CHAPTER 4 EXPERIMENT RESULTS AND DISCUSSION
4.2 P ROPERTIES OF MAGNETIC NANODOT MIS LEAKAGE CHARACTERISTICS
To recognize the effect of magnetic nanodot in capacitor, we measure the characteristic of current density and voltage. Fig.4.5 shows the current density verse voltage of magnetic FePt nanodot MIS capacitor with different Al top electrode area.
Asymmetry curve reveals different leakage mechanism in two current directions.
When positive bias force on 3×104μm2 Al top electrode, the leakage curve is flat to 9.5×10-5 A/cm under ±10V. This is the value that electrons overcome the 10nm block oxide barrier on Si. Beyond +10V, leakage current density increases quickly to 10 A/cm and is not breakdown even under high voltage strength 100V. In this region, MIS capacitor with FePt MND (magnetic nanodot) is with high leakage current which is unusual in MIS capacitor without MND. On the other way, reverse bias force on top electrode, the leakage current also has the same behavior in first -10V. If negative bias is larger than this value, the leakage current mechanism is different. To recognize the leakage current mechanism of the magnetic nandot capacitor, we take five different mechanisms into consideration (see Table 4-3) [30].
4.2.1 Ohmic Conduction Fitting
For the leakage current at very low field (~±10V in MND FePt capacitor), the leakage current in accumulation mode increases linearly with the increase of voltage bias as shown in Fig.4.6(a). The red line is presents a fitting curve of Ohmic
conduction mechanism, which describes the situation of low voltage and high temperature, current is carried by thermally excited electrons hopping from one isolated state to the next. Ohmic conduction takes place when the injected carrier density is far less than the thermally-generated carrier density, which thermally–generated carriers are dominant in conduction. The relation can be expression in (4-1)
⎟⎠
Where a, c is constant, V is applied voltage and T is the absolute temperature. We can also observe from Fig.4.6(b), a plot of J verse V yields a straight line within +10V (depletion mode). When applied voltages are between 10 to -10V, the carriers transport mechanism is Ohmic conduction.
4.2.2 Schottky Emission Fitting
Fig.4.7(a) shows the Schottky emission fitting for FePt nanodots MIS capacitor in accumulation mode. The linear relationship of ln(J/T2) versus V1/2 curve was obtained in the range of -25V to -40V, which gives the slope of 2.504 eV(m/V)1/2 with a goodness of fit functions of 0.9965. The fitting curve in depletion mode is in the range of 13V~20V. Schottky emission mechanism is which leakage current contributed by the carriers that overcome the barrier height between electrodes and dielectric layers, it is also called thermionic emission because the key point is hot carrier emission jump across barrier height. The formula is below:
⎥⎥
Where A* is the Richardson constant, k is the Boltzmann’s constant, T is the absolute temperature (K), E is the applied external electric field , e is electron charge, ε is the permittivity in vacuum, and φBis corresponding to the barrier height between metal/dielectric. Fig.4.7(b) is the thermal relationship of ln(J/T2) and 1/kT, however, extract the slop verse sqrt voltage in Fig.4.8, nonlinear relation between -25 to -49V demonstrates that the leakage current is independent of temperature. Indirectly prove that Schottky emission conduction is not the main mechanism in high field strength.
4.2.3 Frenkel-Poole Emission Fitting
Fig.4.9(a) is Frenkel-Poole emission fitting on reverse bias applied on top Al electrode. Frenkel-Poole emission describes the process for carriers to overcome the barriers resulted from the defect states in dielectric layer, and the barrier lowering is lower in Schottky emission and tunneling. Because its conduction process depends on carrier trapped and detrapped behavior, electric field plays a more important factor than temperature in this mechanism which is verse in Schottky emission. The relationship can be written as following:
( )
Where B is material constant, φt is trap barrier height, and other parameters are the same as those in Schottky emission. The goodness of fitting curve is reach 0.99 in the range of -12V~-20V, and Fig.4.8 shows the thermal relationship of Frenkel-Poole emission, from the slop of ln(J/V) and 1/kT (Fig.4.9(b)), we can figure out the barrier height is 0.049eV from Fig.4.10. It can explain the main mechanism that electrons transport in high electric field is in trap state, which is suspected of not
onlyin oxide but also produced by embedded FePt nanodot. The magnetic of nanodot may provide another force to induce electron conduct through adjacent traps in quantum mechanics effect. That is the reason that capacitor with FePt nanodot has high breakdown voltage nearly 49.2V when reverse bias force on top electrode, which is extremely large than general in dielectric thickness ~34nm (see TEM section monograph in Fig 3.3), it also reveals that electric field strength in dielectric layer is large to 14MV/cm,
Compare with non-MND capacitor in Fig.4.11, it shows the J-V characteristic of MIS capacitor compares with FePt MND and non-MND in 38nm oxide thickness.
Obviously, with the same oxide thickness, magnetic nanodot FePt enhances the electric field intensity in oxide and high voltage breakdown.
4.2.4 Space Charge Limited Current Fitting
Another leakage mechanism is space charge limited current (SCLC), which is attributed to defects in dielectric under high electric field. After charge injection from an electrode, the space charges are formed by trapped carrier. The major SCLC divided into two parts in following:
1
J is current density ,θ is the ratio of free electron to trapped electron ,εis insulator permittivity, d is the film thickness. The current for the unipolar trap-free
case is proportional to the square of the applied voltage. The fitting result in Fig.4.12, the slop in accordance with SCLC mechanism must be 2, but our fitting slop is in proportional line with slope 0.94 from -0.3 to -11V, is less similar fitting in comparison to Ohmic conduction. But in the depletion mode, the bias large than +13V has large fitting goodness 0.991 and slop 1.75 for slop lnJ verse lnV. In high electric field condition, the gate leakage current in the deep depletion region is mainly limited by the generation of minority carriers via the interface states and bulk traps in the depletion region [31], which results in a lower saturation current level under positive bias. In the standard capacitor without FePt NMD, its leakage current saturated at 10-6A over 40V.
4.2.5 Tunneling Fitting
Electron tunneling through thin dielectric layer is another conventional way to conduction electron, Fig.4.13 is the tunneling fitting curve, which is evidently to observe the relationship of following formula between -13V to before breakdown:
( )
⎟ barrier height, and V is the applied voltage. We can calculateψB is 1.07eV in accumulation mode. Obviously, the leakage current of FePt NMD capacitor is not only direct tunneling under high electric field but also transport carriers by jumping or trapping in FePt NMD layer.Above of all, the leakage mechanism of MND is apparent difference in positive and reverse bias. In reverse bias (accumulation mode), low electric field
(<10V), the carrier transition is Ohmic conduction because FePt nanodot embedded in oxide which made dielectric metalize in carrier conduction properties. Beyond -10V, its current density rapidly rise than before and keep high current density in high field strength until to 49V breakdown. This phenomenon is resulting from FePt MND induced parallel magnetic which attract carrier and restrict their conduction path by quantum confinement localization [32]. The mechanism makes leakage current behaviors obey to like Frenkel-Poole emission and tunneling compound features in high field strength in accumulation mode. FePt nanodots in capacitors are the quantum well location which potential lower than oxide. Additionally, the self-induced parallel magnetic field suppresses the tunneling between adjacent wells in FePt nanodot/SiO2 layer by layer structure, which results in the electric field strength must be larger than standard situation in order to prompt carriers tunneling.
These reasons make carrier conduction is like metallic Ohmic conduction mode in low field, trapped or detrapped mode in high electric field. In positive bias applied on top electrode (depletion mode), the leakage current mechanism is also like Ohmic conduction in 0V~+10V, +10~14V is dominated by Frenkel-Poole emission and higher 14V, it is space charge limited current and restricted by minority carrier concentration from depletion region, keep static leakage level until to +100V. Fig.
4.14 is the total fitting result of FePt MND capacitor in accumulation mode and in depletion mode, respectively.