Focus Ion Beam

FIB is a scientific instrument that resembles a scanning electron microscope.

However, the SEM uses a focused beam of electrons to image the sample in the chamber, whereas a FIB instead uses a focused beam of gallium ions. Gallium is chosen because it is easy to build a gallium liquid metal ion source (LMIS). In a Gallium LMIS, gallium metal is placed in contact with a tungsten needle and heated. Gallium wets the tungsten, and a huge electric field (greater than 10⁸V per centimeter) causes ionization and field emission of the gallium atoms.

Unlike an electron microscope, the FIB is inherently destructive to the specimen. When the high-energy gallium ions strike the sample, they will sputter atoms from the surface. Gallium atoms will also be implanted into the top few nanometers of the surface, and the surface will be made amorphous.

Because of the sputtering capability, the FIB is used as a micro-machining tool, to modify or machine materials at the micro- and nano-scale.

The FIB is commonly used to prepare samples for the transmission electron microscope. The TEM requires very thin samples, typically ~100 nanometers.

The nanometer-scale resolution of the FIB allows the exact thin region to be chosen.

2.4.4 Field Emission Transmission Electron Microscope and Energy Dispersive X-Ray Spectrometer

The state-of-the-art JEOL JEM-2100F field emission transmission electron microscope is equipped with an Oxford INCA Energy TEM 200 EDS (energy dispersive X-ray spectrometer) system, a Gatan GIF Tridiem EELS (electron energy loss spectrometer) system and a Fischione high-angle annular dark field detector. Features of the JEM-2100F include a high-brightness Schottky field emission electron gun producing a probe size of less than 0.2 nm. Ultra-high point-to-point TEM resolution is 0.19 nm; atomic scale resolution of 0.136 nm can be achieved using high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) imaging. The facilities are ideally suited for crystallographic and chemical analyses at a sub-nanometer scale, including high-sensitivity EDS and EELS.

Both EDS and EELS are analytical TEM (ATEM) techniques and can provide elemental composition and distribution information. The Oxford INCA Energy TEM 200 EDS system has the following features: automatic peak ID and labeling; element maps and linescans using SmartMap data acquisition; ability to define a line or grid of points for automatic data acquisition; absorption correction for samples of finite thickness and Sitelock image drift correction.

2.4.5 Auger Electron Microscopy (AEM)

Auger Electron Spectroscopy (Auger spectroscopy or AES) was developed in the late 1960's , deriving its name from the effect first observed by Pierre Auger, a French Physicist, in the mid-1920's. It is a surface specific technique utilizing the emission of low energy electrons in the Auger process and is one of the most commonly employed surface analytical techniques for determining the composition of the surface layers of a sample.

2.4.6 Secondary Ion Mass Spectrometer (SIMS)

SIMS is a technique used to analyze the composition of solid surfaces and thin films by sputtering the surface of the specimen with a focused primary ion beam and collecting and analyzing ejected secondary ions. While only charged secondary ions emitted from the material surface through the sputtering process are used to analyze the chemical composition of the material, these represent a small fraction of the particles emitted from the sample. These secondary ions are measured with a mass spectrometer to determine the elemental, isotopic, or molecular composition of the surface. SIMS is the most sensitive surface analysis technique, being able to detect elements present in the parts per billion range.

2.4.7 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.

I. 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.

II. 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.

III. 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.

IV. 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.

Fig. 2-2 Preparation flow of the device.

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

Fig. 2-4 Illustration of the sputter system.

Fig. 2-5 Synthesis flow chart of LNO powder.

Fig. 2-6 Synthesis flow chart of doped SZO 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

Chapter3

Results and Discussion

3.1 Experiment Contents

There are two parts of experiment results and discussion in this chapter. First, we investigate the electrical properties of undoped and V-doped SrZrO3 memory films. The SZO films with higher V dopant concentration shows stable switching properties. And the operated voltages and resistance ratio are increased with V dopant concentration. By these results, we analysis the effects of V-doped SZO memory films, and make a model to discuss the doping effects.

Secondly, we investigate that the pure-SZO (undoped SZO) memory film has the smallest operated voltages, but the switching property is unstable. According to these results, the pure-SZO film was treated by rapid temperature annealing (RTA) to improve the unstable switching property. Then, we discuss the RTA effects by electrical properties and material analysis.

3.2 Properties and Analyses of the LaNiO3 Buffer Layer and SrZrO3 Resistive Films

3.2.1 X-Ray Diffraction and Scanning Electronic Microscope of the LaNiO3 Buffer Layer and SrZrO3 Thin Film

In accordance with the previous studies, the Al/0.3% V-doped SZO/LNO/Pt structure device had good resistive switching properties while the preferred orientations of the SZO film are (100) and (200) [26]. Fig. 3-1 shows the expected

crystallization of the LNO buffer layer grown on the Pt bottom electrode. LNO has (100) and (200) preferred orientations. The peaks of LNO shift toward large angle after SZO deposition process. This indicates that the lattice constant of LNO has been decreased in SZO deposition process. This is due to the SZO film was deposited at high temperature for 1 hour. Then, the expected crystallization of the pure-SZO and V-doped SZO films was grown on the LNO film. The pure-SZO and V-doped SZO has (200) preferred orientation to have obvious resistive switching properties. The device is expected to have good resistive switching properties. Figs.

3-2 and 3-3 show that the surface morphology of the LNO films in 600 and 700^oC RTA, respectively. The surface morphology of the LNO annealed at 600^oC is flat, but that of the 700^oC-annealed film exist some precipitates at the LNO surface.

According to the XRD and SEM analyses, in order to avoid the LNO (110) orientation and precipitates to influence electrical properties of the device, RTA treatment temperature was set at 600^oC in O2 ambience in this thesis.

3.2.2 Transmission Electron Microscope

HRTEM analysis shown in Fig. 3-4 is helpful to recognize the interface between the LNO and SZO films and understand the thickness of respective LNO and SZO films. The LNO buffer layer was sputtered at 250^oC in a gas pressure of 10 mTorr with an Ar:O2 mass ratio of 24:16, and in a period of 85 min. The thickness of LNO buffer layer is about 100nm. The SZO was sputtered at 500^oC in a gas pressure of 10 mTorr with an Ar:O2 mass ratio of 24:16, and in a period of 60 min. The thickness of the SZO film is about 18nm. Fig. 3-4 also shows the interface clearly between the LNO and SZO films. It proves no inter-diffusion between LNO and SZO films.

3.3 Electrical and Physical Properties of the Pure and V-doped SrZrO3

memory films

3.3.1 Nonpolar Switching Property of SrZrO³ Resistive Film

At beginning, the sample is at an Original-state (O-state) lower than L-state.

As shown in Fig. 3-5, when the voltage sweeps to a voltage about -4.3V, the leakage current suddenly increases and switches to the H-state. Then, the nonpolar resistance switching properties can exist without any delay time between every voltage sweep cycles. The first resistive switching process is called the forming process.

After forming process, while the negative voltage is applied on the top electrode from 0 to -6V, the current rapidly increases at -2.4V, and then the device is switched from L-state to H-state. During the measurement, the current is restricted to 1mA to prevent the degradation of the device. While the device is switched from L-state to H-state and limited at 1mA, it does not influence the H-state current of the device. The device altered from L-state to H-state is called as turn-on process.

Subsequently, the bias voltage sweeps from 0V to -2V and the device is switched from H-state back to L-state at -1V. The device altered from H-state to L-state is called as off process. When the positive voltage is applied on the top electrode from 0 to 7V, the device is altered from L-state to H-state at 2.7V. Then, the bias voltage sweeps from 0V to 2V and the device is changed from H-state to L-state at 1.2V.

The resistance ratio between two current states is over 10⁴ measured at -1V. The resistive switching properties of the device altered by either positive or negative bias voltage are called nonpolar resistive switching characteristic.

3.3.2 Forming Process of the Pure and V-doped SrZrO³

Fig. 3-6 shows the forming process of pure-SZO film and 0.1%, 0.2%, 0.3%, 0.4% V-doped SZO films by negative bias voltage. When the voltage sweeps to a voltage about -4.6V, -5.1V, -5.4V, -5.6V and -6.7V for pure-SZO, 0.1%, 0.2%, 0.3% and 0.4% V-doped SZO, the leakage current suddenly increases and switches to the H-state. The statistics of forming voltages for each SZO film is shown in Fig.

3-7. We can investigate the forming voltage was increased with V dopant concentration.

3.3.3 Electrical Properties of the Pure and V-doped SrZrO³

Fig. 3-8 shows the resistive switching I-V curves of pure-SZO film and 0.1%, 0.2%, 0.3%, 0.4% V-doped SZO films in turn-on by negative bias voltage and turn-off by negative bias voltage mode, which is called -on-off mode. While the negative voltage is applied on the top electrode from 0 to -7V, the current rapidly increases at -2.6V, -4.3V, -5.4V, -5.6V and -5.7V, and then the device is switched from L-state to H-state,which was explained due to the formation of current paths by electrical induced defects. During the measurement, the current is restricted to 1mA to prevent the degradation of the device. While the device is switched from L-state to H-state and limited at 1mA, it does not influence the H-state current of the device. The device altered from L-state to H-state is called as on process, and the voltage value is called turn-on voltage. Subsequently, the bias voltage sweeps from 0V to -2V and the device is switched from H-state back to L-state at -0.7V, -1V, -1.1V, -1.3V and -1.5V. The device altered from H-state to L-state is called as off process, and the voltage value is called turn-off voltage. We can investigate that the L-state current was decreased with V dopant concentration, which is due to the

formation of oxygen vacancies was suppressed by Zr⁴⁺ sites being substituted by V⁵⁺[48].

Fig. 3-9 shows the statistics of switching voltages for each SZO film in -on-off mode. We take twenty samples in each film to calculate the average of switching voltages and compile statistics of switching voltages. The turn-on voltages were increased with V dopant concentration. And the pure-SZO has the lowest operation voltages.

The resistance statistics of H-state and L-state for each SZO film is shown in Fig. 3-10. We measure the resistance at 0.3V in H-state and L-state. Correspond to L-state current, the resistance of L-state is increased with V dopant concentration, and the resistance ratio is also increased with V dopant concentration.

3.3.4 Endurance of the Pure and V-doped SrZrO³

Several SZO memory film devices are measured to calculate the uniformity of the device. The variation switching voltage of SZO memory film devices are shown in Fig. 3-11. The variation of turn-on voltage for pure-SZO memory film device is probably within ± 1.5V around -3.5V. The turn-on voltages of V-doped SZO films are stable. The variation of turn-off voltage is almost unchanged for each SZO film.

Fig. 3-12 shows that the variation of two current states of each SZO memory film device. The H-state current is light change for each SZO memory film device. But the variation of L-state current for pure-SZO is the worst. The stability of L-state current would be improved with V-doping. This indicates that the resistive switching property of SZO memory film device would be stabilized by V-doping.

Corresponding to Fig. 3-10, the resistance ratio increases with V dopant concentration.

3.3.5 Retention

For a NVM, the data storage time is also called retention time as an important index. It means how long time the current state can be kept in an acceptable range, once the memory cell is written in one state. As shown in Fig. 3-13, retention test of the pure-SZO memory film device measured at RT after 10⁶s is performed. It is no doubt that there is no current variation after 10⁶s, and the resistance ratio is over 10⁴. It shows good retention performance at RT. Besides, thermal test is executed in order to accelerate degeneration speed of memory device. First, the devices are switched to H-state and L-state, respectively. After 1000s the devices are kept at 85^oC and also measured at 85^oC. Fig. 3-13 also shows that the retention test of the devices kept at 85^oC. H-state and L-state currents are stable at least 10⁶s and the resistance ratio between two state is about 10³. The retention is not affected while measured at 85^oC. According to previous statement, our device shows good retention performance and thermal reliability.

3.3.6 Non-Destructive Readout

Voltage stress test is performed to check reliability for reading data frequently and for affection of unexpected voltage noise. Voltage stress is measured with two applied voltage modes, sweep and pulse voltages. Fig. 3-14 shows that the two states of our device stressed at -0.3V for 3hr are stable and kept the resistance ratio over 10⁴. Voltage stress test of the device switched by sweep or pulse voltage is stable. The device has great non-destructive readout performance. The device is also measured voltage stress of the device at 85^oC in order to accelerate degeneration speed of memory device. As shown in Fig. 3-14, the device is still stable at 85^oC and keeps the resistance ratio over 10⁴. At 85^oC, the device also has great

non-destructive readout performance.

3.3.7 Conduction Mechanisms of Pure and V-doped SrZrO3

Figs. 3-15 to 3-19 show the I-V curves of H-state and L-state in double logarithmic plots for each SZO film. The insets of Figs. 3-15 to 3-19 show the plots of Ln(|I/V|) as a function of |V| of H-state at high field region. For each SZO ^{1/ 2} film, the slope of H-state curve is close to unity, indicating that the H-state current is dominated by Ohmic conduction, which is related to thermally excited electrons hopping from one isolated state to the next one [27]. The slope of the L-state curve at low field is close to unity. This indicates that the L-state current at low field is dominated by Ohmic conduction. The inset shows the plot of Ln(|I/V|) as a function of |V| of L-state at high field region. The linear fittings of the device ^{1/ 2} indicate that the L-state current at high filed region follows the Frenkel-Pool (F-P) emission, which is corresponding to field-enhanced thermal excitation of trapped electrons into the conduction band [27]. Therefore, both H- and L-state conductions are bulk controlled.

3.3.8 Barrier Height

The barrier height of conducting defects for each SZO film was calculated as shown in Fig. 3-20. Φ^B represents the barrier gap between conduction band and the trapping level of defects. ΦB of pure-SZO and 0.1%, 0.2%, 0.3%, 0.4% V-doped SZO film is respective 0.44eV, 0.57eV, 0.63eV, 0.64eV, 0.62eV. The barrier height of defects is increased with V dopant concentration. This result indicates that the shallow trapping level defects are suppressed by V doping.

3.3.9 High-Temperature Measurement

We measure the pure-SZO current of L-state and H-state at room temperature (RT) and high-temperature. Fig 3-21 shows the L-state I-V curves in double nature logarithmic plots at low field region, which were measured at different temperature.

The L-state current at low field increases with measured temperature. The linear fittings indicate that the L-state current at low field is dominated by Ohmic conduction at different temperature. The activation energy (Ea) of L-state measured at -0.3V is about 0.05 eV at low field as shown in Fig. 3-22.

The plots of Ln(|I/V|) as a function of |V| of L-state at high field region ^{1/ 2} with different temperature are shown in Fig. 3-23. The L-state current at high field increases with measured temperature. The linear fittings indicate that the conduction mechanism of L-state at low field is dominated by F-P emission at different temperature. And we plot Ln(|I/V|) versus 1/KT 1/(eV) at -1V for L-state current as shown in Fig. 3-24.

Fig 3-25 shows the H-state I-V curves in double nature logarithmic plots at low field region, which were measured at different temperature. The L-state current at low field increases with measured temperature. The H-state current is dominated by Ohmic conduction at different temperature. And the Ea of H-state measured at -0.3V is about 0.13 eV at low field as shown in Fig. 3-26.

Fig 3-25 shows the H-state I-V curves in double nature logarithmic plots at low field region, which were measured at different temperature. The L-state current at low field increases with measured temperature. The H-state current is dominated by Ohmic conduction at different temperature. And the Ea of H-state measured at -0.3V is about 0.13 eV at low field as shown in Fig. 3-26.