Yufei Meng

Enhanced optical properties of chemical vapor deposited single crystal diamond by low-pressure/high-temperature annealing

Single crystal diamond produced by chemical vapor deposition (CVD) at very high growth rates (up to 150 μm/h) has been successfully annealed without graphitization at temperatures up to 2200 °C and pressures <300 torr. Crystals were annealed in a hydrogen environment by using microwave plasma techniques for periods of time ranging from a fraction of minute to a few hours. This low-pressure/high-temperature (LPHT) annealing enhances the optical properties of this high-growth rate CVD single crystal diamond. Significant decreases are observed in UV, visible, and infrared absorption and photoluminescence spectra. The decrease in optical absorption after the LPHT annealing arises from the changes in defect structure associated with hydrogen incorporation during CVD growth. There is a decrease in sharp line spectral features indicating a reduction in nitrogen-vacancy-hydrogen (NVH−) defects. These measurements indicate an increase in relative concentration of nitrogen-vacancy (NV) centers in nitrogen-containing LPHT-annealed diamond as compared with as-grown CVD material. The large overall changes in optical properties and the specific types of alterations in defect structure induced by this facile LPHT processing of high-growth rate single-crystal CVD diamond will be useful in the creation of diamond for a variety of scientific and technological applications.

Enhanced growth of high quality single crystal diamond by microwave plasma assisted chemical vapor deposition at high gas pressures

Single crystals of diamond up to 18 mm in thickness have been grown by microwave plasma assisted chemical vapor deposition at gas pressures of up to 350 torr. Growth rates of up to 165 μm/h at 300 torr at high power density have been achieved. The processes were evaluated by optical emission spectroscopy. The high-quality single-crystal diamond grown at optimized conditions was characterized by UV-visible absorption and photoluminescence spectroscopy. The measurements reveal a direct relationship between residual absorption and nitrogen content in the gas chemistry. Fabrication of high quality single-crystal diamond at higher growth rates should be possible with improved reactor design that allows still higher gas synthesis pressures.

Pressure tunes electrical resistivity by four orders of magnitude in amorphous Ge2Sb2Te5 phase-change memory alloy

Ge-Sb-Te-based phase-change memory is one of the most promising candidates to succeed the current flash memories. The application of phase-change materials for data storage and memory devices takes advantage of the fast phase transition (on the order of nanoseconds) and the large property contrasts (e.g., several orders of magnitude difference in electrical resistivity) between the amorphous and the crystalline states. Despite the importance of Ge-Sb-Te alloys and the intense research they have received, the possible phases in the temperature–pressure diagram, as well as the corresponding structure–property correlations, remain to be systematically explored. In this study, by subjecting the amorphous Ge2Sb2Te5 (a-GST) to hydrostatic-like pressure (P), the thermodynamic variable alternative to temperature, we are able to tune its electrical resistivity by several orders of magnitude, similar to the resistivity contrast corresponding to the usually investigated amorphous-to-crystalline (a-GST to rock-salt GST) transition used in current phase-change memories. In particular, the electrical resistivity drops precipitously in the P = 0 to 8 GPa regime. A prominent structural signature representing the underlying evolution in atomic arrangements and bonding in this pressure regime, as revealed by the ab initio molecular dynamics simulations, is the reduction of low-electron-density regions, which contributes to the narrowing of band gap and delocalization of trapped electrons. At P > 8 GPa, we have observed major changes of the average local structures (bond angle and coordination numbers), gradually transforming the a-GST into a high-density, metallic-like state. This high-pressure glass is characterized by local motifs that bear similarities to the body-centered-cubic GST (bcc-GST) it eventually crystallizes into at 28 GPa, and hence represents a bcc-type polyamorph of a-GST.

Pressure-induced crystallization of amorphous Ge2Sb2Te5

Using in situ x-ray diffraction, we demonstrate a pressure-induced crystallization of as-deposited amorphous Ge2Sb2Te5 (a-GST) into a body-centered-cubic (bcc) solid solution at 28 GPa, and the back transformation from the bcc-GST to a-GST. A large hysteresis loop was observed, as the bcc-GST was retained until 15 GPa. Comparisons have been made, employing the x-ray data and the structural information obtained from ab initio molecular dynamics simulations, between the as-deposited a-GST and the a-GST obtained from the pressure-induced collapse of the rocksalt GST, both at a high hydrostatic pressure (20 GPa) prior to their crystallization to bcc. The results suggest that both routes have resulted in the same high-pressure amorphous state, which explains their crystallization into bcc-GST at similar pressures.

A body-centered-cubic polymorph of the Ge2Sb2Te5 phase change alloy

In Ge2Sb2Te5 (GST), the prototype phase-change alloy for data storage, in situ x-ray diffraction experiments reveal a pressure-induced crystalline-amorphous-crystalline transition sequence, all at the same fixed composition and in one experimental cycle. A body-centered-cubic polymorph is discovered at high pressures; the formation of this phase is attributable to its high packing density rendered possible by the switch from covalent to metallic bonding as predicted by ab initio calculations.

Developments in synthesis, characterization, and application of large, high-quality CVD single crystal diamond

Single crystal diamond synthesis by microwave plasma chemical vapor deposition at rapid growth rate has considerably advanced in the past few years. Developments have been made in growth, optical quality, and mechanical properties. Of the various types of single crystal diamond that can be produced using these techniques, high quality single crystal CVD diamond can be routinely produced, and this material is playing an increasing role in research on materials under extreme conditions. This article highlights recent developments in single crystal CVD diamond synthesis and characterization, as well as various applications in high-pressure materials research.

Composite chemical vapor deposition diamond anvils for high-pressure/high-temperature experiments

Composite diamond anvils have been developed for high-pressure/high-temperature measurements of diamond anvil cells. The anvils are fabricated using single-crystal chemical vapor deposition (CVD) from previously used and/or slightly damaged anvils made of natural or synthetic diamond. These composite anvils can be fabricated to possess optical characteristics at least comparable to conventional diamond anvils, whereas the single-crystal CVD portion is more durable because of its enhanced toughness relative to natural diamond. The viability of such anvils is demonstrated in measurements on hydrogen at megabar pressures and high temperature.

Single-crystal CVD diamonds as small-angle X-ray scattering windows for high-pressure research

Small-angle X-ray scattering (SAXS) was performed on single-crystal chemical vapor deposition (CVD) diamonds with low nitrogen concentrations, which were fabricated by microwave plasma-assisted chemical vapor deposition at high growth rates. High optical quality undoped 500 µm-thick single-crystal CVD diamonds grown without intentional nitrogen addition proved to be excellent as windows on SAXS cells, yielding parasitic scattering no more intense than a 7.5 µm-thick Kapton film. A single-crystal CVD diamond window was successfully used in a high-pressure SAXS cell.

Hot Spot Formation in Microwave Plasma CVD Diamond Synthesis

Plasma-substrate interactions in diamond synthesis via microwave plasma-assisted chemical vapor deposition (CVD) are an important issue in CVD reactor optimization. The hot spot formation observed during single-crystal diamond synthesis in 2.45-GHz cylindrical cavity reactors is examined after long-run deposition.

Simulation of microwave plasma discharge in 915 MHZ CVD reactor for single crystal diamond deposition

Despite the complexity of deposition processes in microwave plasma-assisted chemical vapor deposition, this technique is still a common choice to produce an excellent quality diamond. Recently, several simulation plasma models have been proposed in order to prescribe the complex deposition process and also to better understand the plasma -microwave energy and plasma - diamond growth surface interactions in the microwave plasma CVD reactors [1–4].