Ternary HfZnO
x
for Multistate Memory Applications
M. K. Hota, and H. N. Alshareef
*
Materials Science and Engineering, King Abdullah University of Science & Technology
(KAUST), Thuwal 23955-6900, Saudi Arabia
*
Email:
Keywords:
High-
k
oxides for electronic applications, Multistate memory, Memristors
Among several dielectrics proposed, HfO
2
-based dielectrics have received growing
interest in the research community, especially for the non-volatile resistive switching memory
applications. On the other hand, ZnO is a cost effective transparent conducting oxide
material, already used as promising memory material in different non-volatile memory
applications. However, atomic layer deposited (ALD) only HfO
2
and ZnO did not show good
stable memory performance. Here, we report a mixed high-
k
oxide of HfZnO
x
(HfO
2
: ZnO=
7:3) deposited by ALD as memory material for resistive switching memory applications. The
rational for choosing this composition is to combine the ability of ZnO to form oxygen
vacancies and related defects relatively easily compared with HfO
2
, which is known to show
resistive memory effect. On the other hand, to increase the bit density and to reduce the cost
per bit, memory devices with multistate characteristics are urgently required. Among several
approaches to build multistate memory, resistive switching based non-volatile memory
devices are found to be promising for multi-level memory operations, due to their simple
metal-insulator-metal capacitor structure, nanoscale scaling possibility, high speed and
controlled multi-level conductance capability. Moreover, the analog-like gradual transition
between different resistance levels under optimized conditions (input impulses) made it
suitable for electronics synaptic memory applications to emulate the plasticity of biological
synapses. The chemical composition of the mixed oxide film was analyzed by X-ray
photoelectron spectroscopy (XPS), and it was found that the amount of oxygen vacancies in
HfO
2
is ~11.5 %, and it increases up to ~21.1 % after incorporation of ZnO into HfO
2
matrix.
Here, ITO and Ti/Au were used as bottom and top electrodes to make the memory devices for
electrical measurements. To achieve the multi-bit memory performance, we used different
current compliance and different RESET voltages. It was found that a 3-bit memory
operation can be made by controlling the current compliance while by controlling RESET
voltage a 4-bit memory operation can be achieved. This multi-bit memory performance was
found highly stable and repeatable. The dc endurance stability was found more than 400
cycles without any degradation and from the cumulative probability study for all memory
states show that the memory states do not overlap with each other, which is mandatory for the
multi-bit memory operation to avoid failure in data storage. The multi-bit memory
performance was further utilized for synaptic memory operations. We have studied the
continuous change (increase/ decrease) in conductivity of the memory device under
sequential application of external DC and A.C. stimulus. The device shows synaptic-like
memory behavior under both DC and A.C. stimulus. We also demonstrated the Ebbinghaus
forgetting nature by applying positive and negative voltage pulses of increasing amplitude.
The formation/rupture of different amount of conducting filaments governed by the oxygen
vacancies under different bias condition leads to the multiple memory states and hence
synaptic memory performance. These findings open up opportunities to explore such ternary
material systems not only for the multi-bit non-volatile memory operation but also for the
next generation synaptic memory applications.
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