- Nano Express
- Open Access
Phase-Change Memory Properties of Electrodeposited Ge-Sb-Te Thin Film
© Huang et al. 2015
- Received: 5 August 2015
- Accepted: 23 October 2015
- Published: 2 November 2015
We report the properties of a series of electrodeposited Ge-Sb-Te alloys with various compositions. It is shown that the Sb/Ge ratio can be varied in a controlled way by changing the electrodeposition potential. This method opens up the prospect of depositing Ge-Sb-Te super-lattice structures by electrodeposition. Material and electrical characteristics of various compositions have been investigated in detail, showing up to three orders of magnitude resistance ratio between the amorphous and crystalline states and endurance up to 1000 cycles.
- Phase-change memory
The development of denser, faster and less energy-consuming non-volatile memory (NVM) is critical to innovations in modern information technology . Among all the competitors for the next generation of NVM, chalcogenide-based phase-change memory (PCM) is one of the most promising candidates for its advantages of high speed, high scalability, high endurance, low power consumption and good compatibility with the complementary metal-oxide semiconductor (CMOS) process [2–4]. Data storage in PCM is achieved by rapidly switching the phase-change material between its amorphous (high-resistance) state and crystalline (low-resistance) state with Joule heating. Materials within the Ge-Sb-Te (GST) ternary phase diagram are generally regarded as suitable phase-change materials, with Ge2Sb2Te5 being the most popular material in this application. The recent development of GeTe/Sb2Te3 super-lattice structure-based interfacial phase-change memory data storage devices has shown even better performance with reduced switching energies, improved write-erase cycle lifetimes and faster switching speeds [5, 6]. The conventional method for the growth of crystalline super-lattice structures using molecular beam epitaxy (MBE) requires a nearly perfect match between the lattice constants of the two materials to allow epitaxial growth, which limits the selection of the super-lattice components. Amorphous GeSbTe super-lattices have been grown previously using ion beam deposition . Unlike MBE-grown crystalline super-lattices, the amorphous super-lattices have no epitaxial or lattice matching , but might still provide many of the advantages in particular related to reducing power in switching of phase-change memory.
Common methods for the deposition of phase-change materials use physical vapour deposition (PVD), chemical vapour deposition (CVD) or atomic layer deposition (ALD) [9–12]. Comparing with those conventional techniques, electrodeposition offers several potentially significant advantages for the growth of semiconductor alloys [13–16]. It is a fast and lower cost alternative to the vapour deposition techniques as it does not require ultra-high vacuum (UHV) equipment or high temperatures, and it can fill high-aspect-ratio cells. Several attempts have been made to grow phase-change materials by electrodeposition. Notable results were reported for the electrodeposition of binary BiTe and SbTe films [17, 18]. However, incorporating the electrochemically challenging germanium for the electrodeposition of ternary GeSbTe materials has proved to be extremely difficult in conventional aqueous solution. We recently reported a new method for the electrodeposition of amorphous ternary GeSbTe materials from a single, highly tuneable, non-aqueous electrolyte . This approach enables excellent control over the composition across the ternary phase diagram and also allows the selective deposition into nanostructures .
In this paper, we report the properties of a series of electrodeposited GeSbTe compounds with various compositions. These compositions can be achieved by varying either the precursor concentrations in the electrolytes or the deposition potentials in a single electrolyte. The latter method opens up the prospect of depositing super-lattice structures by electrodeposition, as has been previously shown for metallic compounds [21, 22]. The electrodeposited phase-change memory cells which have been fabricated are characterised in terms of the on/off ratio of the amorphous and crystalline states, threshold voltage and cyclability.
Three precursors, [NnBu4][GeCl5], [NnBu4][SbCl4] and [NnBu4]2[TeCl6], were prepared and purified for the ternary electrodeposition. All electrochemical experiments and the electrolyte preparation were performed inside a glove box in order to exclude moisture and oxygen (<5 ppm). Electrolytes were prepared in anhydrous CH2Cl2 using 0.1 mol dm−3 tetrabutylammonium chloride ([NnBu4]Cl) as the supporting electrolyte. A Pt gauze was used as the counter electrode, and for the reference electrode, a home-made Ag/AgCl reference electrode, using 0.1 mol dm−3 [NnBu4]Cl in CH2Cl2, was used. All experiments were performed in a standard one-compartment electrochemical cell within a wire mesh Faraday cage.
The deposited GST films were investigated using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). A Zeiss EVO LS 25 microscope equipped with an Oxford INCA x-act X-ray detector was used for the SEM and EDX analyses. For EDX quantification measurements, a sputtered thin film from a Ge2Sb2Te5 target was used for calibration. High-resolution SEM measurements were carried out with a field emission SEM (Jeol JSM 7500F). Films were imaged in high-vacuum mode. Annealing was performed using a rapid thermal annealer (Jipelec JetFirst) in a N2 atmosphere.
Memory Cell Fabrication and Characterisation
Electrodeposition was performed on planar TiN substrates in order to provide a technologically relevant substrate. A 200-nm TiN film was deposited onto a SiO2/Si substrate by reactive sputtering at room temperature. After the electrodeposition of GeSbTe films, another TiN film was sputtered on top of the GeSbTe films and subsequently patterned by photolithography and plasma etching to form the top electrode of the memory cell.
All electrical measurements were performed with a Keithley 4200 semiconductor characterisation system. During the measurements, the programming voltage bias was applied to the top electrode, while keeping the bottom electrode grounded. The probe/point contacts to the top and bottom electrodes of the devices were realised through a pair of Wentworth probe needles, using a Wentworth Laboratories AVT 702 semi-automatic prober. For electrical pulsing, two ns-pulsing measuring units (PMUs) were integrated within the same characterisation setup. The resistance of the devices after applying set or reset voltage pulses was measured at 0.1 V (DC).
EDX quantification results for GeSbTe films electrodeposited from different electrolytes
[NnBu4][GeCl5] (vol. %)
[NnBu4][SbCl4] (vol. %)
[NnBu4]2[TeCl6] (vol. %)
Phase-Change Memory Cells
The phase-change memory properties of electrodeposited GeSbTe thin films have been measured for various compositions. The composition of the resultant films can be tuned by varying the deposition potential in a single electrolyte, making their future application in depositing super-lattice structures possible. Film composition modulation was also achieved by varying the electrolyte concentrations with deposition of four device-quality GeSbTe thin films (Ge0.5Sb1.0Te8.5, Ge2.4Sb2.0Te5.6, Ge3.5Sb1.0Te5.5 and Ge5.0Sb3.5Te1.5). Phase-change memory cells based on the four films have shown promising switching properties with high-resistance ratio (three orders of magnitude) and good durability.
We thank the Engineering and Physical Sciences Research Council (EPSRC) for the support (EP/I010890/1) and for a doctoral prize (R.H. EP/509015FP/1).
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