Tianqi Guo

Sub-band-gap photoconductivity of individual α-Si3N4 nanowires

InN films were grown on sapphire substrate by metalorganic chemical vapor deposition (MOCVD). Different buffer layers were used to investigate their effects on structural properties of InN films. These buffers include the nitrided treatment to the sapphire substrate, a low-temperature InN buffer layer and a thick GaN template. Symmetric high resolution X-ray diffraction (HRXRD) omega/2 theta scans and asymmetric (10 (1) over tilde2) Phi scans confirmed all these InN films had a wurtzite-type structure and the in-plane orientation relationship between InN and Al2O3 was uniquely [10 (1) over bar0](InN)parallel to[11 (2) over bar0](Al2O3) The full-width at half maximum (FWHM) of X-ray (0002) rocking curve decreased from 2.44 degrees to 1.09 degrees by inserting a low-temperature InN buffer. The narrowest FWHM of 0.36 degrees for (0 0 0 2) rocking curve was obtained for InN film grown on a thick GaN template, whose large lateral coherence length is about 311 nin. Atomic force microscopy (AFNI) images revealed that InN films without GaN buffer had a root-mean-square (RMS) roughness of 20 nm while that with a GaN template had a RMS roughness of 80 nm. So, both the low-temperature InN buffer and GaN template can improve InN film features from different aspects. It is necessary to combine the two methods to improve the quality of InN film. (c) 2007 Elsevier B.V. All rights reserved.

Scandium doped Ge2Sb2Te5 for high-speed and low-power-consumption phase change memory

To bridge the gap of access time between memories and storage systems, the concept of storage class memory has been put forward based on emerging nonvolatile memory technologies. For all the nonvolatile memory candidates, the unpleasant tradeoff between operation speed and retention seems to be inevitable. To promote both the write speed and the retention of phase change memory (PCM), Sc doped Ge2Sb2Te5 (SGST) has been proposed as the storage medium. Octahedral Sc-Te motifs, acting as crystallization precursors to shorten the nucleation incubation period, are the possible reason for the high write speed of 6 ns in PCM cells, five-times faster than that of Ge2Sb2Te5 (GST) cells. Meanwhile, an enhanced 10-year data retention of 119 °C has been achieved. Benefiting from both the increased crystalline resistance and the inhibited formation of the hexagonal phase, the SGST cell has a 77% reduction in power consumption compared to the GST cell. Adhesion of the SGST/SiO2 interface has been strengthened, attribut...

Phase change properties of Ti-Sb-Te thin films deposited by thermal atomic layer deposition

Phase change random access memory (PCM) appears to be the strongest candidate for next-generation high density nonvolatile memory. The fabrication of ultrahigh density PCM depends heavily on the thin film growth technique for the phase changing chalcogenide material. In this study, TiSb2Te4 (TST) thin films were deposited by thermal atomic layer deposition (ALD) method using TiCl4, SbCl3, (Et3Si)2Te as precursors. The threshold voltage for the cell based on thermal ALD-deposited TST is about 2.0 V, which is much lower than that (3.5 V) of the device based on PVD-deposited Ge2Sb2Te5 (GST) with the identical cell architecture. Tests of TST-based PCM cells have demonstrated a fast switching rate of ~100 ns. Furthermore, because of the lower melting point and thermal conductivities of TST materials, TST-based PCM cells exhibit 19% reduction of pulse voltages for Reset operation compared with GST-based PCM cells. These results show that thermal ALD is an attractive method for the preparation of phase change materials.

Scandium doping brings speed improvement in Sb 2 Te alloy for phase change random access memory application

Phase change random access memory (PCRAM) has gained much attention as a candidate for nonvolatile memory application. To develop PCRAM materials with better properties, especially to draw closer to dynamic random access memory (DRAM), the key challenge is to research new high-speed phase change materials. Here, Scandium (Sc) has been found it is helpful to get high-speed and good stability after doping in Sb2Te alloy. Sc0.1Sb2Te based PCRAM cell can achieve reversible switching by applying even 6 ns voltage pulse experimentally. And, Sc doping not only promotes amorphous stability but also improves the endurance ability comparing with pure Sb2Te alloy. Moreover, according to DFT calculations, strong Sc-Te bonds lead to the rigidity of Sc centered octahedrons, which may act as crystallization precursors in recrystallization process to boost the set speed.

(GaSb) 0.5 –Ge 1.6 Te Alloys for High-Temperature Phase Change Memory Applications

In this paper, phase change characteristics of (GaSb)0.5–Ge1.6Te alloy were investigated for long data retention phase change memory application. (GaSb)0.5–Ge1.6Te film has high crystallization temperature (357 °C) and large crystallization activation energy (4.57 eV), resulting in a good data retention ability (251 °C for 10 years). The prominent advantages can be seen in comparison with those of pure GeTe and Ge2Sb2Te5. The fine crystal grain size is smaller than GeTe owing to GaSb doping, which is contributed to operation speed. Furthermore, as short as 20 ns electrical pulse can achieve Reset operation. The pulse width of 200 ns requires only a Reset voltage of 1.5 V, which is much lower than that of GeTe-based cells.

Investigation of (SiC)0.85-Sb3Te alloy for high-reliability PCM applications

The reliability and operation speed have long been two great obstacles in phase change memory technology. Thus (SiC)0.85-Sb3Te alloy was proposed to be a new-type phase change material due to its high crystallization temperature (199.7°C) and good data retention ability (118.9°C for 10-year archival life) in this work. The stress accompanying the phase transition in (SiC)0.85-Sb3Te is smaller than those in pure Sb3Te and the traditional material, Ge2Sb2Te5. This is attributed to the fine crystal grain size due to SiC doping, which contributes to the ultrafast reversible operation (5 ns) and good endurance (2.3 × 104 cycles) of (SiC)0.85-Sb3Te based phase change memory cells.

Reactive ion etching effects on carbon-doped Ge2Sb2Te5 phase change material in CF4/Ar plasma

Recently, carbon-doped Ge2Sb2Te5 (CGST) has been proved to be a high promising material for future phase change memory technology. In this article, reactive ion etching (RIE) of phase change material CGST films is studied using CF4/Ar gas mixture. The effects on gas-mixing ratio, RF power, gas pressure on the etch rate, etch profile and roughness of the CGST film are investigated. Conventional phase change material Ge2Sb2Te5 (GST) films are simultaneously studied for comparison. Compared with GST film, 10 % more CF4 is needed for high etch rate and 10% less CF4 for good anisotropy of CGST due to more fluorocarbon polymer deposition during CF4 etching. The trends of etch rates and roughness of CGST with varying RF power and chamber pressure are similar with those of GST. Furthermore, the etch rate of CGST are more easily to be saturated when higher RF power is applied.