Current-Voltage Measurements

The most important part of all is current-voltage measurement. It could understand the electrical properties of the device from current-voltage curve. The electrical measurement system consisted of a probe station, an Agilent 4155C semiconductor parameter analyzer, an Agilent E5250A low leakage switch which are controlled by personal computer with the Agilent VEE software, and GPIB controller.

Our electrical measurements were sorted into five items, static conductivity switching measurement, retention test, stress test, endurance test, and other electrical phenomenon measurement. The aforementioned four items were tested for criteria of our memory device and the last item was executed to understand the fundamental mechanism of our samples.

a. Static Resistive Switching Measurement

The measurement was performed by Agilent 4155C which applied a dc voltage sweeping between two specified voltages to observe the resistive switching of the sample. The measured results could observe the relation of the switching voltage and the H-state or L-state current. Use Agilent 4155C to execute the double voltage sweep function, current-voltage curve was determined with two different-current states associated with the positive applied voltage or the negative one.

b. Retention Test

Retention time is the time of information keeping. The data (1 or 0) is not able to be distinguished beyond retention time. The current of the sample in the H-state or L-state was measured after fixed period. The retention time of the V-doped SZO film was very long. By applying the higher temperature on the device, the retention test is accelerative.

c. Non-Destructive Readout Test

The sample stressed smaller voltage than the switching voltage was able to stay in the same conductivity state. The smaller positive and the smaller negative sweep voltage were applied on the sample all the time and observed that the current changed with sweeping cycles.

d. Endurance Test

The device applied the enough voltage (positive or negative voltage) was able to change the resistance between two states. Of course, the resistance ratio of the device increased after repeat sweeping cycles. The phenomenon, which was the decrease of the H-state current and the increase of the L-state current, was useful for us to explain the conduction mechanism.

Fig. 2-1 Illustration of the experimental flow.

(100) Si substrate

RCA

Oxidation (200nm SiO2)

Deposition of Pt-Ti bottom electrode by electron beam evaporation

Deposition of LNO buffer layer

RTA for LNO buffer layer

Deposition of SZO resistive layer

Deposition of Al top electrode

Fig. 2-2 Preparation flow of the device.

Fig. 2-3 Cross section of the four-layer structure device.

Fig. 2-4 Cross section of the tri-layer structure device.

Fig. 2-5 Illustration of the sputter system.

La2O3 NiO

Mixed and ball-milled in the absolute alcohol for 24hr and dried at the 85^oC oven

Heated at 600^oC for 2hr and then baked at 1300^oC for 10hr

Ball-milled in the absolute alcohol for 24hr and dried by the 150^oC oven for 2hr

Fig. 2-6 Synthesis flow chart of LNO powder.

SrCO3 ZrO2

Mixed and ball-milled in the absolute alcohol for 24hr and dried by the oven

Heated at 600^oCand 800^oC for 2hr and then baked at 1250^oC for 10hr

Ball-milled in the absolute alcohol for 24hr and dried by the oven

Heated at 600^oCand 800^oC for 2hr and then baked at 1400^oC for 10hr

Ball-milled in the absolute alcohol for 24hr and dried by the oven

Dopant oxide

Fig. 2-7 Synthesis flow chart of doped SZO powder.

Chapter 3

Possible Mechanisms of Conductivity Switching Phenomenon

Generally speaking, the basic conduction mechanisms in insulating films are Schottky emission, Frenkel-Poole emission, Tunneling or field emission, Space-charge-limited current, Ohmic conduction and Ionic conduction. The summary of mathematical expressions and voltage versus temperature dependence of these mechanisms are listed in Table. 3-1 [27].

These basic mechanisms are explained briefly as follows.

(1) Schottky emission corresponds to the thermionic emission induced carrier transport across the metal-insulator interface or the insulator-semiconductor interface.

(2) Frenkel-Poole (F-P) emission is caused by field-enhanced thermal excitation of trapped electrons into the conduction band. The expression of trap state is virtually identical to Schottky emission, but the barrier height is instead of the depth of trap potential well. The barrier lowing is twice as large as Schottky emission one because of the immobility of the positive charge.

(3) Tunnel or field emission is due to the current induced by electrons tunneling from the metal fermi-energy into the insulator conduction band.

(4) Space-charge-limited current is caused by the current that result from the carriers injected into insulator didn’t be recombination with any compensating charge.

(5) Ohmic conduction corresponds to the electrons that hop from one isolated state to the next by thermally exciting, that usually happen in low voltage and high temperature condition.

(6) Ionic condition presents ions can’t be readily injected or extracted from the insulator. Positive and negative space charge will build up near interface after an initial current flow. In addition, hysteresis effect happens result from residual

internal field caused some ions to flow back their equilibrium position when the applied voltage is removed.

At present, there are more and more reports to study the possible mechanism of RRAM deeply based on these basic mechanisms. Many researchers illustrate their opinions for the source of conductivity switching characteristics from various materials. From these reports, the possible mechanism could be sorted into six species shown in follows.

(1) Conducting filament (2) Charge transfer

(3) Storage and release of charge carrier

(4) Dipole rearrangement induced polarization (5) Phase transformation

(6) Formation of the depletion layer

The possible mechanism will be introduce and analyze in the following sections.