Phase-change materials (PCMs) switch between amorphous and crystalline solid states. The process can occur at ultrahigh speeds (on the order of MHz), and may be driven thermally, optically, or electrically. Controlling the associated changes in the PCM’s optoelectronic properties is a great motivator to investigate and characterize such systems. As reported in the July 10 issue of Nature (DOI: 10.1038/nature13487; p. 206), Peiman Hosseini and Harish Bhaskaran at the University of Oxford and C. David Wright at the University of Exeter induced phase switching within sputtered Ge2Sb2Te5 (GST) films. These phase changes take place within low-dimensional nanoscale-sized regions, and the optical properties (e.g., color) in these regions change in a reversible, stable manner.
The researchers started by sandwiching GST between two layers of indium tin oxide (ITO), and depositing it on a reflective platinum surface. Crystallization of the GST changes its refractive index, which in turn changes the color of the multilayered stack when illuminated with “white” light. No energy is required after a switching event to maintain the new color, which is a distinct advantage in low-power display applications. In short, the group demonstrated the principle behind a PCM-based, nonvolatile reflective display.
The researchers modeled the stack using a transfer matrix optical computational method, and calculated optoelectronic properties such as color, reflectance, internal electric field, and transmittance. The model simulates the stack’s optical properties for GST in its as-sputtered amorphous state, and also in its crystalline state. Hosseini and colleagues found excellent agreement between their model data and the experimentally measured reflectivity spectra of the films.
The researchers next turned their attention to how the thickness of each of the three films affects the properties of the overall stack. They confirmed a strong dependence of color on film thicknesses. Furthermore, smaller GST thickness enhanced the reflectivity contrast between the amorphous and crystalline states, and fortuitously for display applications, required less power to switch. The researchers also tailored the thickness of the ITO layers to enhance the reflectivity of specific wavelengths; in effect, they demonstrated that it is possible to create a pixel of a specific color for a reflective display.
Working both with a continuous ITO/GST/ITO/Pt film and a similar film with lithographically defined pixels, they used the conductive tip of an atomic force microscope (CAFM) to switch the GST’s phase within nanoscale areas. They changed the color of the affected area, and created images that demonstrate the working principle of a reflective micro-display.
Next, the group moved on from reflective devices to deposit ITO/GST/ITO sandwiches on transparent substrates (quartz). They modulated the optical transmission of such a stack by electrically stimulating the crystallization of the GST layer. The researchers again used a CAFM to serially render high-resolution images, now demonstrating the principle behind a transmissive micro-display. Potential uses of fast, low-power, semi-transparent displays include wavelength-tunable windows, windshield displays, or even synthetic retina devices.
The layered films are very thin, and the techniques described were also demonstrated on flexible substrates and displays, enabling the creation of PCM-based pliable, electronic paper. Harish Bhaskaran, who led the work, said, “This optoelectronic framework has many likely applications, such as ultrafast, entirely solid-state displays, or supple, ‘smart’ contact lenses.”
