Projected memory: reflections on one ..

A custom-made electrical characterization platform with integrated heating stage was used for electrical measurements. The sample was mounted on an invar block with two embedded tungsten heaters. The temperature was measured using a thermocouple inserted into the invar block and controlled via a Eurotherm 2416 temperature controller. The temperature on top of the memory device chip was calibrated using an Omega silicon diode sensor. The measurement temperature was 303K unless specified otherwise. For the annealing steps and the temperature-dependent transport measurements, the tungsten heaters were used. The ambient temperature variation experiment of was performed after 150min of annealing up to 350K. To stabilize the resistance state, this annealing was done for each of the states programmed. Full crystallinity of the phase-change materials in the devices fabricated was verified by resistivity measurements and ensured by annealing steps if required. The devices were electrically contacted using a high-frequency Cascade Microtech Dual-Z probe. A Keithley 2400 SMU was used for DC voltage or current outputs and the measurements of the corresponding current or voltage at the device. The cell resistance was read at a constant read voltage of 0.1V. Device programming was done using an Agilent 81150A Pulse Function Arbitrary Generator. The AC voltage and current signals were measured with a Tektronix DPO4104B oscilloscope. Mechanical relays were used to switch between AC and DC measurements.

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How to cite this article: Koelmans, W.W. et al. Projected phase-change memory devices. Nat. Commun. 6:8181 doi: 10.1038/ncomms9181 (2015).


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In this article, we expand on this idea by proposing a radical rethinking of the memory cell design, namely, the projected memory cell, and present a thorough experimental evaluation of this new concept. First, we introduce the projection concept. Next, we present the design, fabrication and simulation of projected phase-change memory devices. This is followed by experimental results, where we show almost complete elimination of drift and 1/f noise characteristics, thereby proving the efficacy of this concept.


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Second, we present almost analogue storage in the projected memory devices. The rectangular-shaped devices are particulary well suited for this. By increasing the amplitude of the reset pulse, we progressively increased the size of the amorphous region and hence the resulting device resistance. Such a relation between the programmed resistance level and the reset pulse amplitude is typically referred to as a programming curve ().

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Next, we explore the variation in the resistance levels, a key challenge in resistive memories. To conduct a direct comparison with non-projected devices, we characterized identical lateral devices without the projection layer. The AIST-based devices were programmed to several resistance levels, and at each level the resistance of the device was monitored for 1,400s. The temporal evolution of two programmed resistance levels in a non-projected device is shown in . At constant ambient temperature, the resistance versus time is characterized by , where R(t0) is the resistance measured at any time t0 greater than a few hundreds of nanoseconds after programming,. The drift coefficient, νR, was measured to be ≈0.067, which is typical of amorphous AIST. In contrast, the projected memory device exhibited a drift coefficient of only ≈0.0030 (). This remarkable, 22-fold drift reduction occurs because the majority of the current flows through the projection component rather than the amorphous region.