge-Sb-Te) are some of the most promising candidates for next-generation data-storage applications due to excellent thermal stability and distinct amorphous-crystalline phase-transition, which has been successfully used in optical memory devices such as phase change disc (PCD) since the 1990s [5–7].
Although Ge-Sb-Te (GST) is already superior to other chalcogenides in many aspects, there remains some room to obtain better memory performance [8–10]. Recently, it has been investigated that the concentration of Te strongly decrease the crystalline temperature of phase change memory, which possibly lead to unexpected phase separation and reduction of reliability [11,12]. The disadvantageous operation limits drive researchers to study the Te-free Ge-Sb system [13]. Sb-rich GeSb has been of great interests, which performs fast crystallization speed and low power consumption [14,15]. However, there is a stringent need for researches to explore the intrinsic memory property of GeSb and their phase change nature between amorphous and crystalline states. Furthermore, in practical memory applications, Titanium (Ti) adhesion layer or Titanium Nitride (TiN) thermal electrodes are commonly adopted in PCM cells [16–19]. Titanium dioxide (TiO2) is generally used as the dielectric layer in optical disks. The Ti element gradually diffuse into core phase change memory cell during read/write cycling and affect storage performance directly.
In this work, GeSb thin films were fabricated and studied for optical phase change property on crystalline stability, optical phase change contrast resolution, and electronic band structure by both experimental and theoretical investigations. Ti-doped GeSb film was also investigated to extensively identify the influence on the optical phase change properties and the modulation on intrinsic electronic band structure through experimental efforts.
2. Computational and experimental details
2.1 Computational details
The theoretical calculations were performed based on density functional theory (DFT) with the PW91 generalized gradient approximation (GGA) function in Cambridge Sequential Total Energy Package (CASTEP) [20]. A 10 × 10 × 10 k-point grid was adopted for the Brillouin-zone integration. The kinetic energy cutoff was 280 eV. The total energy was calculated with high precision, converged to 10−7 eV/atom. The lattice constants were optimized until the interatomic force is less than 10−2 eV/Å. The maximum stress component tolerance was 0.02 GPa. The corresponding optical properties were determined by the complex dielectric function ɛ(ω) = ɛ1(ω) + iɛ2(ω). The imaginary part was calculated by the joint density of states and the dipole transition matrix, while the real part was evaluated by using Kramers-Krönig transformation [21]. Other optical constants can be derived from the dielectric functions.
2.2 Experimental details
In the aspect of experiments, GeSb and Ti-doped GeSb (TGS) films were deposited on Si(100) substrates by LAB600sp-type magnetron co-sputtering system at room temperature. The deposition environment is Argon (Ar). In order to quantify the concentration of Ti dopant, the deposition power of Ti target was fixed at 20 W. The deposition power of GeSb target was modulated among 70 W, 90 W, and 110 W. The background vacuum pressure was 7.8 × 10−6 mbar and the working pressure was 3.0 × 10−3 mbar. Detailed experimental parameters were summarized in Table 1.
Table 1. Experimental parameters of GeSb and Ti-doped GeSb samples
The thicknesses of as-deposited samples were determined by Veeco Dektak 150 typed surface profiler. The difference of film thickness between undoped GeSb and TGS1 films was induced by the incidence angles of deposition targets. During the preparation process, both GeSb and Ti targets were oppositely tilted to 45° for co-sputtering Ti-doped GeSb samples, while GeSb target was set vertically during the individual deposition process of undoped GeSb samples. For magnetron sputtering system, two targets adopt co-sputtering mode to be tilted in order to fabricate the doped samples. The deposition efficiency loss on co-sputtering sample tray results in the decrease of film thickness. The proportion of Ti dopant were measured by energy dispersive X-ray spectroscopy (EDX), respectively. X-ray diffraction spectrum (XRD) was applied to determine the crystallinity of samples. Scanning range of diffraction angles was set from 20.0° to 60.0°. GeSb thin film samples were then annealed separately at 300°C, 360°C, 420°C and 480°C for 10 minutes in a nitrogen atmosphere to gain the crystalline phase. TGS thin film samples were annealed at 300°C, 360°C, separately. Spectroscopic ellipsometry (SE) and spectrophotometer were used to determine the optical properties of all samples with wavelength from 330 nm to 790 nm so as to identify the optical-induced memory capacities.
3. Results and discussion
Figure 1(a) shows the XRD patterns of GeSb thin films annealed at 300°C, 360°C, 420°C and 480°C, respectively. No diffraction peak was observed in GeSb thin film annealed at 300°C, indicating that the amorphous phase can be stabilized up to 300°C. When annealing temperature was increased to 360°C, the (012), (104) and (110) diffraction peaks were observed as A7 rhombohedral phase of crystalline GeSb studied in previous investigations. The (111) peak was generated at 420°C, which indicated the phase segregation of Ge at a higher temperature [22,23].
Fig. 1 (a) XRD patterns of GeSb annealed at 300°C, 360°C, 420°C and 480°C with indices of crystallographic plane. (b) Crystalline ball-stick structure of GeSb in A7 rhombohedral phase. The green and purple balls are represented to Ge and Sb atoms, respectively. (c) Refractive index n and (d) extinction coefficient ĸ of GeSb thin films annealed at 300°C, 360°C, 420°C and 480°C.
In order to identify the optical phase change capacity of GeSb, the optical properties were measured by SE for GeSb thin films under different annealing temperature. The real part of reflective indices performed comparable as indicated in Fig. 1(c), while imaginary part ĸ performs a strong absorption peak owing to intrinsic absorption in GeSb. The absorption peak of amorphous GeSb was located at 422 nm, while, for crystalline GeSb films, the peaks were located at 497, 519, and 541 nm, respectively. The obvious red-shift trend can be observed in ĸ with the increase of annealing temperature. Since the imaginary part ĸ of reflective index represents interband transition of inside electron, this red-shift trend of absorption peak indicates that the band structure of GeSb was generally changed due to the increase of crystallinity.
Moreover, for crystalline GeSb, the value of extinction coefficient ĸ was increased with the crystallinity of GeSb, which behaves stronger photon absorption with the increase of annealing temperature. To our knowledge, most of chalcogenide materials no longer maintain their optical properties when operated at a higher temperature [24,25]. The stable optical properties of undoped GeSb films can be maintained up to 480 °C, which is heretic to be implemented under high temperature environment.
To further investigate the origin of phase change behavior in GeSb, first principle calculation based on DFT theory was applied on crystalline GeSb. According the XRD spectrum, the appearance of (012), (104) and (110) peaks indicated that the crystalline structure of GeSb was rhombohedral A7 phase, which performed the same structure as Sb [14]. The calculated lattice constant of the unit cell was indicated in Table 2. The relative error between the experimental and theoretical values was acceptable, which suggests that the theoretical model was basically reliable.
Table 2. The comparison between experimental and theoretical lattice constants
The calculated band structures were plotted in Fig. 2(a). It is found that rhombohedral GeSb has a direct narrow band gap of 0.149 eV. As observed in density of states (DOS) that illustrated in Fig. 2(b), s electrons of both Ge atoms and Sb mostly stayed in deep energy levels, which has little influence on intrinsic physical properties of GeSb. The strong sp covalent bonding can be observed in density peaks of both s electrons and p electrons, which directly contributed to insulator behavior of GeSb. However, additional p electrons as active conductive carriers significantly attributed to bands near the Fermi level, possibly leading to the metallic behavior of GeSb. As reported previously, amorphous chalcogenide materials remain similar band structure as crystalline phase in phase change chalcogenide, while the band-gap was speculated to close up when transited from amorphous phase to crystalline phase [27,28]. Thus, the thermal-induced modulation of band-gap in chalcogenide has been generally speculated as the nature of phase change properties between amorphous and crystalline state, in turn conducting distinctive memory levels. and optical absorption characteristics
Fig. 2 (a) Band structure of crystalline GeSb with a narrow band gap of 0.149 eV. (b) Partial and total density of electronic state (DOS) of crystalline GeSb.
Energy band gap and optical transition type of GeSb can be determined experimentally by following relations.
Where A is an energy-independent constant and Eg is the optical bandgap. m is a constant that determines type of optical transitions. For direct allowed transition, such as GeSb with a narrow direct bandgap, m = 1/2 [29,30]. The fitted optical bandgaps of both amorphous and crystalline GeSb are shown in Fig. 3(a)-(b), respectively. Interestingly, the optical band-gap of amorphous GeSb is 0.50 eV, while it closed up and changed to 0.34 eV when transited to crystalline phase. Similar red-shift trend in optical band-gap was observed, in turn, directly reflecting that the phase change nature of GeSb was possibly related to electronic structure modulation during crystallization.Fig. 3 The optical band-gaps of both (a) amorphous and (b) crystalline GeSb that fitted by Tauc plot.
As shown in Fig. 4, the experimental and theoretical dielectric functions are performed from 1.56 eV to 3.75 eV. It is clear that computational dielectric functions well matched with the experimental results, which proves the convincible crystalline GeSb structure calculated in this work. However, owing to the underestimation of band-gap in such a DFT scheme, the calculated dielectric function performed linear correlation with the energy, while experimental results showed the absorption peak of imaginary part at 2.31 eV.
Based on above analysis on GeSb, the reflectivity and its contrast of GeSb were investigated by using spectrophotometer, which should be one of the most essential memory factors so as to obtain a high signal-to-noise ratio (SNR) in phase change storage application. As shown in Fig. 5a, the reflectivity of GeSb thin films increases gradually with the increase of crystallinity. It has been previously investigated that the decrease of Ge concentration in GexSb1-x exhibited higher reflectivity [31]. According to above XRD result, the separation of a small amount of elemental Ge segregation performed an important role to improve the optical reflectivity in GeSb. With the increase of annealing temperature, the annealed GeSb thin films were consisted of crystalline GeSb mixed with Ge crystalline phase, which slightly modulated the concentration of Ge in GeSb thin films, in turn, contributing to a higher optical reflectivity [32].
Fig. 5 (a) Reflectivity spectrum of GeSb thin films annealed at 300°C, 360°C, 420°C and 480°C. (b) Reflectivity contrast between amorphous and crystalline GeSb at the visible spectral region.
To confirm the resolution capability in GeSb-based optical phase change memory, the optical reflectivity contrast (C) was calculated. The reflectivity contrast is defined as the formula (1) [4].
Where Ri and Rf are the reflection value of amorphous and crystalline states of phase change material, respectively. In commercial phase change applications, at least 10% reflectivity contrast should ensure enough optical SNR. Significantly, as shown in Fig. 5(b), high reflectivity contrast in GeSb was observed over 15%, enabling distinct memory levels at visible wavelength from 330 nm to 790 nm. Thus, GeSb can be regarded as a promising candidate of phase change storage due to high optical contrast that can be stabilized under high-temperature environment.Figure 6(a) indicated the XRD spectrum of TGS thin films that annealed at 300°C, 360°C, respectively. There was no diffraction peak in XRD pattern of TGS1 film that annealed at 300°C, indicating their amorphous nature. When annealing temperature was increased to 360°C, (012) diffraction peak was observed, which represented the same rhombohedral phase in crystalline TGS1. With the increase of Ti concentration, (104) and (110) peak were disappeared. Ge (111) diffraction peak gradually disappeared, which represented that Ge segregation can be suppressed through Ti dopants. Meanwhile, Since XRD peaks can reflect the crystallinity and bonding strength in solid crystal, the full width at half maximum (FWHM) of (012) diffraction peak of undoped GeSb and TGS1~3 thin films were studied, which strongly relates the mean grain size that can be calculated by Scherrer equation. The Scherrer equation can be written as,
where d is the mean size of the ordered, crystalline grain; K is a dimensionless shape factor, named Scherrer constant; λ is the X-ray wavelength; β is the line broadening at FWHM; θ is the Bragg angle.Fig. 6 (a) X-ray diffraction spectra of TGS1, TGS2, and TGS3 sample films that annealed at 300°C, 360°C, respectively. (b) Refractive index n of TGS1~3 sample films under amorphous and crystalline states, respectively. (c) Extinction coefficient ĸ of TGS1~3 sample films under amorphous and crystalline states, respectively.
As indicated in Table 3, the intensity of (012) peak was gradually decreased with larger FWHM as the increase of Ti concentration in GeSb. The mean grain size, correspondingly, significantly decreased with the increase of Ti dopant. As reported in previous investigations, the grain size can generally reflect the crystallinity and the order of the solid materials [33]. With the introduction of Ti dopant, Ge-Sb bonding were suppressed. It has been previously investigated that the permeation of Ti elements from Ti-based thermal electrodes during read/write cycles on chalcogenide core media would lead negative influence on their memory behaviors [8]. To this contribution, the optical properties of Ti-doped GeSb was investigated through non-contact optical methods.
Table 3. FWHM of (012) diffraction peak and mean grain size of undoped GeSb and TGS1~3 thin films
The refraction index n was studied by SE for both amorphous and crystalline Ti-doped GeSb thin films as shown in Fig. 6(b). The real part of refractive indices showed no obvious change with Ti dopant, which kept the similar trend as non-doped GeSb. There remained a large refraction index difference in TGS1 between amorphous and crystalline states at visible region, which enabling distinct optical response that can be distinguished. As shown in Fig. 6(c), the absorption peak of the extinction coefficient of amorphous TGS1 was observed at 415 nm, while it located to 539 nm when transited to crystalline phase. Compared to non-doped GeSb, the imaginary part of refractive indices of Ti-doped GeSb indicated an obvious blue shift and the absorption peak of kappa tended to shorter wavelength with the increase of Ti concentration. Since extinction coefficient ĸ was demonstrated for electronic absorption and polarization, it can be speculated that Ti atomic dopant significantly contributed to the modulation of electronic inter-band transition through changing localized bonding environment.
However, with the increase of Ti concentration in TGS2 and TGS3, the optical refractive index difference between two extreme phases gradually became smaller. To further study the inside electronic structure with Ti dopant, the optical band-gaps of TGS1, TGS2 and TGS3 were calculated by Tauc plot as shown in Fig. 7(a)-(c), respectively. Similarly, the blue-shift trend also identified with the increase of Ti crystalline dopant, while the variation of band-gap between amorphous and crystalline states became smaller. Therefore, according to the experimental and theoretical investigations, the phase change behavior in Ti-doped GeSb became weaker due to the suppression on Ge-Sb covalent bonding and the introduction of strong localized carriers, leading to band-gap modulation controlled by Ti dopant concentration.
Fig. 7 Optical band-gap that fitted by Tauc plot of (a) TGS1 under amorphous and crystalline phases, (b) TGS2 under amorphous and crystalline phases, and (c) TGS3 under amorphous and crystalline phases.
As demonstrated in Fig. 8, the reflectivity contrast of Ti-doped GeSb was measured by spectrophotometer. Compared to non-doped GeSb, the reflectivity contrast of Ti-doped GeSb significantly decreased with Ti elemental permeation, which conducted the same conclusion as previous reports. With comparably lower Ti concentration, the occupation sites of Ti atoms should interstitially locate on Ge-Sb bonding, forming dopant defects with generated unbonded carriers, resulting in the increase of metallic behavior. The electronic band-gap closed up, which can be identified in the obvious red-shift of extinction coefficient ĸ compared to GeSb. However, with Ti concentration increased, the occupation of Ti atoms becomes substitutional dopant.
Fig. 8 Reflectivity contrast spectra of TGS1, TGS2, TGS3 thin films between amorphous and crystalline phases at the visible spectral region.
The electronic band-gap became wide due to the formation of Ti-Ge and Ti-Sb bonding, reflecting in the red-shift of optical absorption peaks of extinction coefficient ĸ [34]. Thus, the optical contrast gradually increased with phase change properties remained in TGS2 and TGS3. However, since the wavelength of 405 nm (3.07 eV) is usually chosen in commercial optical storage, it is clearly seen that the reflectivity reduced by nearly 10% with Ti dopant at this region. Thus, the negative influence of phase change behavior in GeSb can be speculated, which may possibly affect storage reliability in GeSb-based phase change memory by using Ti-based functional layers.
4. Conclusions
The intrinsic optical phase change properties of GeSb thin films with equal content of Ge and Sb element were investigated through theoretical and experimental efforts. Distinct optical memory levels can be obtained between amorphous/crystalline phases. High reflectivity contrast (>15%) was generated between two extreme crystallinities at visible spectral region. Through optical band-gap fitted by Tauc plot and theoretical studies by DFT calculation, it can be speculated that the phase change nature originate from the band-gap modulation controlled by thermal phase transition. Moreover, outstanding stability was observed with comparably high crystallization temperature of above 300°C and optical state maintained up to 480°C. However, with the introduction of Ti dopant, crystallization of GeSb was suppressed and optical reflectivity contrast significantly decreased. The negative influence can be speculated with a higher concentration of Ti dopant in GeSb-based phase change memory for storage reliability by using Ti-based functional layers.
Funding
Natural Science Foundation at Shanghai (NSFS) (17ZR1402200, 13ZR1402600); National Natural Science Foundation (NSFC) (60578047, 61427815) and National Major Special Project (2011ZX02402).
Acknowledgments
The authors would like to express their sincere thanks to Prof. Chen LY and Prof. Xu M for their effective backup.
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