CHAPTER 4 EXPERIMENT RESULTS AND DISCUSSION
4.3.2 Thermal Stress on FePt MND Capacitor
Fig.4.18(a) shows capacitance density of the FePt MND capacitor at 1MHz from 25℃ to 125℃. After thermal stress, the capacitance density increases with temperature at 1MHz and more obviously at 1KHz (Fig.4.18(b)). The major reason is considered to be the interface defect density existed between FePt nanodot and oxide matrix medium increasing during the thermal stress process. The leakage current density also affected on thermal stress in depletion mode, it can reveal that FePt nanodot structure is relatively stable under thermal stress in accumulation mode.
Fig.4.19 shows the effect on current density with thermal stress. In accumulation mode, current density increases slightly with temperature before breakdown voltage.
In depletion mode, current density increases nearly one order in magnitude beyond +13V on the rise per of temperature per 25℃. It is worth noting that current density did not rise when temperature exceed 100℃. As mentioned previously, carrier conduction is major limited by the generation of minority carriers via the interface states and bulk traps, and the generation and recombination rates of minority carriers are proportional increasing with temperature. [40]
Figure 4.1 XRD pattern of FePt thin film with different annealing temperature for 30min (a)400℃ (b)500℃ (c)600℃
Figure 4.2 Magnetic hysteresis loop of FePt thin film parallel magnetic field with different element ratio
Figure 4.3 Magnetic hysteresis loop of FePt thin film fabricate on different underlayer
Table 4-1 Different element concentration ratio of Fe:Pt measured by ICP-MS
Estimative ratio by thin film thinkness (Å)
Real ratio (ppm)
Pt:Fe Pt Fe
25:20 0.7114(26%) 2.0246 (47%)
25:25 0.657(39%) 1.040 (61%)
51:40 2.764(53%) 2.432(47%)
51:40 0.577 (48%) 0.617(52%)
Figure 4.4 Magnetic hysteresis loop of MND MIS capacitor after magnetic annealing 700℃ 60 minutes
Table 4-2 Magnetization value of FePt MND Magnetic hysteresis Physics quantity
(unit)
Parallel magnetic field
Vertical magnetic field
Ms(emu) 6.433×10-4(0.26T) 7.086×10-4 (0.29T) Mr(emu) 2.457×10-4 (0.1T) 7.854×10-5(0.03T)
Hs(Oe) 12724.86 13846.89 Hn(Oe) 10271.99 13414.54 Hc(Oe) 968 716.46
Figure 4.5 J-V ofmagnetic FePt nanodot MIS capacitor with different Al top electrode area
Table 4-3 Cnduction mechanisms[30]
Conduction mechanism
Characteristic Temperature dependence
Frenkel-Poole emission
( )
⎟⎟
(a)
(b)
Figure 4.6 Ohmic Conduction Fitting (a)accumulation mode (b)depletion mode
(a)
Schottky emission
1/kT
28 30 32 34 36 38 40
-15 -14 -13 -12 -11 -10 -9 -8 -7
25.0V 30.3V 36.0V 42.3V 49.0V
ln (J/T
2) (A /c m
2-T
2)
(b)
Figure 4.7 Schottky emission fitting
(a)J-V (b) the Schottky relationship of ln(J/T2) verse 1/kT
Figure 4.8 Extract slop from Fig.4.7(b)
(a)
(b)
Figure 4.9 Frenkel-Poole emission fitting (a)J-V (b) Frenkel-Poole thermal relationship
Figure 4.10 Extract slop from Fig.4.9(b)
Figure 4.11, J-V characteristic of MIS capacitor compares with FePt MND and non-MND
(a)
(b)
Figure 4.12 SCLC fitting (a)accumulation mode (b)depletion mode
(a)
(b)
Figure 4.13Tunneling fitting (a)accumulation mode (b)depletion mode
(a)
(b)
Figure 4.14 Leakage current mechanism fitting (a) accumulation mode (b) depletion mode
Figure 4.15 Current density with 20V constant voltage stress during 10000 second
(a)
(b)
Figure 4.16 J-V with constant voltage stress (a)accumulation mode (b)depletion mode
Figure 4.17 C-V with constant voltage stress 20V during 10000s
(a)
(b)
Figure 4.18 FePt MND capacitor C-V characteristic with thermal stress at (a)1MHz (b)1kHZ
Figure 4.19 FePt MND capatior J-V characteristic with thermal stress
Chapter 5
Conclusion and Future Work
5.1 Conclusion
From quantum mechanics, magnetic field has a confinement effect that restricts the charge distribution and improves the insulating behavior. We take COMSOL as simulation tool to present this effect and fabricate FePt magnetic nanodots capacitors to proof this phenomenon.
The capacitor with magntic FePt nanodots has high breakdown electric strength;
its leakage mechanism is different to general non-metal-dot capacitors. In accumulation mode, tunneling and Frenkel-Poole emission is the main mechanism because FePt nanodot is a possible trap provider. And magnetic field from FePt nanodots may make carrier tunnel though dielectric material difficultly that in result of high breakdown strength.
FePt nanodots existing in dielectric layer also enhance the static dielectric constant of silicon oxide. The electric polarizability is greatly increased because of the interface of FePt metal dots and silicon oxide and its magnetic properties. And under constant voltage stress +20V, the FePt MND capacitor has good characteristic in electric endurance.
5.2 Future Work
In future work, we can research the relationship of MND capacitor with different magnetic field strength and then progress to discuss the magnetic nanodots quantum confinement effect in different structure. Verify the role of parallel or vertical magnetic field in capacitor. Furthermore, this structural possibility for future flash memory [41], we can replace different material to study on its cell characteristics.
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