Characterization of ferroelectricity in metalÕferroelectricÕinsulatorÕ
semiconductor structure by pulsed
C
–
V
measurement; Ferroelectricity
in YMnO
3
ÕY
2
O
3
ÕSi structure
Takeshi Yoshimura,
a)
Norifumi Fujimura, Daisuke Ito, and Taichiro Ito
Department of Applied Materials Science, College of Engineering, Osaka Prefecture University,
1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan
͑Received 1 September 1999; accepted for publication 23 December 1999͒
Ferroelectric properties of the metal/ferroelectric/insulator/semiconductor ͑MFIS͒ structure were
investigated using the Pt/YMnO
3
/Y
2
O
3
/Si structure. The ferroelectric C–V hysteresis was
observed for not only the Pt/YMnO
3
(0001)/Y
2
O
3
/Si capacitor but also the Pt/amorphous-
YMnO
3
/Y
2
O
3
/Si capacitor. The polarization evaluated by conventional C–V measurement should
include interfacial polarization and rearrangement of the space charge together with the spontaneous
polarization by ferroelectricity. To eliminate the generation of the interfacial polarization and the
rearrangement of the space charge, the shorter charging time should be used to evaluate the
ferroelectricity of the MFIS capacitor. Therefore, we propose the pulsed C–V measurement as a new
method for evaluating the MF͑I͒S capacitor. © 2000 American Institute of Physics.
͓S0021-8979͑00͒02407-5͔
I. INTRODUCTION
Recently, ferroelectric random access memory ͑FeRAM͒
has attracted much attention, because of its nonvolatile op-
eration and the high access speed.
1,2
FeRAMs are classified
into two types. One is one transistor and one capacitor ͑1T–
1C͒ type. Although low density ͑below 1 Mbit͒ 1T–1C type
has been already industrialized, a ferroelectric material with
larger remanent polarization is required to produce 1T–1C
type FeRAMs with higher density. Another type of FeRAM
consists of metal-ferroelectric-semiconductor field effect
transistor ͑MFSFET͒.
3–6
The ferroelectric films are used as a
gate insulator of the FET. Compared to the 1T–1C type,
MFSFET has several advantages, such as nondestructive
readout and decreasing memory cell size. The ferroelectric
thin films do not need to have large spontaneous polariza-
tion, because the spontaneous polarization directly controls
the channel conductivity of the FET. Since the capacitance
of the ferroelectric gate is not important on the operation of
MFSFET, memory cell can be downsized.
However, MFSFET has serious problems such as the
formation of an amorphous SiO
2
layer with a low dielectric
constant at the film/Si interface and interdiffusion between
the film and Si.
7
To solve them, a metal-ferroelectric-
insulator-semiconductor ͑MFIS͒ structure
8
and a metal-
ferroelectric-metal-insulator-semiconductor ͑MFMIS͒
structure
9
have been demonstrated using various insulators
such as SiO
2
, CeO
2
, and Bi
2
SiO
5
.
10–16
On the MFI͑M͒S
structure, the bias voltage of the top electrode is not effec-
tively applied to the ferroelectric layer, because the bias volt-
age is also applied to the insulator layer. Therefore, the ferro-
electric films with low dielectric constant are required.
We have proposed the use of RMnO
3
͑R: rare earth ele-
ments͒ thin films for MF͑I͒SFET type FeRAM.
17–19
When R
is Y, Er, Ho, Tm, Yb, and Lu, RMnO
3
has a hexagonal
structure with an unipolarization axis along ͓0001͔.
20–24
Since RMnO
3
has a low permittivity and does not include
volatile elements such as Bi and Pb, it is expected to have
several advantages over Pb(Zr
X
Ti
1ϪX
)O
3
and SrBi
2
Ta
2
O
9
.
We reported that ͑0001͒ oriented Y(Yb)MnO
3
films were
obtained on Y
2
O
3
͑111͒/Si(111). The C–V characteristics of
the Pt/Y(Yb)MnO
3
/Y
2
O
3
/Si capacitor showed a ferroelec-
tric polarization switching type hysteresis loop. To confirm if
the origin of the hysteresis loop is the ferroelectricity of
YMnO
3
, we performed detailed analyses of the C–V
characteristics.
25–28
The relationships between the memory
window of the hysteresis loop and the sweep rate or the
sweep width of the bias voltage has been investigated. How-
ever, some issues still exist in the characterization of the
MFIS capacitor using conventional C–V measurement. In
the case of the MFIS capacitor, as the ferroelectric material
should have lower remanent polarization than that of the
MFM capacitor, conventional C–V characteristics are more
influenced by interfacial polarization and rearrangement of
the space charge. Although there are many reports on the
MFIS capacitor having ferroelectricity,
29,30
those results are
not sufficient as the proof of the ferroelectricity by only con-
ventional C–V measurement.
In this article, problems of conventional C–V measure-
ment for evaluating MFIS capacitor are discussed using two
types of YMnO
3
/Y
2
O
3
/Si(111) samples with different crys-
tallinity of YMnO
3
. It is shown that pulsed C–V measure-
ment is effective for evaluating the MFIS capacitor.
II. EXPERIMENT
A n-type Si͑111͒ single crystal wafer with 1 ⍀ cm resis-
tivity was used as a substrate. The substrate was chemically
cleaned using hot solutions ͑80 °C͒ of NH
4
OH:H
2
O
2
:H
2
O
͑ϭ1:1:5͒, and HCl:H
2
O
2
:H
2
O ͑ϭ1:1:6͒, and 1% HF acid.
a͒
Electronic mail: fujim@ams.osakafu-u.ac.jp
JOURNAL OF APPLIED PHYSICS VOLUME 87, NUMBER 7 1 APRIL 2000
34440021-8979/2000/87(7)/3444/6/$17.00 © 2000 American Institute of Physics