U.S. patent application number 11/378719 was filed with the patent office on 2006-08-31 for pcmo thin film with controlled resistance characteristics.
This patent application is currently assigned to Sharp Laboratories of America, Inc.. Invention is credited to David R. Evans, Sheng Teng Hsu, Tingkai Li, Wei-Wei Zhuang.
Application Number | 20060194403 11/378719 |
Document ID | / |
Family ID | 35187653 |
Filed Date | 2006-08-31 |
United States Patent
Application |
20060194403 |
Kind Code |
A1 |
Li; Tingkai ; et
al. |
August 31, 2006 |
PCMO thin film with controlled resistance characteristics
Abstract
PrCaMnO (PCMO) thin films with predetermined memory-resistance
characteristics and associated formation processes have been
provided. In one aspect the method comprises: forming a
Pr.sup.3+.sub.1-xCa.sup.2+.sub.xMnO thin film composition, where
0.1<x<0.6; in response to the selection of x, varying the
ratio of Mn and O ions as follows: O.sup.2- (3.+-.20%); Mn.sup.3+
((1-x).+-.20%); and, Mn.sup.4+ (x.+-.20%). When the PCMO thin film
has a
Pr.sup.3+.sub.0.70Ca.sup.2+.sub.0.30Mn.sup.3+.sub.0.78Mn.sup.4+.sub.0.22O-
.sup.2-.sub.2.96 composition, the ratio of Mn and O ions varies as
follows: O.sup.2- (2.96); Mn.sup.3+ ((1-x)+8%); and, Mn.sup.4+
(x-8%). In another aspect, the method creates a density in the PCMO
film, responsive to the crystallographic orientation. For example,
if the PCMO film has a (110) orientation, a density is created in
the range of 5 to 6.76 Mn atoms per 100 .ANG..sup.2 in a plane
perpendicular to the (110) orientation.
Inventors: |
Li; Tingkai; (Vancouver,
WA) ; Zhuang; Wei-Wei; (Vancouver, WA) ;
Evans; David R.; (Beaverton, OR) ; Hsu; Sheng
Teng; (Camas, WA) |
Correspondence
Address: |
SHARP LABORATORIES OF AMERICA, INC.;C/O LAW OFFICE OF GERALD MALISZEWSKI
P.O. BOX 270829
SAN DIEGO
CA
92198-2829
US
|
Assignee: |
Sharp Laboratories of America,
Inc.
|
Family ID: |
35187653 |
Appl. No.: |
11/378719 |
Filed: |
March 17, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10836689 |
Apr 30, 2004 |
7060586 |
|
|
11378719 |
Mar 17, 2006 |
|
|
|
Current U.S.
Class: |
438/385 ;
257/E45.003; 438/382; 438/384 |
Current CPC
Class: |
H01L 45/1233 20130101;
G11C 13/0007 20130101; H01L 45/04 20130101; H01L 45/147 20130101;
G11C 2213/31 20130101 |
Class at
Publication: |
438/385 ;
438/384; 438/382 |
International
Class: |
H01L 21/20 20060101
H01L021/20 |
Claims
1-8. (canceled)
9. A method for forming a
Pr.sup.3+.sub.1-xCa.sup.2+.sub.xMn.sup.3+.sub.(1-x).+-.20%Mn.sup.4+.sub.x-
.+-.20%O.sup.2-.sub.3.+-.20% (PCMO) thin film memory-resistance
device, the method comprising: forming a bottom electrode; forming
a PCMO thin film with a crystallographic orientation overlying the
bottom electrode; and, creating a lattice mismatch in the PCMO
film, responsive to the orientation, between the PCMO thin film and
the bottom electrode.
10. The method of claim 9 wherein forming a PCMO thin film with a
crystallographic orientation includes forming a PCMO film in a
(110) orientation; and, wherein creating a lattice mismatch in the
PCMO film includes creating a lattice mismatch of less than 30%
between the PCMO thin film and the bottom electrode.
11. The method of claim 9 wherein forming a PCMO thin film with a
crystallographic orientation includes forming a PCMO film in a
(001) orientation; and, wherein creating a lattice mismatch in the
PCMO film includes creating a lattice mismatch of less than 30%
between the PCMO thin film and the bottom electrode.
12. The method of claim 9 wherein forming a bottom electrode
includes forming a bottom electrode from a material selected from
the group including Pt, Ir, Al, and TiN.
13. The method of claim 9 further comprising: forming a buffer
layer interposed between the PCMO thin film and the bottom
electrode formed from a material selected from the group including
InO2 and ZnO.
14. A method for forming a
Pr.sup.3+.sub.1-xCa.sup.2+.sub.xMn.sup.3+.sub.(1-x).+-.20%Mn.sup.4+.sub.x-
.+-.20%O.sup.2-.sub.3.+-.20% (PCMO) thin film with predetermined
memory-resistance characteristics, the method comprising: forming a
PCMO thin film with a crystallographic orientation; and, creating a
PCMO film with a selectable resistance state, responsive to the
orientation.
15. The method of claim 14 wherein forming a PCMO thin film with a
crystallographic orientation includes forming a PCMO film in a
(001) orientation; and, wherein creating a PCMO film with a
selectable resistance state, responsive to the orientation,
includes: writing a high resistance in the range of 10 to 1000 kilo
ohms, using .+-.(2 to 10) volt (V) pulse, with a duration in the
range of 5 nanosecond (ns) to 50 microseconds; and, resetting a low
resistance in the range of 500 ohms to 10 kilo ohms, using .+-.(2
to 10) V pulse, with a duration in the range of 5 ns to 50
microseconds.
16. The method of claim 14 wherein forming a PCMO thin film with a
crystallographic orientation includes forming a PCMO film in a
(110) orientation; and, wherein creating a PCMO film with a
selectable resistance state, responsive to the orientation,
includes: writing a high resistance in the range of 10 to 1000 kilo
ohms, using .+-.(2 to 10) V pulse, with a duration in the range of
5 ns to 50 microseconds; and, resetting a low resistance in the
range of 500 ohms to 10 kilo ohms, using .+-.(2 to 10) V pulse,
with a duration in the range of 5 ns to 50 microseconds.
17-31. (canceled)
32. A
Pr.sup.3+.sub.1-xCa.sup.2+.sub.xMn.sup.3+.sub.(1-x).+-.20%Mn.sup.4+-
.sub.x.+-.20%O.sup.2-.sub.3.+-.20% (PCMO) thin film
memory-resistance device, the device comprising: a bottom
electrode; a PCMO thin film overlying the bottom electrode having:
a crystallographic orientation; and, a lattice mismatch between the
PCMO thin film and the bottom electrode, responsive to the
crystallographic orientation.
33. The device of claim 32 wherein the PCMO thin film has a (110)
orientation; and, wherein the PCMO thin film has a lattice mismatch
of less than 30%.
34. The device of claim 32 wherein the PCMO thin film has a (001)
orientation; and, wherein the PCMO thin film has a lattice mismatch
of less than 30%.
35. The device of claim 32 wherein the bottom electrode is a
material selected from the group including Pt, Ir, Al, and TiN.
36. The device of claim 32 further comprising: a buffer layer
interposed between the PCMO thin film and the bottom electrode
formed from a material selected from the group including InO2 and
ZnO.
37. A
Pr.sup.3+.sub.1-xCa.sup.2+.sub.xMn.sup.3+.sub.(1-x).+-.20%Mn.sup.4+-
.sub.x.+-.20%O.sup.2-.sub.3.+-.20% (PCMO) thin film with
predetermined memory-resistance characteristics, the PCMO thin film
comprising: a crystallographic orientation; and, a selectable
resistance state, responsive to the crystallographic
orientation.
38. The PCMO thin film of claim 37 wherein the crystallographic
orientation is in a (001) orientation; and, wherein the selectable
resistance state includes: a high resistance in the range of 10 to
1000 kilo ohms, using .+-.(2 to 10) volt (V) pulse, with a duration
in the range of 5 nanosecond (ns) to 50 microseconds; and, a low
resistance in the range of 500 ohms to 10 kilo ohms, using .+-.(2
to 10) V pulse, with a duration in the range of 5 ns to 50
microseconds.
39. The PCMO thin film of claim 37 wherein the crystallographic
orientation is in a (110) orientation; and, wherein the selectable
resistance state includes: a high resistance in the range of 10 to
1000 kilo ohms, using .+-.(2 to 10) V pulse, with a duration in the
range of 5 ns to 50 microseconds; and, a low resistance in the
range of 500 ohms to 10 kilo ohms, using .+-.(2 to 10) V pulse,
with a duration in the range of 5 ns to 50 microseconds.
40-46. (canceled)
Description
RELATED APPLICATIONS
[0001] This application is a Divisional of a pending patent
application entitled, PCMO THIN FILM WITH RESISTANCE RANDOM ACCESS
MEMORY (RRAM) CHARACTERISTICS, invented by Tingkai Li et al., Ser.
No. 10/836,689, filed Apr. 30, 2004, which is incorporated herein
by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention generally relates to integrated circuit (IC)
memory fabrication and, more particularly, to a
Pr.sub.xCa.sub.1-xMnO.sub.3 (PCMO) memory film and associated film
process.
[0004] 2. Description of the Related Art
[0005] In order to meet the requirements of low power, low
operation voltage, high-speed, and high-density memory
applications, electrically alterable resistors are being
investigated that can be fabricated from PCMO. Metalorganic
chemical vapor deposition (MOCVD), sputtering, laser ablation, and
metalorganic deposition (MOD) methods can be used, along with
appropriate surface treatments, to deposit PCMO on various
substrates. The different deposition methods are being investigated
to address the control of specific interfacial properties of the
PCMO, such as lattice mismatch, trapped charge density, as well as
bulk properties such as grain size and film. Reversible and
non-volatile resistance changes in the PCMO film are desirable
using either bipolar or unipolar pulsed biasing. Therefore, an
understanding the relationship between memory characteristics and
PCMO thin film material structure is sought. PCMO also exhibits
colossal magnetoresistive (CMR) properties. These are also strongly
affected by thin film crystal structure. An understanding of the
relationship between crystal structure and an electrically
alterable resistance suitable for RRAM devices is desired.
[0006] It would be advantageous if the memory resistance
characteristics of a PCMO film could be controlled and optimized by
the judicious choice of crystallization and orientation of PCMO
thin films.
SUMMARY OF THE INVENTION
[0007] The present invention describes processes for forming PCMO
resistance-memory characteristics that are responsive to
controllable deposition attributes such as film composition,
structure, and crystallographic orientation.
[0008] Accordingly, a method is provided for forming a PrCaMnO
(PCMO) thin film with predetermined memory-resistance
characteristics. In one aspect the method comprises: forming a
Pr.sup.3+.sub.1-xCa.sup.2+.sub.xMnO thin film composition, where
0.1<x<0.6; in response to the selection of x, varying the
ratio of Mn and 0 ions as follows: [0009] O.sup.2- (3.+-.20%);
[0010] Mn.sup.3+ ((1-x).+-.20%); and, [0011] Mn.sup.4+
(x.+-.20%).
[0012] For example, when the PCMO thin film has a
Pr.sup.3+.sub.0.70Ca.sup.2+.sub.0.30Mn.sup.3+.sub.0.78Mn.sup.4+.sub.0.22O-
.sup.2-.sub.2.96 composition, the ratio of Mn and O ions varies as
follows: [0013] O.sup.2- (2.96); [0014] Mn.sup.3+ ((1-x)+8%); and,
[0015] Mn.sup.4+ (x-8%).
[0016] In another aspect, the method comprises forming a PCMO thin
film with a crystallographic orientation; and, creating a density
in the PCMO film, responsive to the orientation. For example, if
the PCMO film has a (110) orientation, a density is created in the
range of 5 to 6.76 Mn atoms per 100 .ANG..sup.2 in a plane
perpendicular to the (110) orientation.
[0017] In another aspect, the method comprises: forming a bottom
electrode; forming a PCMO thin film with a crystallographic
orientation overlying the bottom electrode; and, creating a lattice
mismatch in the PCMO film, responsive to the orientation, between
the PCMO thin film and the bottom electrode. For example, if the
PCMO film has a (110) orientation, then a lattice mismatch of less
than 30% is created between the PCMO thin film and the bottom
electrode.
[0018] In a different aspect, the method comprises: forming a PCMO
thin film with a crystallographic orientation; and, creating a PCMO
film with a selectable resistance state, responsive to the
orientation. For example, if the PCMO film has a (001) orientation,
then the PCMO film resistance state can be written to a high
resistance in the range of 10 to 1000 kilo ohms, using .+-.(2 to
10) volt (V) pulse, with a duration in the range of 5 nanosecond
(ns) to 50 microseconds. Further, the (001) PCMO can be reset to a
low resistance in the range of 500 ohms to 10 kilo ohms, using
.+-.(2 to 10) V pulse, with a duration in the range of 5 ns to 50
microseconds.
[0019] In another aspect, the method comprises: forming a PCMO thin
film with a crystal grain size; and, creating a PCMO film with a
selectable resistance state, responsive to the grain size. For
example, if the PCMO film is formed with an average crystal grain
size in the range of 3 to 40 nanometers (nm), then the PCMO film
can be written to a high resistance of greater than 225 kilo ohms,
using 5 V pulse of less than 100 ns, or reset to a low resistance
of less than 10 kilo ohms, using a 3 V pulse of less than 10
microseconds.
[0020] In one aspect, the method comprises creating a super lattice
structure with an ordered distribution in the PCMO film, responsive
to the PCMO film composition. For example, if the PCMO thin film
has a Pr.sup.3+.sub.0.50Ca.sup.2+.sub.0.50MnO.sub.0.3 composition,
then a distribution is created as follows:
[0021] Z(Mn.sup.3+)PrMnO.sub.3:Z(Mn.sup.4+)CaMnO.sub.3 subunits,
where Z is a natural number.
[0022] Additional details of the above-described methods, and a
PCMO device with memory characteristics responsive to film
attributes such as composition, structure, and crystallographic
orientation, are presented below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a drawing illustrating perovskite cubic
structures.
[0024] FIG. 2 is a drawing illustrating the PrMnO.sub.3 lattice
structure.
[0025] FIG. 3 is a drawing depicting the Gibbs free energies for
the formation of Pr and Mn oxides.
[0026] FIGS. 4A and 4B are drawings illustrating the mechanism of
reversible resistive switching properties in a CaMnO.sub.3
unit.
[0027] FIG. 5 is a drawing depicting the relationship between
composition and lattice structures of
Pr.sub.1-xCa.sub.xMnO.sub.3.
[0028] FIG. 6 is a drawing depicting the band gap as a function of
x value and oxygen vacancies.
[0029] FIG. 7 is a diagram depicting the distribution of
PrMnO.sub.3 and CaMnO.sub.3 subunits, as a function of x, in
Pr.sub.1-xCa.sub.xMnO.sub.3 materials.
[0030] FIGS. 8A and 8B are drawings showing the direction from
Mn.sup.3+ ions in PrMnO.sub.3, to oxygen vacancies, oriented toward
a top electrode.
[0031] FIGS. 9A and 9B are drawings depicting the direction of
electron flow when opposite polarity pulses are applied to the top
electrode.
[0032] FIGS. 10A and 10B are drawings showing the directionality of
electron jumps between Mn.sup.3+ and Mn.sup.4+ ions.
[0033] FIGS. 11A and 11B show electron flow when the PrMnO.sub.3
and CaMnO.sub.3 subunit layers are reversed with respect to the
bottom electrode.
[0034] FIGS. 12A, 12B, 12C depict the (110), (011), and (101)
orientations, respectively, of PCMO thin films.
[0035] FIG. 13 shows an x-ray pattern of PCMO with partial (110)
orientation.
[0036] FIGS. 14A and 14B depict the (100) and (010) orientations of
PCMO thin films, respectively.
[0037] FIG. 15 shows the (001) orientation (marked ABCD) of PCMO
thin films.
[0038] FIG. 16 shows the x-ray pattern of PCMO thin films with
(001) orientation.
[0039] FIG. 17 depicts the EDX pattern of the PCMO thin film of
FIG. 16.
[0040] FIG. 18 depicts the switching properties of c-axis oriented
PCMO thin films.
[0041] FIG. 19 shows the x-ray pattern of a PCMO thin film with
nanosized polycrystalline materials.
[0042] FIG. 20 depicts the unipolar switching properties of the
PCMO thin film of FIG. 19.
[0043] FIG. 21 is a partial cross-sectional view of the present
invention
Pr.sup.3+.sub.1-xCa.sup.2+.sub.xMn.sup.3+.sub.(1-x).+-.20%Mn.sup.4+.sub.x-
.+-.20%O.sup.2-.sub.3.+-.20% (PCMO) thin film memory-resistance
device.
[0044] FIGS. 22 through 27 are flowcharts illustrating different
aspects of the present invention method for forming a PrCaMnO
(PCMO) thin film with predetermined memory-resistance
characteristics.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0045] Some general relationships between PCMO physical attributes,
responsive to deposition procedures, and resistance-memory
characteristics will be initially presented. Detailed analysis of
these general relationships is presented below in the Functional
Description Section of the application.
[0046] The first aspect of the present invention should be
considered with respect to FIGS. 4A, 4B, and Table 1, below. A
PrCaMnO (PCMO) thin film with predetermined memory-resistance
characteristics comprises a Pr.sup.3+.sub.1-xCa.sup.2+.sub.xMnO
composition, where 0.1<x<0.6; and, a ratio of Mn and O ions
is responsive to the composition as follows: [0047] O.sup.2-
(3.+-.20%); [0048] Mn.sup.3+ ((1-x).+-.20%); and, [0049] Mn.sup.4+
(x.+-.20%).
[0050] When the composition is
Pr.sup.3+.sub.0.70Ca.sup.2+.sub.0.30Mn.sup.3+.sub.0.78Mn.sup.4+.sub.0.22O-
.sup.2-.sub.2.96, then the ratio of Mn and O ions is as follows:
[0051] O.sup.2- (2.96); [0052] Mn.sup.3+ ((1-x)+8%); and, [0053]
Mn.sup.4+ (x-8%).
[0054] Further, the PCMO film has a 13.3% oxygen vacancy.
[0055] In a second aspect of the invention, considered with respect
to Table 1 and FIGS. 12A-12C, a
Pr.sup.3+.sub.1-xCa.sup.2+.sub.xMn.sup.3+.sub.(1-x).+-.20%Mn.sup.4+.sub.x-
.+-.20%O.sup.2-.sub.3.+-.20% (PCMO) thin film comprises a
crystallographic orientation, and a density responsive to the
orientation. When the crystallographic orientation is a (110)
orientation, the density is in the range of 5 to 6.76 Mn atoms per
100 .ANG..sup.2 in a plane perpendicular to the (110) orientation
(the <110>). Further, a distance of 3.855 .ANG. exists
between closest (the two closest) Mn atoms, responsive to the (110)
orientation.
[0056] When the crystallographic orientation is in a (001)
orientation, the density is in the range of 5 to 6.73 Mn atoms per
100 .ANG..sup.2 in a plane perpendicular to the (001) orientation,
and a distance of 3.840 .ANG. exists between the two closest Mn
atoms, responsive to the (001) orientation.
[0057] FIG. 21 is a partial cross-sectional view of the present
invention
Pr.sup.3+.sub.1-xCa.sup.2+.sub.xMn.sup.3+.sub.(1-x).+-.20%Mn.sup.4+.sub.x-
.+-.20%O.sup.2-.sub.3.+-.20% (PCMO) thin film memory-resistance
device. The device 100 comprises a bottom electrode (BE) 102, a
PCMO thin film 104 overlying the bottom electrode 102, and a top
electrode (TE) 106. The bottom electrode 102 can be a material such
as Pt, Ir, Al, or TiN. In some aspects, a buffer layer 108 is
interposed between the PCMO thin film 104 and the bottom electrode
102 formed from a material such as InO2 or ZnO.
[0058] The PCMO thin film 104 has a crystallographic orientation,
and a lattice mismatch between the PCMO thin film 104 and the
bottom electrode 102 (or buffer layer 108), responsive to the
crystallographic orientation. When the PCMO thin film 104 has a
(110) orientation, then the lattice mismatch of less than 30%.
Likewise, when the PCMO thin film 104 has a (001) orientation, the
lattice mismatch is less than 30%.
[0059] In a different aspect (see FIGS. 18 and 20), the
Pr.sup.3+.sub.1-xCa.sup.2+.sub.xMn.sup.3+.sub.(1-x).+-.20%Mn.sup.4+.sub.x-
.+-.20%O.sup.2-.sub.3.+-.20% (PCMO) thin film has a
crystallographic orientation and a selectable resistance state,
responsive to the crystallographic orientation. When the
crystallographic orientation is in a (001) orientation, the
selectable resistance state is a high resistance in the range of 10
to 1000 kilo ohms, using .+-.(2 to 10) volt (V) pulse, with a
duration in the range of 5 nanosecond (ns) to 50 microseconds, or a
low resistance in the range of 500 ohms to 10 kilo ohms, using
.+-.(2 to 10) V pulse, with a duration in the range of 5 ns to 50
microseconds.
[0060] If the PCMO crystallographic orientation is in a (110)
orientation, the selectable resistance state is a high resistance
in the range of 10 to 1000 kilo ohms, using .+-.(2 to 10) V pulse,
with a duration in the range of 5 ns to 50 microseconds, or a low
resistance in the range of 500 ohms to 10 kilo ohms, using .+-.(2
to 10) V pulse, with a duration in the range of 5 ns to 50
microseconds.
[0061] In another aspect, see FIGS. 19 and 20, the
Pr.sup.3+.sub.1-xCa.sup.2+.sub.xMn.sup.3+.sub.(1-x).+-.20%Mn.sup.4+.sub.x-
.+-.20%O.sup.2-.sub.3.+-.20% (PCMO) thin film comprises a crystal
grain size, and a selectable resistance state, responsive to the
grain size. If the crystal grain size is in the range of 3 to 40
nanometers (nm), then the selectable resistance state is a high
resistance of greater than 225 kilo ohms, using 5 V pulse of less
than 100 ns, or a low resistance of less than 10 kilo ohms, using a
3 V pulse of less than 10 microseconds.
[0062] If the crystal grain size in the range of 40 nm to
epitaxial, then the selectable resistance state can be a high
resistance of greater than 300 kilo ohms, using -5 V pulse of less
than 50 microseconds, or a low resistance of less than 10 kilo
ohms, using a +5 V pulse of less than 50 microseconds.
[0063] In another aspect, see FIG. 7, a
Pr.sup.3+.sub.1-xCa.sup.2+.sub.xMn.sup.3+.sub.(1-x).+-.20%Mn.sup.4+.sub.x-
.+-.20%O.sup.2-.sub.3.+-.20% (PCMO) thin film comprises a
composition, and a super lattice structure of with an ordered
distribution, responsive to the composition. For example, if the
composition is Pr.sup.3+.sub.0.50Ca.sup.2+.sub.0.50MnO.sub.0.3,
then the super lattice structure ordered distribution is:
[0064] Z(Mn.sup.3+)PrMnO.sub.3:Z(Mn.sup.4+)CaMnO.sub.3 subunits,
where Z is a natural number.
[0065] If the composition is
Pr.sup.3+.sub.0.67Ca.sup.2+.sub.0.33MnO.sub.0.3, then the super
lattice structure ordered distribution is:
[0066] 2Z(Mn.sup.3+)PrMnO.sub.3:Z(Mn.sup.4+)CaMnO.sub.3 subunits,
where Z is a natural number.
[0067] If the composition is
Pr.sup.3+.sub.0.75Ca.sup.2+.sub.0.25MnO.sub.0.3, then the super
lattice structure of ordered distribution is:
[0068] 3Z(Mn.sup.3+)PrMnO.sub.3:Z(Mn.sup.4+)CaMnO.sub.3 subunits,
where Z is a natural number.
Functional Description
[0069] FIG. 1 is a drawing illustrating perovskite cubic
structures. Pr.sub.1-xCa.sub.xMnO.sub.3 is an electrically
alterable resistive material whose memory resistance
characteristics are related to the PCMO lattice structure. The unit
cell of Pr.sub.1-xCa.sub.xMnO.sub.3 consist of two subunits units:
PrMnO.sub.3 and CaMnO.sub.3. The ideal lattice of each unit has the
same perovskite cubic structure, as shown in the figure.
[0070] In the case of the Pr.sub.1-xCa.sub.xMnO.sub.3 series, the
end members PrMnO.sub.3 and CaMnO.sub.3 have an orthorhombic
structure belonging to the same space group Pbnm, but the
distortion of the latter is almost negligible so that its structure
may be regarded as quasicubic. The change of the lattice parameters
when passing from x=0 to x=1 is, however, not monotonic and
exhibits some anomalies. Similar peculiarities, though different in
detail, seem to exist in other rare earth manganese perovskites.
Further, both PrMnO.sub.3 and CaMnO.sub.3 are antiferromagnetically
ordered and insulating at low temperatures (types A.sub.y and G,
respectively). But in the range of 0<x<0.4 a spontaneous
ferromagnetic moment appears that has been ascribed to the strong
double exchange interactions Mn.sup.3+--O.sup.2--Mn.sup.4+, which
results in a material transformation from (Mott) insulator to
metallic conductor.
[0071] FIG. 2 is a drawing illustrating the PrMnO.sub.3 lattice
structure. PrMnO.sub.3 units have Pr.sup.3+ and Mn.sup.3+ ions. The
deviations of Mn--O.sub.I--Mn and Mn--O.sub.II--Mn angles from
180.degree. (which is the angle in ideal cubic perovskites)
manifest the buckling and are the cause of the lattice distortion
(especially the elongation of the b axis) in orthorhombic
perovskites. The increase of the b parameter is markedly enhanced
in PrMnO.sub.3 and other samples with high Mn.sup.3+ content due to
the e.sub.g orbital ordering shown in FIG. 2.
[0072] FIG. 3 is a drawing depicting the Gibbs free energies for
the formation of Pr and Mn oxides. According to simple calculations
of the Gibbs free energy of oxides, both Mn.sup.3+ and Mn.sup.4+
and Pr.sup.3+ and Pr.sup.4+ are stable at room temperature, as
shown. In PrMnO.sub.3 units, the Pr.sup.3+ and Mn.sup.3+ ions are
stable. PrMnO.sub.3 are antiferromagnetically ordered and
insulating at low temperatures. Under certain conditions, such as
those existing with CaMnO.sub.3, or with oxygen vacancies in
PrMnO.sub.3 units, the bond gap of the PrMnO.sub.3 material
decreases. Then, the Mn.sup.3+ ion can lose a free electron and
change to Mn.sup.4+ as shown in equation 1.
Mn.sup.3+.fwdarw.Mn.sup.4++e.sup.- (1)
[0073] Because the Mn.sup.3+ in PrMnO.sub.3, and Mn.sup.4+ in
CaMnO.sub.3 subunits have almost the same octahedral cage formed by
oxygen ions enclosing Mn ions, double exchange interactions
Mn.sup.3+--O.sup.2--Mn.sup.4+ may occur. Oxygen vacancies further
enhance the double exchange interactions
Mn.sup.3+--O.sup.2--Mn.sup.4+. Therefore, doping Ca into
PrMnO.sub.3 to form PCMO, in particular Pr.sub.1-xCa.sub.xMnO.sub.3
(0<x<0.4), results in an electrically conductive material.
Moreover, the CaMnO.sub.3 subunit formed in the cubic perovskite
ABX.sub.3 structure has a lattice constant of 3.73 .ANG.. Mn ions
are sixfold coordinated with oxygen ions along the Cartesian axis,
while Ca ions are twelvefold coordinated with oxygen ions lying
along its (110) directions. Six oxygen ions form an octahedral cage
for the Mn ions, and the cage is linked by oxygen ions. Naturally,
the CaMnO.sub.3 subunits have Ca.sup.2+ and Mn.sup.4+ ions. Because
of the strong electronegativity of oxygen, essentially no electrons
are available to transform an Mn.sup.4+ ion into an Mn.sup.3+
within these subunits. CaMnO.sub.3 has a large band gap and bulk
CaMnO.sub.3 is found to be an insulator. On the other hand, due to
formation of defects such as oxygen vacancies, the band gap of
CaMnO.sub.3 may be reduced, in which case the Mn.sup.4+ ion may
accept an electron to form Mn.sup.3+ in these subunits as shown in
equation 2. Mn.sup.4++e.sup.-.fwdarw.Mn.sup.3+ (2)
[0074] In this way, Mn.sup.3+ ions may form within CaMnO.sub.3
subunits with oxygen vacancies, as shown in equation 3.
CaMnO.sub.3.fwdarw.2[Mn.sup.3+]'.sub.[Mn.sub.4+.sub.]+O.sub.o+V.sup..cndo-
t..cndot..sub.o (3)
[0075] FIGS. 4A and 4B are drawings illustrating the mechanism of
reversible resistive switching properties in a CaMnO.sub.3 unit. If
the valence electrons of the Mn.sup.3+ ions in Mn.sup.4+ positions
get energy, by any means such as thermal or electrical energy, the
electrons can jump to oxygen vacancies or jump back to the
Mn.sup.3+ ions in Mn.sup.4+ positions. On the other hand, when
PrMnO.sub.3 units exist in CaMnO.sub.3 material, the valence
electrons of the Mn.sup.3+ ions in PrMnO.sub.3 units that get
energy (thermal, electrical energy, etc.), can jump to Mn.sup.4+
ions in CaMnO.sub.3 units, or jump back to the Mn.sup.3+ ions in
CaMnO.sub.3 units, which is the double exchange interaction
Mn.sup.3+--O.sup.2--Mn.sup.4+. Oxygen vacancies provide the space
for the double exchange interactions Mn.sup.3+--O.sup.2--Mn.sup.4+.
In this way, Pr.sub.1-xCa.sub.xMnO.sub.3 materials can have both
lower resistance and higher resistance states. In the state where
electrons jump into oxygen vacancies or the Mn.sup.4+ position in
CaMnO.sub.4 units (FIG. 4A), a lower resistance results. In the
state where electrons jump back to Mn.sup.3+ ions, a higher
resistance results, as shown in FIG. 4B.
[0076] For Pr.sub.1-xCa.sub.xMnO.sub.3 (0<x<1) materials, the
lattice constants, orthorhombic distortion D, and the valence
distribution are shown in Table 1, cross-referenced to an
increasing value of x (increasing CaMnO.sub.3 subunits to PCMO).
From Table 1, it follows that when x=0.3 or 0.6, PCMO materials
have a maximum concentration of Mn.sup.3+ ions (8%) in CaMnO.sub.3
subunits and oxygen vacancies (4%).
The Room Temperature Lattice Constants, Orthorhombic Distortion D,
and the Valance Distribution in the Pr.sub.1-xCa.sub.xMnO.sub.3
Materials
[0077] TABLE-US-00001 TABLE 1 x a .ANG.) b .ANG.) c .ANG.) D (%)
Valence distribution 0
Pr.sup.3+.sub.0.994Mn.sup.3+.sub.0.958Mn.sup.4+.sub.0.036O.sup.2-.su-
b.3 (4) 0.1 5.442 5.617 7.635 1.59
Pr.sup.3+.sub.0.89Ca.sup.2+.sub.0.11Mn.sup.3+.sub.0.91Mn.sup.4+.sub.0.09O-
.sup.2-.sub.2.985 (3) 0.2 5.442 5.552 7.657 1.01
Pr.sup.3+.sub.0.80Ca.sup.2+.sub.0.2Mn.sup.3+.sub.0.81Mn.sup.4+.sub.0.19O.-
sup.2-.sub.2.995 (3) 0.3 5.426 5.478 7.679 0.41
Pr.sup.3+.sub.0.70Ca.sup.2+.sub.0.30Mn.sup.3+.sub.0.78Mn.sup.4+.sub.0.22O-
.sup.2-.sub.2.96 (3) 0.4 5.415 5.438 7.664 0.17
Pr.sup.3+.sub.0.60Ca.sup.2+.sub.0.40Mn.sup.3+.sub.0.64Mn.sup.4+.sub.0.36O-
.sup.2-.sub.2.98 (3) 0.5 5.395 5.403 7.612 0.14
Pr.sup.3+.sub.0.50Ca.sup.2+.sub.0.50Mn.sup.3+.sub.0.55Mn.sup.4+.sub.0.45O-
.sup.2-.sub.2.975 (3) 0.6 5.375 5.375 7.60 --
Pr.sup.3+.sub.0.40Ca.sup.2+.sub.0.60Mn.sup.3+.sub.0.48Mn.sup.4+.sub.0.52O-
.sup.2-.sub.2.96 (3) 0.7 5.349 5.349 7.543 --
Pr.sup.3+.sub.0.30Ca.sup.2+.sub.0.70Mn.sup.3+.sub.0.33Mn.sup.4+.sub.0.67O-
.sup.2-.sub.2.985 (3) 0.8 5.318 5.318 7.521 --
Pr.sup.3+.sub.0.20Ca.sup.2+.sub.0.80Mn.sup.3+.sub.0.25Mn.sup.4+.sub.0.75O-
.sup.2-.sub.2.97 (3) 0.9 5.300 5.300 7.495 --
Pr.sup.3+.sub.0.10Ca.sup.2+.sub.0.90Mn.sup.3+.sub.0.23Mn.sup.4+.sub.0.77O-
.sup.2-.sub.2.93 (3) 1
Ca.sup.2+Mn.sup.3+.sub.0.16Mn.sup.4+.sub.0.84O.sup.2-.sub.2.92
(4)
[0078] FIG. 5 is a drawing depicting the relationship between
composition and lattice structures of Pr.sub.1-xCa.sub.xMnO.sub.3.
Due to the absence of double exchange interactions:
Mn.sup.3+--O.sup.2--Mn.sup.4+, for x=0.6, a composition of
Pr.sub.1-xCa.sub.xMnO.sub.3 with x=0.3 may be the best candidate
material for an RRAM device in some applications. From the figure,
in the composition range 0<x<0.4, Pr.sub.1-xCa.sub.xMnO.sub.3
materials exhibit a larger lattice distortion. In contrast, for
x>0.4, a smaller lattice distortion can be found.
[0079] Next, the mechanism of reversible resistive switching
properties in a PCMO material is described. Based on above
analysis, the electrically alterable resistive properties of PCMO
materials may come from electron traps and double exchange
interactions: Mn.sup.3+--O.sup.2--Mn.sup.4+.
The Case of Pr.sub.0.7Ca.sub.0.3MnO.sub.3
[0080] FIG. 6 is a drawing depicting the band gap as a function of
x value and oxygen vacancies. According to experimental data,
Pr.sub.0.7Ca.sub.0.3MnO.sub.3 has a maximum concentration of
Mn.sup.3+ ions (8%) in CaMnO.sub.3 subunits and oxygen vacancies
(4%), as summarized in Table 1. Moreover, the composition of
Pr.sub.0.7Ca.sub.0.3MnO.sub.3 is located at the boundary between
larger and smaller orthorhombic distortion, D, as x is varied in
Pr.sub.1-xCa.sub.xMnO.sub.3. In PCMO materials, in the composition
range 0<x<0.4, the materials are known to have lower
resistance. Conversely, for x>0.4, PCMO materials have higher
resistance. Similarly, Pr.sub.0.7Ca.sub.0.3MnO.sub.3 is also
located at the boundary between higher and lower resistance
materials. From the characteristics of
Pr.sub.0.7Ca.sub.0.3MnO.sub.3 materials, it can be concluded that
for x>0.4, the band gap of PCMO materials is so large that it is
difficult to electrically alter the material resistance.
Conversely, in the composition range 0<x<0.2, PCMO materials
are leaky or conductive.
[0081] In the composition range 0.2<x<0.4, the resistance of
the material becomes metastable, which allows electrical alteration
of the resistance. Also, a pulsed electric field can cause
electrons to jump into oxygen vacancies, or to transfer from
Mn.sup.3+ to Mn.sup.4+. In this case, the resistance of PCMO
increases. Conversely, if in a pulsed electrical field, electrons
transfer back from oxygen vacancies to Mn.sup.3+, or from Mn.sup.4+
back into Mn.sup.3+, then the resistance of PCMO decreases.
Therefore, electrical alteration of the resistance in PCMO may be
caused by electron trapping in oxygen vacancies and double exchange
interactions: Mn.sup.3+--O.sup.2--Mn.sup.4+. In addition, as
temperature is increased, thermally excited electrons may transfer
into oxygen vacancies or make double exchange interactions
Mn.sup.3+--O.sup.2--Mn.sup.4+. Thus, the resistance of PCMO
materials decreases as a function of increasing temperature, as is
observed.
[0082] FIG. 7 is a diagram depicting the distribution of
PrMnO.sub.3 and CaMnO.sub.3 subunits, as a function of x, in
Pr.sub.1-xCa.sub.xMnO.sub.3 materials. In PCMO for any value of x,
there are ordered distributions of PrMnO.sub.3 and CaMnO.sub.3
subunits. PrMnO.sub.3 and CaMnO.sub.3 subunits are separately grown
layer-by-layer depending on the ratio of PrMnO.sub.3 to
CaMnO.sub.3. For x=1/2, PrMnO.sub.3 subunits alternate with
CaMnO.sub.3 subunits. For Pr.sub.0.7Ca.sub.0.3MnO.sub.3 materials,
which is close to x=1/3, typically two PrMnO.sub.3 subunit layers
are separated by one CaMnO.sub.3 subunit layer. Likewise, for
x=1/4, three PrMnO.sub.3 subunit layers are separated by one
CaMnO.sub.3 subunit layer. Because CaMnO.sub.3 subunits have
Mn.sup.3+ ions and oxygen vacancies, a double exchange
interactions: Mn.sup.3+--O.sup.2--Mn.sup.4+ may occur between of
PrMnO.sub.3 and CaMnO.sub.3 subunits. Pr.sub.0.7Ca.sub.0.3MnO.sub.3
may be changed by an applied electric field from an insulator to a
conductor, or vice versa. Therefore, these materials exhibit a
non-volatile, electrically alterable resistance.
Oxygen Vacancies
[0083] FIGS. 8A and 8B are drawings showing the direction from
Mn.sup.3+ ions in PrMnO.sub.3, to oxygen vacancies, oriented toward
a top electrode. In FIG. 8A, a positive voltage is applied to the
top electrode. In FIG. 8B, a negative voltage is applied to the top
electrode. From above analysis, the ordering and orientation of
PrMnO.sub.3 and CaMnO.sub.3 units and distribution of oxygen
vacancies are very important for the electrically alterable
resistive properties of PCMO materials. If the distribution of
PrMnO.sub.3 and CaMnO.sub.3 subunits and oxygen vacancies are
random, then material does not show a strong electrically alterable
resistance. According to equation 3, CaMnO.sub.3 subunits may have
Mn.sup.3+ ions and oxygen vacancies under certain conditions. Due
to the distortion of PrMnO.sub.3 subunits, oxygen vacancies may be
formed on the boundaries between CaMnO.sub.3 and PrMnO.sub.3
units.
[0084] FIGS. 9A and 9B are drawings depicting the direction of
electron flow when opposite polarity pulses are applied to the top
electrode. The free valence electrons of the Mn.sup.3+ ions
transfer to oxygen vacancies if a positive pulsed voltage is
applied to the top electrode (FIG. 9A). In this case, the material
shows conductor properties. On the contrary, free valence electrons
in oxygen vacancies transfer back to the Mn.sup.3+ ions if a
negative pulsed voltage is applied to the top electrode at and the
material shows insulating properties (FIG. 9B). If the directions
from the Mn.sup.3+ ions to oxygen vacancies are oriented toward the
bottom electrode, then the behavior relative to the potential of an
applied pulse is reversed. In this way, electrically alterable
resistive properties of PCMO materials may be obtained.
Double Exchange Interactions Mn.sup.3+--O.sup.2--Mn.sup.4+
[0085] FIGS. 10A and 10B are drawings showing the directionality of
electron jumps between Mn.sup.3+ and Mn.sup.4+ ions. Alternatively,
the formation of Mn.sup.3+ ions and oxygen vacancies in CaMnO.sub.3
units enhances the double exchange interactions
Mn.sup.3+--O.sup.2--Mn.sup.4+ between CaMnO.sub.3 and PrMnO.sub.3
unit layers. The free valence electrons of the Mn.sup.3+ ions in
PrMnO.sub.3 subunit layers transfer to CaMnO.sub.3 subunit layers
if a positive pulsed voltage is applied to the top electrode, and
the material shows conductor properties. On the contrary, the free
valence electrons in CaMnO.sub.3 subunit layers transfer back to
PrMnO.sub.3 subunit layers if a negative pulsed voltage is applied
to the top electrode, and the material shows insulating
properties.
[0086] FIGS. 11A and 11B show electron flow when the PrMnO.sub.3
and CaMnO.sub.3 subunit layers are reversed with respect to the
bottom electrode. Then, the material shows a behavior opposite of
the behavior shown in FIGS. 10A and 10B.
The Relationship Between PCMO Composition and Electrically
Alterable Resistance
[0087] Based on the above analysis, the composition of the
Pr.sub.1-xCa.sub.xMnO.sub.3 thin films significant affect
electrically alterable resistance properties. Experimental results
show that PCMO materials with 0.1<x<0.6 exhibit electrically
alterable resistance, and, in particular, PCMO materials with
x=0.3, or close to this value, show the largest effect.
Crystallographic Orientation
[0088] The orientations of PCMO thin films show a significant
affect upon electrically alterable resistance properties. Perfectly
oriented and crystallized PCMO films, or epitaxial (single-crystal)
PCMO with a smaller concentration of Mn.sup.3+ ions, have a higher
resistance, and the largest change in resistance responsive to the
application of an electric field. Due to higher activation energy
(or barrier energy) for the transfer of electrons between Mn.sup.3+
and Mn.sup.4+, a higher operation voltage is required and the
non-volatile (retention, endurance, etc.) properties are good. Of
course, a reduction of the film thickness can reduce operation the
voltage, which allows the resistance of the alterable resistor to
be set at an optimum value. For perfectly oriented and crystallized
PCMO films or epitaxial PCMO thin films, the main electrically
alterable resistance properties of PCMO materials come from double
exchange interactions Mn.sup.3+--O.sup.2--Mn.sup.4+. Oxygen
vacancies enhance the double exchange interactions
Mn.sup.3+--O.sup.2--Mn.sup.4+. This is the so-called bipolar case,
for which resistance is raised and lowered reversibly by the
application of short duration pulses of opposite polarity.
[0089] The orientation plays an important role in the bipolar
switching properties. Table 2 shows the density of Mn atoms and the
nearest distance between two Mn atoms, along with orientations.
According to calculations, the PCMO thin films with (110) and (001)
orientations have the best bipolar switching properties.
The Densities of Mn Atoms and Distance Between Two Mn Atoms of PCMO
Materials
[0090] TABLE-US-00002 TABLE 2 Distance between two Mn Densities of
Mn atoms atoms Orientation (number per 100 .ANG..sup.2) (.ANG.)
(100) 4.75 5.426 (010) 4.80 5.478 (001) 6.73 3.840 (110) 6.76 3.855
(011) 3.91 6.690 (101) 3.36 6.647
[0091] FIGS. 12A, 12B, 12C depict the (110), (011), and (101)
orientations, respectively, of PCMO thin films. FIG. 12A shows the
(110) orientation (marked ABCD) of a PCMO thin film. From the
figure, the highest density of 6.76 per 100 .ANG..sup.2 area of Mn
atoms and shorter distance (about 3.855 .ANG.) between two Mn atoms
lies along the (110) direction. Therefore, the strongest
electrically alterable resistance can be obtained. Due to the lower
density of Mn atoms and longer distance along (011) or (101),
electrically alterable resistive properties will be weaker for
films oriented in these directions, as shown in FIG. 12B and 12C,
respectively.
[0092] FIG. 13 shows an x-ray pattern of PCMO with partial (110)
orientation. The sample shows good bipolar switching
properties.
[0093] FIGS. 14A and 14B depict the (100) and (010) orientations of
PCMO thin films, respectively. FIG. 14A shows the (100) orientation
(marked ABCD) of PCMO thin films. Compared with (110) orientation,
the density of 4.75 per 100 .ANG..sup.2 area of Mn atoms along the
(100) direction is lower than in the (110) orientation, and the
distance (about 5.426 .ANG.) between two Mn atoms of (100)
orientation is longer than that of (110) orientation. Therefore,
the switching properties of (110) orientation are better than (100)
orientation. In addition, the (010) orientation is similar to (100)
orientation, as shown in FIG. 14B.
[0094] FIG. 15 shows the (001) orientation (marked ABCD) of PCMO
thin films. Compared with (110) orientation, the density of 6.73
per 100 .ANG..sup.2 area of Mn atoms along with the (001) direction
is similar to (110) orientation, and the distance about 3.84 .ANG.
between two Mn atoms is also similar to (110) orientation.
Therefore, the switching properties of (110) orientation are
similar to the (001) orientation.
[0095] FIG. 16 shows the x-ray pattern of PCMO thin films with
(001) orientation.
[0096] FIG. 17 depicts the EDX pattern of the PCMO thin film of
FIG. 16.
[0097] FIG. 18 depicts the switching properties of c-axis oriented
PCMO thin films. The sharp (002) peak (FIG. 16) confirms the
formation of C-oriented PCMO thin film. The energy dispersion x-ray
analysis (EDX) measurement shows that the composition of the PCMO
thin film is close to Pr.sub.0.7Ca.sub.0.3MnO.sub.3, as shown in
FIG. 17. The PCMO thin film shows very good bipolar switching
properties, as shown in FIG. 18, which is consistent with the above
calculations.
[0098] The use of oriented polycrystalline PCMO films may result in
a reduction in the operation voltage. But the resistance and ratio
of higher resistive state to the lower resistive state is also
decreased, and retention and endurance may be degraded.
[0099] By increasing Mn.sup.3+ ions, defects such as oxygen vacancy
occur, and the barrier energy for transformation of electrons
between Mn.sup.3+ and Mn.sup.4+ decreases. Therefore, resistance
and operation voltage may decrease, but retention properties may be
degraded. If the ratio of Mn.sup.3+/Mn.sup.4+ ions is too high, the
CMR materials are conductive.
[0100] The amount, distribution, and orientation of oxygen
vacancies and defects have an effect upon switching properties. The
distribution and orientation of groups of PrMnO.sub.3 and
CaMnO.sub.3 have an affect upon the switching properties. There are
optimum ratios of about 1-4 of Mn.sup.3+/Mn.sup.4+ and Pr/Ca for
RRAM applications.
Crystallization and Switching Properties
[0101] When perfectly crystallized PCMO materials change into
polycrystalline or amorphous PCMO materials, the oxygen vacancies
and defects increase. The main reversible resistive switching
mechanism of PCMO materials includes electron exchange or electron
trapping between Mn.sup.3+--O.sup.2--Mn.sup.4+, and oxygen
vacancies or defects, resulting in unipolar switching.
[0102] FIG. 19 shows the x-ray pattern of a PCMO thin film with
nanosized polycrystalline materials. Based on calculations, the
PCMO gain size is about 40 nm.
[0103] FIG. 20 depicts the unipolar switching properties of the
PCMO thin film of FIG. 19. The write, high resistance state is
achieved by applying 5V for 100 ns. The erase, lower resistance
state results from applying 3V for 10 .mu.s.
[0104] FIG. 22 is a flowchart illustrating the present invention
method for forming a PrCaMnO (PCMO) thin film with predetermined
memory-resistance characteristics. Although the method is depicted
as a sequence of numbered steps for clarity, no order should be
inferred from the numbering unless explicitly stated. It should be
understood that some of these steps may be skipped, performed in
parallel, or performed without the requirement of maintaining a
strict order of sequence. The method starts at Step 200.
[0105] Step 202 forms a Pr.sup.3+.sub.1-xCa.sup.2+.sub.xMnO thin
film composition, where 0.1<x<0.6. Step 204, in response to
the selection of x, varies the ratio of Mn and O ions as follows:
[0106] O.sup.2- (3.+-.20%); [0107] Mn.sup.3+ ((1-x).+-.20%); and,
[0108] Mn.sup.4+ (x.+-.20%).
[0109] If Step 202 forms a PCMO thin film with a
Pr.sup.3+.sub.0.7Ca.sup.2+.sub.0.3Mn.sup.3+.sub.0.78Mn.sup.4+.sub.0.22O.s-
up.2-.sub.2.96 composition, then Step 204 varies the ratio of ions
as follows: [0110] O.sup.2- (2.96); [0111] Mn.sup.3+ ((1-x)+8%);
and, [0112] Mn.sup.4+ (x-8%).
[0113] In one aspect, a further step, Step 206 forms a 13.3% oxygen
vacancy in the PCMO film.
[0114] FIG. 23 is a flowchart illustrating a second aspect of the
present invention method for forming a
Pr.sup.3+.sub.1-xCa.sup.2+.sub.xMn.sup.3+.sub.(1-x).+-.20%Mn.sup.4+.sub.x-
.+-.20%O.sup.2-.sub.3.+-.20% (PCMO) thin film with predetermined
memory-resistance characteristics. The method starts at Step 300.
Step 302 forms a PCMO thin film with a crystallographic
orientation. Step 304 creates a density in the PCMO film,
responsive to the orientation.
[0115] In one aspect, Step 302 forms a PCMO film in a (110)
orientation. Then, Step 304 creates a density in the range of 5 to
6.76 Mn atoms per 100 .ANG..sup.2 in a plane perpendicular to the
(110) orientation. In one aspect a further step, Step 306, creates
a distance of 3.855 .ANG. between the two closest Mn atoms in
response to the (110) orientation.
[0116] If Step 302 forms a PCMO film in a (001) orientation, then
Step 304 creates a density in the range of 5 to 6.73 Mn atoms per
100 .ANG..sup.2 in a plane perpendicular to the (001) orientation,
and Step 306 creates a distance of 3.840 .ANG. between the two
closest Mn atoms in response to the (001) orientation.
[0117] FIG. 24 is a flowchart the present invention method for
forming a
Pr.sup.3+.sub.1-xCa.sup.2+.sub.xMn.sup.3+.sub.(1-x).+-.20%Mn.sup.4+.sub.x-
.+-.20%O.sup.2-.sub.3.+-.20% (PCMO) thin film memory-resistance
device. The method starts at Step 400. Step 402 forms a bottom
electrode from a material such as Pt, Ir, Al, or TiN. Step 404
forms a PCMO thin film with a crystallographic orientation
overlying the bottom electrode. Step 406 creates a lattice mismatch
in the PCMO film, responsive to the orientation, between the PCMO
thin film and the bottom electrode. In some aspects, Step 403 forms
a buffer layer interposed between the PCMO thin film and the bottom
electrode formed from a material such as InO2 or ZnO.
[0118] If Step 402 forms a PCMO film in a (110) orientation, then
Step 404 creates a lattice mismatch of less than 30% between the
PCMO thin film and the bottom electrode. If Step 402 forms a PCMO
film in a (001) orientation, Step 404 also creates a lattice
mismatch of less than 30% between the PCMO thin film and the bottom
electrode.
[0119] FIG. 25 is a flowchart illustrating a fourth aspect of the
present invention method for forming a
Pr.sup.3+.sub.1-xCa.sup.2+.sub.xMn.sup.3+.sub.(1-x).+-.20%Mn.sup.4+.sub.x-
.+-.20%O.sup.2-.sub.3.+-.20% (PCMO) thin film with predetermined
memory-resistance characteristics. The method starts at Step 500.
Step 502 forms a PCMO thin film with a crystallographic
orientation. Step 504 creates a PCMO film with a selectable
resistance state, responsive to the orientation.
[0120] If Step 502 forms a PCMO film in a (001) orientation, then
Step 504 creates a PCMO film with a selectable resistance state,
responsive to the orientation, by: [0121] writing a high resistance
in the range of 10 to 1000 kilo ohms, using .+-.(2 to 10) V pulse,
with a duration in the range of 5 ns to 50 microseconds; or, [0122]
resetting a low resistance in the range of 500 ohms to 10 kilo
ohms, using .+-.(2 to 10) V pulse, with a duration in the range of
5 ns to 50 microseconds.
[0123] If Step 502 forms a PCMO film in a (110) orientation, then
Step 504 creates a PCMO film with a selectable resistance state,
responsive to the orientation, by: [0124] writing a high resistance
in the range of 10 to 1000 kilo ohms, using .+-.(2 to 10) V pulse,
with a duration in the range of 5 ns to 50 microseconds; or, [0125]
resetting a low resistance in the range of 500 ohms to 10 kilo
ohms, using .+-.(2 to 10) V pulse, with a duration in the range of
5 ns to 50 microseconds.
[0126] FIG. 26 is a flowchart illustrating a fifth aspect of the
present invention method for forming a
Pr.sup.3+.sub.1-xCa.sup.2+.sub.xMn.sup.3+.sub.(1-x).+-.20%Mn.sup.4+.sub.x-
.+-.20%O.sup.2-.sub.3.+-.20% (PCMO) thin film with predetermined
memory-resistance characteristics. The method starts at Step 600.
Step 602 forms a PCMO thin film with a crystal grain size. Step 604
creates a PCMO film with a selectable resistance state, responsive
to the grain size.
[0127] If Step 602 forms a PCMO film with a crystal grain size in
the range of 3 to 40 nm, then Step 604 creates a PCMO film with a
selectable resistance state, responsive to the orientation, by:
[0128] writing a high resistance of greater than 225 kilo ohms,
using 5 V pulse of less than 100 ns; or, [0129] resetting a low
resistance of less than 10 kilo ohms, using a 3 V pulse of less
than 10 microseconds.
[0130] If Step 602 forms a PCMO film with a crystal grain size in
the range of 40 nm to epitaxial, then Step 604 creates a PCMO film
with a selectable resistance state, responsive to the orientation,
by: [0131] writing a high resistance of greater than 300 kilo ohms,
using -5 V pulse of less than 50 microseconds; or, [0132] resetting
a low resistance of less than 10 kilo ohms, using a +5 V pulse of
less than 50 microseconds.
[0133] FIG. 27 is a flowchart illustrating a sixth aspect of the
present invention method for forming a
Pr.sup.3+.sub.1-xCa.sup.2+.sub.xMn.sup.3+.sub.(1-x).+-.20%Mn.sup.4+.sub.x-
.+-.20%O.sup.2-.sub.3.+-.20% (PCMO) thin film with predetermined
memory-resistance characteristics. The method starts at Step 700.
Step 702 forms a PCMO thin film with a composition. Step 704
creates a super lattice structure with an ordered distribution in
the PCMO film, responsive to the composition.
[0134] If Step 702 forms a PCMO thin film with a
Pr.sup.3+.sub.0.50Ca.sup.2+.sub.0.50MnO.sub.0.3 composition, then
Step 704 creates a distribution of:
[0135] Z(Mn.sup.3+)PrMnO.sub.3:Z(Mn.sup.4+)CaMnO.sub.3 subunits,
where Z is a natural number.
[0136] If Step 702 forms a PCMO thin film with a
Pr.sup.3+.sub.0.67Ca.sup.2+.sub.0.33MnO.sub.0.3 composition, Step
704 creates a distribution of:
[0137] 2Z(Mn.sup.3+)PrMnO.sub.3:Z(Mn.sup.4+)CaMnO.sub.3 subunits,
where Z is a natural number.
[0138] If Step 702 forms a PCMO thin film with a
Pr.sup.3+.sub.0.75Ca.sup.2+.sub.0.25MnO.sub.0.3 composition, Step
704 creates a distribution of:
[0139] 3Z(Mn.sup.3+)PrMnO.sub.3:Z(Mn.sup.4+)CaMnO.sub.3 subunits,
where Z is a natural number.
[0140] PCMO thin films and associated formation processes have been
presented. Some examples of resistance-memory characteristics, and
related features, have been presented cross-referenced to PCMO film
attributes to clarify the invention. However, the invention is not
limited to merely these examples. Other variations and embodiments
of the invention will occur to those skilled in the art.
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