The growth of AlGaN has been extensively studied, but corresponding research related to the effect of AlN substrate surface has rarely been reported in literature. In this article, the effects of AlN substrate surface on deposition of AlGaN films were investigated by molecular dynamics (MD) simulations. (0001) Al-terminated and $\left(000\overline{1}\right)$ N-terminated AlN were considered as substrates. The quality of surface morphology and atomic scale structure of deposited AlGaN film are discussed in detail. The results show that the surface morphology and crystal quality of AlGaN film grown on (0001) Al-terminated AlN surface are better than for that grown on $\left(000\overline{1}\right)$ N-terminated AlN surface under various growing temperatures and Al/Ga injection ratios between Al and Ga. This can be attributed to the higher mobility of Al and Ga adatoms on the (0001) Al-terminated AlN surface. These findings can provide guidance for the preparation of high-quality AlGaN thin films on AlN substrate.

A 2.45 GHz microwave-plasma chemical-vapor deposition (MPCVD) reactor was designed and built in-house by collaborating with Guangdong TrueOne Semiconductor Technology Co., Ltd. A cylindrical cavity was designed as the deposition chamber and a circumferential coaxial-mode transformer located at the top of the cavity was adopted as the antenna. Two quartz-ring windows that were placed far away from the plasma and cooled by water-cooling cavity walls were used to affix the antenna to the cavity and act as a vacuum seal for the reactor, respectively. This design improved the sealing and protected the quartz windows. In addition, a numerical simulation was proposed to predict the electric-field and plasma-density distributions in the cavity. Based on the simulation results, a microwave-plasma reactor with TM₀₂₁ mode was built. The leak rate of this new reactor was tested to be as low as 1 × 10⁻⁸ Pa·m³·s⁻¹, and the maximal microwave power was as high as 10 kW. Then, single-crystal diamond films were grown with the morphology and crystalline quality characterized by an optical microscope, atomic force microscope (AFM), Raman spectrometer, photoluminescence (PL) spectrometer, and high-resolution X-ray diffractometer. It was shown that the newly developed MPCVD reactor can produce diamond films with high quality and purity.

Heteroepitaxial growth of aluminum nitride (AIN) has been explored by experiments, but the corresponding growth mechanism is still unrevealed. Here, we use molecular dynamics simulations to study effects of temperature and N : Al flux ratio on deposited AlN. When the temperature increases from 1000 K to 2000 K with an N : Al flux ratio of 2.0, the growth rate of the AlN film decreases. The crystallinity of the deposited AlN is distinctly improved as the temperature increases from 1000 K to 1800 K and it becomes saturated between 1800 K and 2000 K. The crystallinity of the deposited film at 1800 K increases with an increase in the N : Al flux ratio from 0.8 to 2.4, and this degraded a little at an N : Al flux ratio of 2.8. In addition, stoichiometry is closely related to crystallinity of deposited films. Film with good crystallinity is connected with a near 50% N fraction. Furthermore, the average mean biaxial stress and mean normal stress at 1800 K with N : Al flux ratios of 2.0, 2.4 and 2.8 are calculated, indicating that the deposited film with lowest stress has the best crystal quality and the defects appear where stresses occur.

In this work, we investigated the deposition of AlN film on GaN substrate by using molecular dynamics (MD) simulations. The effects of GaN substrate surface, growth temperature, and injected N: Al flux ratio on the growth of AlN film were simulated and studied. Consequently, the deposited AlN film on the (0001) Ga-terminated GaN surface achieves better surface morphology and crystallinity than that on the (000-1) N-terminated GaN surface due to the different diffusion ability of Al and N adatoms on two GaN surfaces. Furthermore, with the increase of growth temperature, the surface morphology and crystallinity of AlN film were improved owing to the enhanced mobility of adatoms. At the optimised injected N: Al flux ratio of 1, comparatively good surface morphology and crystallinity of deposited AlN films were realised. This method lays a foundation for the follow-up real-time study of defects and stress evolution of AlN on GaN and can be applied to film growth of other materials. GRAPHICAL ABSTRACT

During single-crystal diamond growth by microwave plasma chemical vapor deposition (MPCVD), polycrystalline diamonds are prone to growing at the edge regions. This substantially reduces the usable area of the grown diamond. In recent years, a series of experimental studies have been carried out to solve this, and some achievements have been obtained. However, to understand the diamond growth mechanism more deeply, the relationship between growth quality and various influencing factors still needs to be studied quantitatively through simulations. The plasma density and substrate temperature are important factors that affect the quality of diamond crystallization. In this paper, the growth conditions of the diamond were simulated and compared with the experiments. The results showed that the temperature distribution on the substrate surface was uniform in the axial direction, while the CH₃, H, and electron number density decreased significantly at the edge regions. When the substrate holder mentioned in the literature was used for growth, it was found that the number density uniformity of CH₃, H, and electrons on the substrate surface improved significantly, without changing the temperature uniformity, and so did the edge quality of the grown diamond. However, the average growth rate decreased significantly and, combined with the approximate growth rate formula, it was found that the number density of CH₃ and H on the substrate surface decreased significantly. The main reason for this was that the substrate holder was too high, which led to the substrate reaching the growth temperature at lower deposition conditions. Therefore, a substrate holder with a 6 mm height reduction was designed for simulation and experiment. The edge quality of the grown diamond with this substrate holder was better than that found using the previous substrate holder, and the growth rate was significantly improved. However, there was still a gap in the growth rate compared with results without the substrate holder. Finally, a substrate ring was proposed, to achieve the goal of high quality and high growth rate at the same time.

Contact configuration modification at carbon nanotube (CNT)-metal interface during nanowelding is investigated using molecular dynamics simulation. It is found that the nanowelding could be accomplished at a temperature much lower than the melting point of the metal, during which a side contact structure and core filling nanowires form at the CNT-metal interface. Metal atoms prefer entering into the core to moving along the outer surface of the CNTs. Also, the filling of the metal atoms into the CNT can enhance the local melting of the metal at the interface, which would facilitate the metal atoms wetting on the outer walls and dramatically increasing the contact length.