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Significance Statement
Germanium telluride (GeTe) is a phase-change material or chalcogenide that quickly changes its crystalline structure when heated to the material’s transition temperature. Specicially, GeTe’s conductivity varies by, as much as, six orders of magnitude when transitioning from amorphous to crystalline states. In order to fully realize germanium telluride’s potential as a terahertz (THz) modulator, methods of quickly switching between the conductive and insulating states are needed. Like other chalcogenides, Germanium telluride can be crystallized and re-amorphized with laser pulses. Short, high-power pulses can amorphize Germanium telluride, and steadier, lengthier pulses will crystallize it. In addition, joule heating provides another avenue for GeTe switching. Similar to the laser pulse mechanism, short voltage pulses with sufficient power can melt-quench and amorphize Germanium telluride, while longer, weaker pulses crystallize the material. The figure below, based on El-Hinnawy et al., is a design layout and optical image of a microelectromechanical systems (MEMS) fabricated indirect joule heating test structure we are using to investigate GeTe conductivity and switching for inclusion in THz modulators [1, 2].
References:
[1] El-Hinnawy et al., Applied Physics Letters, 105, 013501 (2014). [2] Gwin et al., Applied Physics Letters, 107, 031904, (2015).
Figure Legend: Design layout (a) and optical image (b) of a microelectromechanical systems (MEMS) fabricated indirect joule heating test structure used to investigate Germanium telluride conductivity and switching.
Journal Reference
Appl. Phys. Lett. 107, 031904 (2015).
H. Gwin1, C. H. Kodama1, T. V. Laurvick1, R. A. Coutu Jr.1, P. F. Taday2
[expand title=”Show Affiliations”]- Department of Electrical and Computer Engineering, Air Force Institute of Technology, Wright-Patterson AFB, Ohio 45433, USA
- Applications Group, TeraView Ltd., Cambridge CB4 0WS, United Kingdom
Abstract
We demonstrate improved terahertz (THz) modulation using thermally crystallized germanium telluride (GeTe) thin films. GeTe is a chalcogenide material that exhibits a nonvolatile, amorphous to crystalline phase change at approximately 200 °C, as well as six orders of magnitude decreased electrical resistivity. In this study, amorphous GeTe thin films were sputtered on sapphire substrates and then tested using THz time-domain spectroscopy (THz-TDS). The test samples, heated in-situ while collecting THz-TDSmeasurements, exhibited a gradual absorbance increase, an abrupt nonvolatile reduction at the transition temperature, followed by another gradual increase in absorbance. The transition temperature was verified by conducting similar thermal tests while monitoring electrical resistivity. THz transmittance modulation data were investigated between 10 and 110 cm−1 (0.3–3.3 THz). A peak modulation of approximately 99% was achieved at 2.3 THz with a 100 nm GeTe film on a sapphire substrate. After isolating the sapphire and the crystalline GeTe (c-GeTe) absorbance contributions, the results showed THz modulations ranging from 88.5% to 91.5% that were attributed solely to the single layer of transitioned c-GeTe. These results strongly motivate using GeTe or other chalcogenide thin films in THz modulators, filters, and metamaterial applications.
© 2015 Author(s)
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