Phase Change Memories
Phase change memories are optical or electric non-volatile data storage devices that store information by switching a suitable material locally between an amorphous and a crystalline phase. These two phases can easily be distinguished by their reflectivity and conductivity respectively. Switching is achieved by inducing heat into the bit that is to be switched, either by a laser pulse or a current pulse. The principle is illustrated and explained below.
- The working principle of phase change memories relies on the contrast in physical properties between an amorphous and a crystalline phase of the employed material. In order to switch between the phases for example in optical devices, laser pulses of varying intensity and duration are applied. The phase transition from crystalline to amorphous takes place by melting the material illuminated by a short high-intensity laser spot. Due to the temperature gradient with respect to the surrounding material, the heat can dissipate quickly and the melt solidifies, leaving behind an amorphous mark. The process back to the crystalline phase is achieved by a long pulse of moderate intensity that allows the amorphous mark to relax into the crystalline state.
Already today, most available non-volatile rewritable optical data storage devices ( e.g. CD-RW, DVD+-RW, DVD-RAM, BD-RE) succesfully employ the phase change principle. In the near future also electrical memories, so called phase change RAM (PCRAM or PRAM), will compete with existing techniques such as Flash, outperforming it with regard to prospective data density, durability and greatly improved speed. All major companies in the field of semiconductors concentrate on developing PCRAM prototypes.
- DVD-RAM
The materials employed for phase change applications are characterized by a unique combination of physical properties. To mention a few, the amorphous phase needs to be sufficiently stable against crystallization to avoid data loss, e.g. as a result of sun illumination, and guarantee the reliability of the devices. Naturally, the optical or electrical contrast respectively between the two phases should be pronounced to allow for easy detection of the state of the bit. Moreover, the switch between the phases must be very rapid to facilitate high data transfer rates. For mobile applications, the energy consumption upon reading and writing must be small. Finally, the materials must meet economical and ecological demands.
A number of suitable materials are available that were found by extensive trial-and-error testing, but a deeper understanding of the physical background of their properties is an active field of research. Due to the lack of periodicity, the physics of amorphous solids are a scientific challenge leading to exciting effects. Among these is the so called 'threshold switching' effect, here an amorphous solid exhibits a drastic deviation from the usual linear response on the application of an electric field beyond a certain threshold field.
The phase change group of the I.Institute of Physics (IA) stands out because of the combination of a wide variety of experimental techniques with theoretical models. If you are interested to find out more about our work, please consult our list of publications or contact us directly. Infomation on how to participate in our research efforts, e.g. as a Master- or PhD-student, can be found on our page about open positions.