Investigating the Influence of Resonant Bonding on the Optical

Oct 4, 2017 - These results support the assumption that resonant bonding is crucial for successful optical phase change memory materials. View: ACS ...
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Article Cite This: Chem. Mater. 2017, 29, 9320-9327

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Investigating the Influence of Resonant Bonding on the Optical Properties of Phase Change Materials (GeTe)xSnSb2Se4

Christine Koch,† Gerrit Schienke,† Melf Paulsen,† Dominik Meyer,‡ Martin Wimmer,‡ Hanno Volker,‡ Matthias Wuttig,‡ and Wolfgang Bensch*,† †

Institute of Inorganic Chemistry, University of Kiel, Max-Eyth-Strasse 2, 24118 Kiel, Germany Institute of Physics, RWTH Aachen University, Sommerfeldstrasse 14, 52056 Aachen, Germany

‡

ABSTRACT: Thin film samples of Ge 9 SnSb 2 Te 9 Se 4 , Ge4.5SnSb2Te4.5Se4, Ge2.5SnSb2Te2.5Se4, and GeSnSb2TeSe4 were prepared via co-sputtering of GeTe and SnSb2Se4 and compared to the well-investigated phase change material GeTe. All samples were obtained in an amorphous state. Temperature-dependent in situ X-ray diffraction experiments reveal a crystallization temperature that increases with an increasing SnSb2Se4 content, leading to a higher stability of the amorphous phase. The electrical contrast between the amorphous and metastable crystalline state investigated via the van der Pauw method is as large as 4 orders of magnitude up to a 4.5:1 GeTe:SnSb2Se4 ratio. Increasing the SnSb2Se4 content leads to a decrease in the electrical contrast. Investigations of the samples by applying Fourier transform infrared spectroscopy and variable incident angle spectroscopic ellipsometry show that the optical properties of the amorphous phase are not affected by changes in stoichiometry. In striking contrast, the impact of SnSb2Se4 on the optical properties of the crystalline phases is significant: all optical constants decrease because of the reduction in the level of resonant bonding, which leads to a reduced absolute reflectivity of the crystalline phase resulting in a decrease in the optical contrast. These results support the assumption that resonant bonding is crucial for successful optical phase change memory materials.

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INTRODUCTION The evolution of modern optical data storage media started with the examination of GeTe for reversible phase change optical data storage.1 GeTe has almost perfect properties with regard to the phase change process and the metastable phases. The amorphous phase and the crystalline phase show a high optical contrast, because of the pronounced reflectivity of the crystalline phase.2,3 Furthermore, a good electrical contrast of ∼4 orders of magnitude was reported.4,5 With a high activation energy for crystallization and a crystallization temperature of >150 °C from the amorphous to the metastable crystalline structure (space group R3m),6 the amorphous phase is stable enough for long-term data storage.1,7 As a result of this excellent combination of properties, the structures of both metastable phases have been further investigated and are well understood.6,8,9 It was found that GeTe can be switched over 105 cycles,10 with the disadvantage being the relatively slow switching velocity, as soon as the GeTe stoichiometry differs from an exact 50:50 composition.11 This disadvantage was eliminated by using materials on the pseudobinary line between GeTe and Sb2Te3 (GST materials). In 1987, it was reported that these GST materials show ultrafast switching velocities with an increase in Sb2Te3 content (50 ns for Ge2Sb2Te5, 40 ns for GeSb2Te4, and 30 ns for GeSb4Te7) compared to the velocities of 100−300 ns for GeTe. A decrease in the © 2017 American Chemical Society

crystallization time for GeTe was reported in 2009 by investigating the melt-quenched instead of the as-deposited amorphous phase.1,10−12 On the basis of the results published for GST materials, new research in the field of pseudobinary phase change materials has intensified.13−19 Because of the improved properties,20 the GST materials were commercially quite successful and are used as active materials in optical data storage devices such as compact discs (CDs), digital versatile discs (DVDs), and Blu-ray discs. Moreover, they are currently being investigated for recent and future applications like nonvolatile electrical or multilevel data storage devices and have been investigated with regard to their thermoelectric properties.21−30 The reason for the large optical contrast was not understood until 2008, when Wuttig and co-workers2,31 suggested that all phase change materials should be arranged by their tendency to hybridization and ionicity in a so-called “treasure map”. It was pointed out that all phase change materials are located in a very small area of this map with defined values for hybridization and ionicity. Because of the octahedral-like environment of the atoms, only these compositions show pronounced resonant Received: August 4, 2017 Revised: October 1, 2017 Published: October 4, 2017 9320

DOI: 10.1021/acs.chemmater.7b03299 Chem. Mater. 2017, 29, 9320−9327

Article

Chemistry of Materials Table 1. Powers and Deposition Times for the Synthesis of the Investigated Phase Change Materialsa GeTe (a) Ge9SnSb2Te9Se4 (b) Ge4.5SnSb2Te4.5Se4 (c) Ge2.5SnSb2Te2.5Se4 (d) GeSnSb2TeSe4 (e) a

GeTe:SnSb2Se4 ratio

power GeTe target

power SnSb2Se4 target

deposition time

1:0 9:1 4.5:1 2.5:1 1:1

20 20 20 20 10

− 10 20 35 40

4750 4000 3100 2150 2300

All compositions were determined via EPMA measurements of the as-deposited amorphous films.

Figure 1. In situ XRD patterns of (a) GeTe, (b) Ge9SnSb2Te9Se4, (c) Ge4.5SnSb2Te4.5Se4, (d) Ge2.5SnSb2Te2.5Se4, and (e) GeSnSb2TeSe4. In the asdeposited state, all samples are amorphous. The phase changes can be seen by the appearance and disappearance of reflections.

high optical contrast of the samples.40,41 Comparable results were obtained by the simultaneous substitution of Ge with Sn and Te with Se.42 The last mentioned samples can be synthesized as thin films via co-sputtering of GeTe and SnSb2Se4. Compound SnSb2Se4 has been characterized quite little, because it does not show any electrical or optical phase change abilities.43 This observation is in line with the treasure map: SnSb2Se4 is far outside the region of successful optical data storage compounds, and therefore, no pronounced optical contrast can be expected between the amorphous and metastable crystalline states. On the contrary, it was reported that samples containing small amounts of SnSb2Se4 in GeTe [e.g., (GeTe)7SnSb2Se4] exhibit a pronounced optical contrast of 74%.42 This result is surprising and in contrast with what is expected from the data in the treasure map. Hence, we decided to investigate the influence of varying SnSb2Se4 contents in GeTe. Therefore, we synthesized thin film samples of Ge9SnSb2Te9Se4 (b), Ge4.5SnSb2Te4.5Se4 (c), Ge2.5SnSb2Te2.5Se4 (d), and GeSnSb2TeSe4 (e) and compared the electrical and optical properties of these materials to that of pure GeTe (a).

bonding in the metastable crystalline state, which seems to be the reason for the high optical contrast. Since then, many authors have accepted the model of resonant bonding that seems to be crucial for the high optical contrast of phase change materials.23,32−36 Nevertheless, some authors argued that there are other compounds of phase change materials (Ga2Te3) with a tetrahedral-like environment in the metastable crystalline phase.37,38 This would exclude resonant bonding as the reason for the large optical contrast. However, only the successful phase change and a good electrical contrast were reported, while the optical properties of the samples were not thoroughly studied. Therefore, some doubts about whether these materials show an adequate optical contrast between the two metastable phases remain. As a result, the model of resonant bonding that is responsible for the optical contrast has not yet been disproved. Many studies examined the substitution or doping of GST materials. Among these studies, those reporting the effects of substitution of Te with Se that leads to greatly enhanced electrical properties, which improve the potential of these materials for future electrical data storage applications,39 are most relevant for our investigation. Other publications dealt with the substitution of Ge with Sn, revealing the exceptionally 9321

DOI: 10.1021/acs.chemmater.7b03299 Chem. Mater. 2017, 29, 9320−9327

Article

Chemistry of Materials

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EXPERIMENTAL SECTION

Table 2. First and Second Phase Change Temperatures of Samples a−e and Annealing Temperatures for Further Investigations

Thin film samples of GeTe (a), Ge 9 SnSb 2 Te 9 Se 4 (b), Ge4.5SnSb2Te4.5Se4 (c), Ge2.5SnSb2Te2.5Se4 (d), and GeSnSb2TeSe4 (e) were prepared via DC magnetron sputtering using stoichiometric targets GeTe and SnSb2Se4 (umicore, 99.99%) bonded to watercooled copper plates. All deposition runs were performed with a base pressure of 4.5:1. Furthermore, band gap tuning can be performed by changing the electrical behavior of the crystalline samples with co-sputtering of GeTe and SnSb2Se4 from a conducting to a semiconducting state. Optical Properties. Figure 4 displays the “treasure map” of common GST phase change materials. The coordinate r′σ describes the iconicity amount of bonds comparable to Pauling’s electronegativity difference, while rπ−1 defines the degree of covalency, depending on the difference between the radii of the s and p states.31 All phase change materials are located in a very small area of this map (gray) and show 9323

DOI: 10.1021/acs.chemmater.7b03299 Chem. Mater. 2017, 29, 9320−9327

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Chemistry of Materials

Therefore, no pronounced optical contrast can be expected for these materials. For investigation of the optical properties, the same assumptions as in refs 2, 39, and 42 were made. The dielectric function, consisting of real part ε1 and imaginary part ε2, the index of refraction (n), and the extinction coefficient (k) are related as follows (eqs 1−3): ε = ε1 + ε2 (1) ε1 = n2 − k 2

(2)

ε2 = 2nk

(3)

Optical constant ε∞, which is linked to the degree of polarization, can be determined at energies of 0.05 eV for ε1. Optical band gaps were defined with the α-10000 method (Table 4).2 The values for GeTe agree well with literature data.2,9 As mentioned above, the substitution of Te with Se leads to more pronounced semiconducting behavior that cannot be compensated by substitution of Ge with Sn. Consequently, the optical band gap increases with a higher SnSb2Se4 content in the amorphous and crystalline phases. The Drude contribution was subtracted for all crystalline samples to determine the optical constants and band gaps, as shown in Figures 6 and 7. The impact of resonant bonding can be estimated with eq 4. The larger the value of ζ, the larger the contribution of resonant bonding. GeTe shows good optical properties, having a value of 1.62 for ζ, which is comparable to the value of 1.60 for Ge9SnSb2Te9Se4. With an increase in SnSb2Se4 content, the level of resonant bonding decreases significantly and the values of ζ decrease as well from 1.31 for Ge4.5SnSb2Te4.5Se4 to 0.93 for Ge2.5SnSb2Te2.5Se4 to 0.53 for GeSnSb2TeSe4. This is consistent with the XRD investigations, showing less crystallinity and larger distortions for SnSb2Se4 rich crystalline phases, resulting in less resonant bonding. With the optical constants in hand, the absolute reflectivities of the different phases were calculated, as well as the total optical contrast (5). Because the level of resonant bonding is reduced significantly with an increase in SnSb2Se4 content, the consequences for the optical contrast should be pronounced if resonant bonding influences the optical contrast at all.

Figure 4. Treasure map for phase change materials. Black symbols denote well-investigated phase change materials of the pseudobinary line between GeTe and Sb2Te3. The gray area denotes the region in the map in which pronounced resonant bonding in the crystalline state occurs and most phase change materials are located. Colored stars denote the compositions investigated herein.

Figure 5. Scheme of resonant bonding in antimony. The left and right panels show the two limiting cases (covalent bonding). Crystalline phase change materials (e.g., antimony) minimize the energy by forming a hybrid wave function leading to resonant bonding (middle). Red dots represent electrons. Adapted from ref 2.

pronounced resonant bonding (Figure 5), that being the reason for a distinct delocalization of electrons in the metastable crystalline state. This leads to a high electronic polarizability and therefore to an increased index of refraction and an unusually high dielectric constant, which is the reason for the high reflectance of optical light in the metastable crystalline phase of phase change materials.2,31 The map indicates that samples d and e are far outside of this region because the distortion of the octahedral environment of the atoms leads to a misalignment of the p orbitals. According to the model, this destabilization of the resonant bonding in the metastable crystalline phase causes a decrease in the optical contrast.

ζ = ε∞crystalline /ε∞amorphous − 1

(4)

R total = (R crystalline − R amorphous)/R amorphous

(5)

Figures 6 and 7 show the energy-dependent behavior of dielectric constants ε1 and ε2, the index of refraction (n), and the extinction coefficient (k). All amorphous samples behave in a similar way, almost regardless of their compositions. There is a slight trend in the values of ε1: with an increase in SnSb2Se4

Table 4. Dielectric Functions ε∞ and Energy Gaps of the Amorphous (a) and Crystalline (c) Samples optical constant ε∞ GeTe (a) Ge9SnSb2Te9Se4 (b) Ge4.5SnSb2Te4.5Se4 (c) Ge2.5SnSb2Te2.5Se4 (d) GeSnSb2TeSe4 (e) a

optical band gap Eg

tempa (°C)

a

c

ζb

a

c

% decrease

210 235 250 250 250

12.8 11.1 10.6 10.4 10.4

33.5 28.9 24.5 20.1 15.9

1.62 1.60 1.31 0.93 0.53

0.84 0.98 1.05 1.09 1.15

0.56 0.64 0.66 0.70 0.74

33 35 37 36 36

Annealing temperature for the crystalline samples. bζ according to eq 4. 9324

DOI: 10.1021/acs.chemmater.7b03299 Chem. Mater. 2017, 29, 9320−9327

Article

Chemistry of Materials

for ε2 is not strongly affected. Just a small shift from 2.5 to 2.8 eV can be observed. The values for the extinction coefficients are quite similar. The differences in the crystalline samples are much more pronounced. With an increase in SnSb2Se4 content, the values of the maxima for ε1 and ε2 are reduced from 50 and 5.4 for GeTe to 20 and 2.8 for GeSnSb2TeSe4, respectively. The same trend is observed for n and k, with values of 7.4 and 5.4 for GeTe and 4.6 and 2.8 for GeSnSb2TeSe4, respectively. This demonstrates the drastic influence of SnSb2Se4 on the optical properties. With a larger SnSb2Se4 content, the values for the optical constants, n, and k are becoming similar to the values of the amorphous phase. With these results, the absolute reflectivity of the amorphous and crystalline phases was calculated (Figure 8). As already

Figure 6. Dielectric functions of ε1 (left) and ε2 (right) for crystalline (top) and amorphous (bottom) samples of (a) GeTe, (b) Ge9SnSb2Te9Se4, (c) Ge4.5SnSb2Te4.5Se4, (d) Ge2.5SnSb2Te2.5Se4, and (e) GeSnSb2TeSe4. All functions are depicted without the Drude term. As a result, ε2 is zero for energies below the band gap.

Figure 8. Calculated absolute reflectivity of the amorphous (bottom) and crystalline films (top) of (a) GeTe, (b) Ge9SnSb2Te9Se4, (c) Ge4.5SnSb2Te4.5Se4, (d) Ge2.5SnSb2Te2.5Se4, and (e) GeSnSb2TeSe4.

predicted, the amorphous phases show comparable values of absolute reflectivity. Only GeTe has a reflectivity slightly higher than those of samples b−e (40−45% for GeTe vs 35−40% for samples containing SnSb2Se4). For the crystalline samples, the absolute reflectivity differs quite significantly from ∼70% for GeTe to ∼60% for b, ∼55% for c, ∼50% for d, and finally 45− 50% for e, depending on the wavelength. These values of absolute reflectivity result in a total optical contrast of ∼30% for GeTe, ∼23% for b, 19% for c, 14% for d, and ∼7.5% for e (Figure 9). This trend agrees with the assumption that resonant bonding in the crystalline phase is crucial for a huge optical contrast and matches very well with the predictions of the “treasure map”.2,31

Figure 7. Index of refraction n (left) and extinction coefficient k (right) for crystalline (top) and amorphous (bottom) samples of (a) GeTe, (b) Ge 9 SnSb 2 Te 9 Se 4 , (c) Ge 4. 5 SnSb 2 Te 4. 5 Se 4 , (d) Ge2.5SnSb2Te2.5Se4, and (e) GeSnSb2TeSe4.

content, the values for energies of