Pixel-level Microsecond Electrical Switching of Infrared Transparent Phase Change Materials

IntroductionPreviously, we have reported on a novel class of chalcogenide phase change materials, based on Ge-Sb-Se-Te (GSST) alloys, with excellent infrared transparency from 1.5 to > 10 micron wavelengths. As shown in Fig.... [ view full abstract ]

Introduction

Previously, we have reported on a novel class of chalcogenide phase change materials, based on Ge-Sb-Se-Te (GSST) alloys, with excellent infrared transparency from 1.5 to > 10 micron wavelengths. As shown in Fig. 1(a), increasing Se substitution leads to a progressive lowering of extinction coefficient.

A material in this family with a particular useful stoichiometry, Ge₂Sb₂Se₄Te₁ (GSS4T1), possesses broadband transparency through the infrared range while maintaining a large index contrast between amorphous and crystalline phases (Fig. 1(b)).

Methods

Here, for the first time, we report reversible electrical switching of GSS4T1 pixelated devices.  The devices are fabricated in a CMOS-compatible clean room on 8 inch wafers utilizing a full Si-based fabrication capabilities. Devices of pixel sizes from 1 to 30 micron were fabricated (Fig. 2(a)). Additionally, fabrication of more complicated sub-pixel metasurfaces was demonstrated (Fig. 2(a)). The pixel cross-section includes a 50-nm thick tungsten heater layer, 50-nm thick GSS4T1 layer, and a thin capping layer (Fig. 2b). Electrical switching measurement is obtained by applying electrical pulse trains through a programmable voltage generator into the gate of a high powered transistor (as shown in Fig. 2(b)), while measuring the reflection of a 1550-nm laser off the surface using an InGaAs imager.  

Results and Discussion

Microsecond switching of 30-micron and 1-micron pixels in a wafer-scale device has been demonstrated.  Figure 3 shows 1550-nm laser reflectivity as the 30-micron pixel is cycled between amorphous (high reflectance) and crystalline (low reflectance) phases. Amorphization with a single 1-microsecond pulse with 50% duty cycle at 25 Volts is demonstrated. Crystallization requires a train of 15 V pulses with a period of 1 ms, 50% duty cycle, with a total pulse train duration of 50 ms. The results represent the first embodiment of a pixelated electrical switched display for applications from short wave to longwave infrared. With further optimization, we expect sub-microsecond switching times for both crystallization and amorphization times, enabling realization of megahertz switching speeds.

We anticipate that these devices will enable a variety of applications in the infrared photonics such as ultrafast spatial light modulators, holographic devices, beam steering and neuromorphic networks for reservoir computing