Enhanced threshold voltage of
Zn-doped Ge
2
Sb
2
Te
5
phase-change
memory deposited by electron-beam evaporation
Rui Li
1,2
, Yifan Jiang
1
, Ling Xu
*
,1
, Zhongyuan Ma
1
, Fei Yang
1
, Jun Xu
1
, and Weining Su
3
1
National Laboratory of Solid State Microstructures and School of Electronic Science and Engineering, Nanjing University,
Nanjing 210093, P. R. China
2
College of Electronic Science and Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210003, P. R. China
3
National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, P. R. China
Received 2 June 2013, revised 30 July 2013, accepted 16 August 2013
Published online 30 September 2013
Keywords doping, electron-beam evaporation, Ge
2
Sb
2
Te
5
, phase-change materials, phase-change memory, thin films
*
Corresponding author: e-mail okxuling@gmail.com, Phone: þ86 25 8362 1278, Fax: þ86 25 8359 5535
Zn-doped Ge
2
Sb
2
Te
5
(GST) thin films are deposited on glass
substrates by an electron-beam evaporation technique in an
ultra-high vacuum. GST mixed with 5%, 10%, 20% Zn is used.
Through in situ resistance measurements, an increase of
crystalline temperature in Zn-doped GST was observed. From
absorption spectra, the optical bandgap values of 10% Zn-
doped GST in different structures are estimated to be 1.1, 0.9,
and 0.7 eV, respectively, which are slightly wider than that of
pure GST. A prototype phase-change memory (PCM) device
using GST thin film doped with Zn was fabricated. The current–
voltage test results of the devices show better switching
performances in both SET and RESET processes than that of
the device using pure GST film. The threshold voltage is greatly
increased with the adding of Zn atom with fixed proportion,
which can weaken the interaction during the read and write
operation of PCM.
ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Phase-change memory (PCM) is one
of the most promising candidates as the next-generation
memory [1]. It has been five decades since the phase-change
phenomenon was first presented by Ovshinsky [2]. Since
PCM can meet the commercial demands of scaling down and
high storage density for nonvolatile memory, Ge
2
Sb
2
Te
5
(GST) alloy materials, the key component of PCM, have
attracted a great number of researcher’s attention. GST phase-
change materials have an amorphous state with the feature of
high resistivity and a crystalline state with the feature of low
resistivity, and the two states can be transformed to each other
under certain conditions [3]. Many research workers have
made a lot of efforts to optimize the characteristics of phase-
change material [4]. One of the most effectual methods to
manipulate the properties of phase-change materials is
doping. Recently, many elements such as Si [5], N [6, 7],
O[8],Zn[9–11], Ag [12], Sn [13], and so on have been doped
into GST to change optical and electrical properties by many
other research groups using RF magnetic sputtering. For
instance, Men et al. [9] reported work focusing on the optical
properties of Zn-doped GST. In addition, Skelton et al. [10]
has simulated the Zn incorporation in a GST network, which
requires further experiments to support the conclusions. Wang
et al. [11] show some results that Zn-doped GST prepared
by RF magnetic sputtering exhibits a higher crystallization
temperature. However, we have not found any literature
reporting the application of Zn-GST material in PCRAM
devices using an electron-beam evaporation (EBE) technique.
The EBE technique possesses many advan tages such as
low cost, convenience and being able to deposit large-area
thin films. At present, few efforts have been reported on the
deposition of GST thin film by an EBE technique.
In this paper, Zn is chosen to dope into GST to prepare
Zn-doped GST (Zn-GST) films by using an EBE method. The
optical bandgap and activation energies of crystallization (E
a
)
of the Zn-GST films are also estimated. A prototype PCM
device was fabricated and the I–V characteristic of the device
using GST and Zn-GST films were also studied in the paper.
2 Experiment Three hundred nanometers thick Zn-
GST thin films have been deposited by EBE. The thin films
were deposited on quartz or glass substrates by a high-vacuum
Phys. Status Solidi A 210, No. 12, 2650–2655 (2013) / DOI 10.1002/pssa.201329381
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ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim