Novel Sn-Based Contact Structure for GeTe Phase Change Materials

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Functional Inorganic Materials and Devices

Novel Sn-based Contact Structure for GeTe Phase Change Materials Hamed Simchi, Kayla A Cooley, Zelong Ding, Alex Molina, and Suzanne E. Mohney ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02933 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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ACS Applied Materials & Interfaces

Novel Sn-based Contact Structure for GeTe Phase Change Materials Hamed Simchi*,1, Kayla A. Cooley2, Zelong Ding3, Alex Molina4, Suzanne E. Mohney*,5 Department of Materials Science and Engineering and Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, USA 1

Orcid ID 0000-0003-4004-1360, 2 Orcid ID 0000-0002-6598-4296, 3 Orcid ID 0000-0002-4838-

3077, 4 Orcid ID 0000-0003-3999-8907, 5 Orcid ID 0000-0001-5649-7640 KEYWORDS: Contact resistance, Germanium telluride, Phase change materials, Radio frequency switches, Thermal Stability

Abstract Germanium telluride (GeTe) is a phase change material (PCM) that has gained recent attention due to its incorporation as an active material for radio frequency (RF) switches, as well as memory and novel optoelectronic devices. Considering PCM-based RF switches, parasitic resistances from Ohmic contacts can be a limiting factor in device performance. Reduction of the contact resistance (Rc) is therefore critical for reducing the ON-state resistance in order to meet the requirements of high-frequency RF applications. To engineer the Schottky barrier between the metal contact and GeTe, Sn was tested as an interesting candidate to alter the composition of the semiconductor near its surface, potentially forming narrow bandgap (0.2 eV) SnTe or a

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graded alloy with SnTe in GeTe. For this purpose, a novel contact stack of Sn/Fe/Au was employed and compared to a conventional Ti/Pt/Au stack. Two different pre-metallization surface treatments of HCl and deionized (DI) H2O were employed to make the Te-rich and Gerich interface, respectively. Contact resistance values were extracted using the refined transfer length method (RTLM). The best results were obtained with DI H2O for the Sn-based contacts but HCl treatment for the Ti/Pt/Au contacts. The as-deposited contacts had Rc (ρc) of 0.006 Ω.mm (8 × 10-9 Ω-cm2) for Sn/Fe/Au and 0.010 Ω.mm (3 × 10-8 Ω-cm2) for Ti/Pt/Au. However, the Sn/Fe/Au contacts were stable and their resistance decreased further to 0.004 Ω.mm (4 × 10-9 Ω-cm2) after annealing at 200 oC. In contrast, the contact resistance of the Ti/Pt/Au stack increased to 0.012 Ω.mm (4 × 10-8 Ω-cm2). Transmission electron microscopy (TEM) was used to characterize the interfacial reactions between the metals and GeTe. It was found that formation of SnTe at the interface, in addition to Fe-diffusion (doping) into GeTe, are likely responsible for the superior performance of Sn/Fe/Au contacts, resulting in the lowest reported contact resistance on GeTe.

1. Introduction Phase change materials have attracted a lot of interest due to their unique characteristics such as fast transformation between crystalline or amorphous states, large change in reflectivity between the crystalline and amorphous states, and large resistance change between the states.1 Historically, they are alloys of Ge, Sb, and Te (with Ag, In) and have shown application in rewritable CDs & DVDs,1–3 non-volatile resistance state memories,4,5 and radio frequency (RF) switches.6–8


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Among the studied GexSbyTez (GST) alloys for RF switches, GeTe is one of the most suited stoichiometries due to its very low crystalline (on-state) resistivity (10-4 Ω-cm),9 large dynamic range (107),10 and relatively high crystallization temperature (190 oC).11 Parasitic resistances from the Ohmic contacts were shown to be ~25-50% of the ON-state resistance of the devices.11,12 Reduction of the contact resistance is critical for reducing the ONstate resistance to maximize the cut-off frequency for RF applications. Furthermore, a high contact resistance results in more power dissipation at the contact, limiting efficiency. Previous studies have reported a variety of contact metals and structures for applications in RF switches. The conventional Ti/Pt/Au stack has been employed as an Ohmic contact to III-V semiconductors such as GaAs,13 GaN,14 InGaAsP,15 as well as phase change materials.12,16 For the GeTe, the Ti/Pt/Au stack revealed a contact resistance (Rc) = 9.85 × 10-3 Ω.mm.12 But, Ti interaction with GeTe results in formation of TiTe2, affecting the endurance of devices with Ti contacts.16,17 According to the Ti-Te and Ti-Ge phase diagrams,18 several intermetallic phases of Ti-Te and Ti-Ge could indeed form, and significant interdiffusion between layers was reported at the interface.19,20 In another study, Ni/Al contact with Rc = 5.3 × 10-3 Ω.mm,21 was fabricated. In this case, the circular transfer length method (CTLM) test structures was used, which may cause some artifacts in measuring very low resistance values due to the influence of the metal sheet resistance. We demonstrated that pre-metallization surface treatments have a big influence on the resistance of Ni-based contacts to GeTe.22 It was found that due to formation of a nickel telluride phase at the interface, Ge-rich surfaces obtained by Ar+ plasma, H2O, and (NH4)2S treatment provided lower Rc compared to a Te-rich surface (after HCl treatment).22 However, this difference was not


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observed for Au contacts in which no reaction occurred at the metal/semiconductor interface.23 For the Au contacts, Rc of 7 ×10-3 Ω.mm was obtained for all surface treatments. Recently, an even lower contact resistance of 4 ×10-3 Ω.mm as reported for Mo/Ti/Pt/Au.24 To engineer the Schottky barrier between the metal and GeTe, one approach might be to grade the composition of the semiconductor near its surface, maintaining the favorable properties of GeTe for the switch, but reducing the band gap at the metal/semiconductor interface. SnTe has a smaller band gap (0.2 eV) than GeTe (0.7 eV) and thus can be considered as a potential candidate for the near-surface grading. In addition, having the SnTe at the interface may improve adhesion to metal electrodes and mitigate delamination during ON-OFF cycles.25 In this study, two different pre-metallization surface preparations (HCl and DI H2O) were used to modify the surface chemistry of GeTe films. We have shown previously that H2O and HCl treatments tune the surface to be Ge-rich and Te-rich, respectively.22 For the first time, a novel contact stack of Sn/Fe/Au was employed to incorporate Sn at the interface and form SnTe or an alloy with GeTe in contact to GeTe. Contact resistance values were extracted using the refined transfer length method (RTLM) and compared to those of a conventional Ti/Pt/Au stack. The composition, structure, and morphology of the as-deposited and annealed contacts were studied using scanning transmission electron microscopy (STEM).

2. Experimental

Polycrystalline p-type GeTe (with rhombohedral crystal structure) with a high hole concentration of ~1020 cm-3 (110 nm thick), provided by Northrop Grumman Electronic Systems, was sputtered on a Si substrate with a 100 nm thick amorphous Si3N4 barrier layer for this study. Details of


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sputtering process and film characterization can be found in a previous report.12 Samples were degreased in acetone, isopropanol, and deionized (DI) water for 5 min with ultra-sonication for 10 s each and then dried with compressed nitrogen. After degreasing, samples were oxidized inside a PR-100 UV-Ozone Photoreactor (UVP Inc., San Gabriel, CA) for 10 min at 1 SLPM to remove organic contaminants, although it does cause oxide to grow on the surface. Afterwards, the samples were treated with hydrochloric acid (HCl), or DI water with the intent of removing the oxide and tuning the chemistry of the surface. For HCl-treated samples, a dilution of (10:1) H2O:37%HCl was used for 2 min and then samples were rinsed with DI water for 15 s. DI water treatment was done for 5 min. After each treatment, the sample was blown dry with N2. For the electrical measurements, “refined” transfer length method (RTLM) test structures26 were patterned with nominal gap spacings of 0.6–20 µm. Details of the lithography process were reported previously [14]. Afterwards, samples were loaded in an e-beam evaporation system with base pressure < 2×10-7 Torr. Then, the contact stacks of Sn/Fe/Au (5/15/100 nm) or Ti/Pt/Au (20/15/100 nm) were deposited. Au provides low sheet resistance needed for the probing the electrodes, while Fe was found to be a reasonable barrier for the interdiffusion of Sn and Au layers, and was better than the other barrier (Mo) that we tested. Before annealing, a 10 nm SiO2 protective layer was deposited on samples using e-beam evaporation. Annealing was performed at 200 °C for 30 min in flowing Ar in a 1 in. diameter quartz tube furnace. A four-point probe technique was used to measure resistances using a Keithley 237 parameter analyzer with linear current sweep and voltage measurement. For each treatment, at least 4 sets of RTLM were measured for two different samples. Gap spacing and mesa widths were measured via scanning electron microscopy (SEM). The reported values are the average of those eight sets and reported errors are the standard deviation.


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At the final step, samples were examined via cross-sectional transmission electron microscopy (XTEM). Cross-sections from the metal/semiconductor edge were prepared in a Helios Nanolab 660 system. Before cross sectioning, a protective layer of C (1 µm thick) was deposited via ionbeam gas-assisted chemical vapor deposition. High angle annular dark field (HAADF) scanning TEM (STEM) micrographs were taken in a FEI Talos transmission electron microscope using an accelerating voltage of 200 kV. In addition, x-ray energy dispersive spectroscopy (XEDS) was performed with a built-in Super X-EDS system for elemental analysis.

3. Results and Discussion A summary of the extracted data (Rc), specific contact resistance (ρc), semiconductor sheet resistance (Rsh), and transfer length (Lt)) from current-voltage measurements for the as-deposited and annealed contacts is provided in Table 1 and shown in Figure 1. We note that extraction of ρc using the transmission line model assumes that Rsh under and between the contact remains the same. In our study, this assumption is not strictly true, but reaction between the metallization and GeTe is shallow, so we expect the error to be small. For the as-deposited Ti/Pt/Au contacts, similar Rc (ρc) values were obtained by HCl and DI H2O treatments at 0.011 Ω.mm (2.8 × 10-8 Ω.cm2) and 0.010 Ω.mm (2.6 × 10-8 Ω.cm2), respectively. Annealing at 200 oC resulted in slightly higher Rc at 0.013 Ω.mm (4.1 × 10-8 Ω.cm2) for HCl treated samples and 0.012 Ω.mm (3.5 × 10-8 Ω.cm2) for DI H2O treated samples. The average Rsh for these samples was between 41.6 - 43.6 Ω/sq. We also note that we have sometimes observed small micro-cracks in the GeTe between the contacts after annealing, which can explain small increases in the sheet resistance with annealing.


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Compared to Ti/Pt/Au contacts, Sn/Fe/Au contacts provided significantly lower resistance values, with DI H2O treated Sn-based contacts exhibiting the lowest Rc values of this study. For the as-deposited condition, DI H2O treatment produced Rc (ρc) values of 0.005 Ω.mm (7.5 × 10-9 Ω.cm2). Contact resistance values of Sn-based contacts decreased after annealing at 200 oC for 30 min for both pre-metallization treatments. In fact, Rc (ρc) of DI H2O treated Sn contacts reached extremely low values of 0.0037 Ω.mm (3.8 × 10-9 Ω.cm2), which is about 40% lower than the best Ti-based contact performance of this study. Table 1. A summary of the electrical properties of different as-deposited (AD) and annealed Ti/Pt/Au and Sn-based contacts to GeTe.

Rc (Ω.mm)

ρc (Ω.cm2)

AD

0.010 ± 0.003

(2.6 ± 1.1) x 10-8

41.6 ± 5.8

DI H2O

200 oC

0.012 ± 0.002

(3.5 ± 0.8) x 10-8

43.2 ± 6.4

Ti/Pt/Au

HCl

AD

0.011 ± 0.005

(2.8 ± 2.1) x 10-8

43.2 ± 8.9

Ti/Pt/Au

HCl

200 oC

0.013 ± 0.004

(4.1 ± 2.4) x 10-8

43.6 ± 12.0

Sn/Fe/Au

DI H2O

AD

0.0055 ± 0.001

(7.5 ± 1.5) x 10-9

39.8 ± 0.9

Sn/Fe/Au

DI H2O

200 oC

0.0037 ± 0.002

(3.8 ± 2.0) x 10-9

42.6 ± 0.7

Sn/Fe/Au

HCl

AD

0.0068 ± 0.002

(2.6 ± 0.5) x 10-8

40.6 ± 1.7

Sn/Fe/Au

HCl

200 oC

0.0058 0.0001

(7.8 ± 2.3) x 10-9

43.1 ± 0.7

Contact

Treatment

Anneal

Ti/Pt/Au

DI H2O

Ti/Pt/Au

±

Rsh(Ω/sq.)


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Figure 1. a) Contact resistance (Rc) and b) specific contact resistance (ρc) for Ti/Pt/Au and Snbased contacts to GeTe. We have recently shown that both DI H2O and HCl treatments are effective in removing surface oxides GeO2 and TeO2, though they do not passivate the surface. Ge-rich and Te-rich surfaces obtained by DI H2O and HCl treatment, respectively.22 Therefore, a greater amount of SnTe or GexSn1-xTe alloy at the interface might form after the HCl treatment. In order to better understand the role of surface treatments and the subsequent interface stoichiometry on the contact resistance, TEM analysis was performed on as-deposited and annealed Sn/Fe/Au contacts, treated with either DI H2O or HCl. The high angle annular dark field (HAADF) STEM images and XEDS map of DI H2O-treated Sn-based contacts are shown in Figure 2. The microstructures of both as-deposited and 200 oC-annealed samples appear similar (Figures 2a and 2b). However, the annealed sample appears to have a slightly reduced Sn thickness, suggesting more interdiffusion of Sn into other layers. XEDS maps of the as-deposited sample (Figures 2a) shows that Sn overlaps both the Fe and GeTe layers. On the other hand, XEDS maps of the annealed sample reveal that Sn diffused into the GeTe layer, forming the SnTe or GexSn1-xTe at the contact interface. The corresponding fast Fourier transform (FFT)


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pattern (Figure 3a, inset) is consistent with the SnTe phase that has a cubic crystal structure (space group Fm3m). In addition, Fe has reacted with GeTe and formed a Fe-Ge layer above the SnTe. Figure 4 shows the STEM images and XEDS maps of HCl-treated Sn-based contacts. The microstructures of GeTe films in both as-deposited and 200 oC-annealed samples look similar. For the as-deposited sample, compared to the DI H2O-treated sample, the Sn signal is weaker suggesting more diffusion/reaction between Sn and GeTe layers. When annealed at 200 oC, the Sn has completely diffused into the whole stack, but Fe has remained more localized. The FFT pattern (Figure 3b) again confirms the formation of SnTe at the interface. In contrast to the DI H2O-treated sample, however, no evidence was found for the formation of any Fe-Ge layer in the HCl-treated sample.

Figure 2. Effect of annealing on the microstructure and elemental distribution for Sn-based contact after DI H2O treatment. HAADF STEM image and XEDS map of (a) as-deposited and (b) 200 oC-annealed contacts.


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Figure 3. HRTEM of the Sn/GeTe interface for the a) DI H2O- and b) HCl-treated samples. (Inset: corresponding FFT of the boxed area.)

Figure 4. Effect of annealing on the microstructure and elemental distribution for Sn-based contact after HCl treatment. HAADF STEM image and XEDS map of (a) as-deposited and (b) 200 oC-annealed contacts. The FFT patterns of GeTe phase near the interface (not shown here) for both DI H2O and HCltreated samples were also well-matched with the GeTe phase. Lack of alteration of lattice parameters of SnTe and GeTe (to within the 1-2 % accuracy typical for these measurements)


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suggests only limited dissolution of GeTe and SnTe into each other if it occurs. The Sn-Te phase diagram also reveals that Sn and Te are not in thermodynamic equilibrium under the conditions of our experiment, and only limited solubility of GeTe in SnTe is possible. 18 By comparing the x-ray energy dispersive spectroscopy (XEDS) maps of annealed Sn contacts after DI H2O and HCl treatments, we again see that SnTe was formed at the interface in both cases. However, formation of a Fe-Ge layer was more pronounced for the DI H2O-treated sample compared to the HCl-treated one. Bates et al.27 have shown that SnTe is stable against reaction with Fe. Since the HCl treatment leaves the surface Te-rich,22 the subsequent SnTe layer formed is thicker in that sample compared to the DI H2O-treated sample (Ge-rich surface). The thicker SnTe may serve as a barrier to formation of a Fe-Ge layer above the SnTe and reduce diffusion of Fe into the GeTe. Interestingly, Fe-substitution on Ge sites has been shown to be energetically favorable.28 This substitution might happen in our samples, especially since they have a very high hole concentration of ~1020 cm-3, and therefore presumably a high Ge vacancy concentration. It has been shown that doping of GeTe with other transition metals, i.e. V, Cr, and Mn, results in halfmetallicity.29 In particular, Mn-doping of GeTe produces a narrow bandgap, or semimetal, depending on the structure being formed.29 A similar effect might occur by doping GeTe with Fe atoms, reducing the bandgap. Consequently, the contact to GeTe in samples with Sn/Fe/Au contacts was made by SnTe or a SnTe-rich alloy of GexSn1-xTe, along with limited Fe diffusion into GeTe, which appears to aid current transport (reduce the contact resistance). This effect was less pronounced for the HCl-


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treated samples in which a Te-rich surface promotes SnTe formation, which might act as a barrier for the diffusion for Fe into GeTe.30 In order to study the synergetic effect of Sn and Fe, a new set of samples with Sn/Au and Fe/Au contacts were fabricated (DI H2O treatment). Figure 5 shows that these bilayer contacts do not provide resistance as low as that of the Sn/Fe/Au contacts. The resistance values were also increased by annealing at 200 oC, in contrast to Sn/Fe/Au contacts. Therefore, simultaneous presence of both Sn and Fe is necessary to achieve the ultralow resistance values. Replacing the Fe barrier layer with Mo did not provide as low of a resistance as well. In fact, Xray photoelectron spectroscopy (XPS) depth profiling (not shown here) revealed a significant interdiffusion of all layers causing the poor performance of Sn/Mo/Au contacts to GeTe.

Figure 5. a) The contact resistance (Rc) and b) specific contact resistance (ρC) for different stacks after DI H2O treatment.

4. Conclusions For the first time, Sn/Fe/Au contacts to GeTe were proposed and compared with conventional Ti/Pt/Au contacts. Prior to metallization, two different surface treatments of HCl and DI H2O


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were employed to make the surface Te-rich and Ge-rich, respectively. RTLM test structures were used to investigate the influence of different pre-metallization surface preparation and annealing on the contact resistance. It was found that as-deposited Sn/Fe/Au contacts have lower resistances than Ti/Pt/Au contacts, at 0.005 Ω.mm (7.5 × 10-9 Ω-cm2) and 0.010 Ω.mm (2.6 × 108

Ω-cm2), respectively. Resistance of Sn/Fe/Au contacts decreased further after annealing at

200oC for 30 min, reaching an extremely low value of 0.0037 Ω.mm (3.8 × 10-9 Ω-cm2). TEM analysis revealed that SnTe (or a SnTe-rich allow with GeTe) formed at the interface of the Snbased contacts, and it was thicker in the HCl-treated sample (with the Te-rich surface) than in the DI H2O-treated sample (Ge-rich surface). As a result, Fe diffusion into the GeTe in the HCltreated sample was less significant than in the DI H2O-treated sample. Finally, the presence of both Sn and Fe atoms at the interface appears to be necessary to achieve the ultralow resistance in contacts to GeTe.

AUTHOR INFORMATION Corresponding Authors Hamed Simchi and Suzanne E. Mohney, E-mail: [email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources We appreciate funding from the Office of Naval Research, award ONR N00014-15-12395.


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ACKNOWLEDGMENT The authors acknowledge support from ONR through N00014-15-12395. The authors are also grateful to Northrop Grumman Corporation for providing GeTe layers and their fruitful discussions. ABBREVIATIONS GeTe, Germanium telluride; RF, radio frequency; RTLM, refined transfer length method; CTLM, circular transfer length method; DI, deionized; XTEM, cross-sectional transmission electron microscopy; HAADF, high angle annular dark field; XEDS, x-ray energy dispersive spectroscopy. REFERENCES (1)

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ToC figure


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Novel Sn-Based Contact Structure for GeTe Phase Change Materials

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