Feng Song-Lin

Raman spectra and XPS studies of phase changes in Ge2Sb2Te5 films

Ge2Sb2Te5 film was deposited by RF magnetron sputtering on Si (100) substrate. The structure of amorphous and crystalline Ge2Sb2Te5 thin films was investigated using XRD, Raman spectra and XPS. XRD measurements revealed the existence of two different crystalline phases, which has a FCC structure and a hexagonal structure, respectively. The broad peak in the Raman spectra of amorphous Ge2Sb2Te5 film is due to the amorphous –Te–Te– stretching. As the annealing temperature increases, the broad peak separates into two peaks, which indicates that the heteropolar bond in GeTe4 and the Sb–Sb bond are connected with four Te atoms and other units such as (TeSb) Sb–Sb (Te2) and (Sb2) Sb–Sb (Te2), where some of the four Te atoms in the above formula are replaced by Sb atoms, remain in crystalline Ge2Sb2Te5 thin film. And from the results of Raman spectra and XPS, higher the annealing temperature, more Te atoms bond to Ge atoms and more Sb atoms substitute Te in (Te2) Sb–Sb (Te2).

Structure and Electrical Properties of Ge2Sb2Te5 Thin Film Used for Ovonic Unified Memory

Annealing temperature effects on the structure and electrical resistance of Ge2Sb2Te5 thin film were studied. The crystallization and melting temperature of the thin film are about 175 degrees C and 610 degrees C respectively. The structure of the as-deposited and the annealed film at different annealing temperatures are identified by the x-ray diffraction method. The thin film changes from an amorphous state to the fcc structure at about 200 degrees C, and then changes into the hexagonal close-packed (hcp) structure at about 400 degrees C. Electrical resistance of as-deposited thin film reaches as high as 1.7×108 Ω/sq. Then it decreases to 1.6×104 Ω/sq and 165 Ω/sq after the annealing process at 250 degrees C and 400 degrees C respectively. The current–voltage curve of the Ge2Sb2Te5 thin film is also investigated. Due to the large resistance difference between the amorphous, the fcc crystalline and the hcp crystalline states, Ge2Sb2Te5 film may be used to realize multilevel storage for Ovonic unified memory.

Characteristics of Sn-Doped Ge2Sb2Te5 Films Used for Phase-Change Memory

Sn-doped Ge2Sb2Te5 thin films deposited on Si(100)/SiO2 substrates by rf magnetron sputtering are investigated by a differential scanning calorimeter, x-ray diffraction and sheet resistance measurement. The crystallization temperatures of the 3.58 at.%, 6.92 at.% and 10.04 at.% Sn-doped Ge2Sb2Te5 thin films have decreases of 5.3, 6.1 and 0.9°C, respectively, which is beneficial to reduce the switching current for the amorphous-to-crystalline phase transition. Due to Sn-doping, the sheet resistance of crystalline Ge2Sb2Te5 thin films increases about 2–10 times, which may be useful to reduce the switching current for the amorphous-to-crystalline phase change. In addition, an obvious decreasing dispersibility for the sheet resistance of Sn-doped Ge2Sb2Te5 thin films in the crystalline state has been observed, which can play an important role in minimizing resistance difference for the phase-change memory cell element arrays.

Simulation of the propagation property of metal wires Terahertz waveguides

The propagation property of metal wires terahertz waveguides is studied and simulated under the framework of the Sommerfeld model. The group velocity dispersion, attenuation amplitude, transverse magnetic mode and propagating energy have been obtained by numerically solving the complex eigenvalue equation for the propagation constant. It is found that the group velocity dispersion and attenuation amplitude decrease with the increasing radii of metal wires. The energy power within the dielectric layer increase with the increase of radiation frequency.

Electrical properties of ag-doped Ge2Sb2Te5 films used for phase change random access memory

Ag-doped Ge2Sb2Te5 films were deposited by rf magnetron sputtering on SiO2/Si substrates. The content of Ag ranging from 4.5 to 11.3 at.% is determined by inductively coupled plasma atomic emission spectrometry. The crystallization temperature of Ag-doped Ge2Sb2Te5 increases with the increasing Ag content and the stability of phase change film is improved greatly. Structures were measured by x-ray diffraction and the face-centered-cubic structure was more stable after Ag doping. Four-point probe was used to measure the sheet resistance of Ag-doped Ge2Sb2Te5 films annealed at different temperatures and it is indicated that Ag atoms increase the sheet resistance of Ge2Sb2Te5 thin film when the annealing temperature is higher than about 360 degrees C, which is beneficial for reducing the reset current. Current-voltage curves were tested and it is demonstrated that 4.5 at.% Ag doping into the Ge2Sb2Te5 film can reduce the threshold current from 1.46 mA to 0.25 mA and can only increase the threshold voltage slightly, which is very useful for low energy consumption.

Si doping effect on self-organized InAs/GaAs quantum dots

In situ ultra high vacuum scanning probe microscopy (SPM) and low-temperature :photoluminescence (PL) studies have been performed on Si-doped self-organized InAs/GaAs quantum dots samples to investigate the Si doping effects. Remarkably, when Si is doped in the sample, according to the SPM images, more small dots are formed when compared with images from undoped samples. On the PL spectra, high-energy band tail which correspond to the small dots appear, with increasing doping concentration, the integral intensity of the high-energy band tail account for the whole peak increase too. We relate this phenomenon to a model that takes the Si atom as the nucleation center for QDs formation

Damascene Array Structure of Phase Change Memory Fabricated with Chemical Mechanical Polishing Method

A damascene structure of phase change memory (PCM) is fabricated successfully with the chemical mechanical polishing (CMP) method, and the CMP of Ge2Sb2Te5 (GST) and Ti films is investigated. The polished surface of wafer is analysed by scanning electron microscopy (SEM) and an energy dispersive spectrometer (EDS). The measurements show that the damascene device structure of phase change memory is achieved by the CMP process. After the top electrode is deposited, dc sweeping test on PCM reveals that the phase change can be observed. The threshold current of array cells varies between 0.90 mA and 1.15 mA.

Simulation of Phase-Change Random Access Memory with Ring-Type Contactor for Low Reset Current by Finite Element Modelling

A three-dimensional finite element models for phase change random access memory (PCRAM) is established to simulate thermal and electrical behaviours during RESET operation. The RESET behaviours of the conventional structure (CS) and the ring-type contact in bottom electrode (RIB) are compared with each other. The simulation results indicate that the RIB cell has advantages of high heat efficiency for melting phase change material in cell, reduction of contact area and lower RESET current with maintaining good resistance contrast. The RESET current decreases from 1.26mA to 1.2mA and the heat consumption in GST material during programming increases from 12% to 37% in RIB structure. Thus the RIB structure PCRAM cell is suitable for future device with high heat efficiency and smaller RESET current.

Reactive Ion Etching as Cleaning Method Post Chemical Mechanical Polishing for Phase Change Memory Device

In order to improve nano-scale phase change memory performance, a super-clean interface should be obtained after chemical mechanical polishing (CMP) of Ge2Sb2Te5 phase change films. We use reactive ion etching (RIE) as the cleaning method. The cleaning effect is analysed by scanning electron microscopy and an energy dispersive spectrometer. The results show that particle residue on the surface has been removed. Meanwhile, Ge2Sb2Te5 material stoichiometric content ratios are unchanged. After the top electrode is deposited, current-voltage characteristics test demonstrates that the set threshold voltage is reduced from 13 V to 2.7V and the threshold current from 0.1mA to 0.025mA. Furthermore, we analyse the RIE cleaning principle and compare it with the ultrasonic method.

Fabrication and Characterization of ZnO-Based Film Bulk Acoustic Resonator with a High Working Frequency

We report on the fabrication and characterization of a ZnO-based film-bulk acoustic-resonance device utilized as biosensor. The device has a multilayer structure which consists of piezoelectric element (Au/ZnO/Pt) and a Bragg-reflection-layer acoustic isolation consisting of multilayers of ZnO/Pt. Dielectric measurements have revealed that the device has a very high working frequency (up to ~3.1 GHz), meaning that the device may have a higher sensitivity than the devices reported in the literature.