Studies on Doping Concentration Variations in 4H-SiC Substrates Using X-ray Contour Mapping

For thin-film sensor applications, the mechanical properties, such as the hardness and elastic modulus, of the sensing material are important in addition to its electrical properties. The mechanical properties of the material need to be known at the design stage of the sensor because these properties are influenced by doping. The effect of in situ nitrogen doping on the mechanical properties of 3C-SiC (111) thin films are presented in this paper. These films are deposited at a pressure of 2.5 mbar and a temperature of 1040 °C on thermally oxidized Si (100) substrates from methyltrichlorosilane and ammonia using a resistively heated vertical hot-wall low-pressure chemical vapour deposition reactor. The effect of in situ nitrogen (0, 9 and 17 atomic per cent of nitrogen) doping on the mechanical properties of the material is investigated using nanoindentation. The x-ray diffraction patterns of 3C-SiC thin films show a decrease in the crystallanity and the intensity of the peak (111), with an increase in the dopant concentration from 0 to 17 atomic percent (%). AFM investigations show a decrease in the roughness and an increase in grain size of the 3C-SiC thin films with an increase in the nitrogen concentration. Nanoindentation measurements revealed that the elastic modulus and hardness of nitrogen doped 3C-SiC thin films decreased from 353 ± 5 to 178 ± 3 GPa and from 35 ± 1.4 to 22 ± 0.6 GPa, respectively, with an increase in the nitrogen doping concentration. This study shows that the mechanical properties of films strongly depend on the grain size, which is influenced by the effects of nitrogen doping. The elastic modulus and hardness are found to be 178 GPa and 22 GPa, respectively, for 3C-SiC thin films doped with 17 atomic % of nitrogen concentration, making it suitable as a sensing material for sensor applications.

The role of Be incorporation in the modulation of the N doping ZnO

The hardness values of 4H-SiC silicon with different nitrogen and vanadium doping concentrations is measured by nanoinderter. It is found that the hardness value is positively correlated with the nitrogen doping concentration ranges from 3.50×1017 to 8.02×1018cm-3, and high vanadium doping concentration will reduce the hardness value of the silicon carbide. Base on the first principles, the elastic constants of Vasp software are simulated and calculated, and the bulk modulus and shear modulus of non-doped, nitrogen-doped and vanadium-doped 4H-SiC are estimated by Voigt-Reuss-Hill. The calculation results show that the shear modulus of vanadium-doped silicon carbide and nitrogen-doped silicon carbide are lower than that of non-doped silicon carbide.

To date, research on defect-induced magnetism in SiC has been conducted to clarify the relationship between the ferromagnetic signal and the carrier concentration. It has been experimentally proven that there is an interaction between the d0 magnetic moment and the hole carrier in p-type 4H-SiC. However, for n-type SiC, the existing theoretical predictions are insufficient to explain the variation in magnetization with the doping concentration. To solve this problem, we prepared 4H-SiC epitaxial layers with different nitrogen doping concentrations and introduced defects by ion implantation. By measuring and analyzing the magnetic properties, we found that the ferromagnetic composition depends on both the implantation dose and the doping concentration. By performing first-principles calculations, we studied the magnetic moments and interactions of defects with different charge states, which is related to defect-induced ferromagnetism. These defects include not only the paramagnetic centers reported in previous studies, such as silicon vacancies and divacancies, but also the NCVSi complex defect of recent interest, which are indicated by positron annihilation experiments. Combining experimental observations with theoretical calculations, we explained the relationship between magnetic properties and the nitrogen doping concentration in the epitaxial samples. Our research will help us to better understand the physical mechanism of defect-induced magnetism in doped semiconductors and provide a potential platform for the control of defect-induced magnetism by carrier density modulation and the fabrication of SiC spintronic devices without transition metals.

Effects of bimetallic catalysts on synthesis of nitrogen-doped carbon nanotubes as nanoscale energetic materials

High-temperature diamond capacitor

Recent advances made towards the improvement of the quality of 4H silicon carbide (4H-SiC) have enabled many successful applications in the power device industry. While defect densities in homoepitaxial active layers have been lowered significantly, there is still a strong interest in understanding the formation mechanism of crystal defects such as dislocations preferably through direct observation. During 4H-SiC homoepitaxy and post-growth annealing, the process of stress relaxation has previously been reported to lead to the formation of misfit dislocations or interfacial dislocations (IDs). [1] The driving force for such a relaxation process is believed to be the lattice misfit induced by the difference in nitrogen doping concentrations across the substrate/epilayer interface. According to van der Merwe [2], at initial stage of epitaxy growth, misfit is accommodated by elastic strain in epilayers; as the growth continues, misfit strain builds up until a critical amount when crystals start to deform plastically with misfit dislocations introduced at the substrate/epilayer interface, relaxing additional elastic strain and minimizing the free energy of the epi-substrate system. Matthews and Blakeslee, [3] from a mechanics point of view, predict the onset of stress relaxation process to be the moment when the mismatch stress applied on a single threading glide dislocation in the epilayer (basal plane dislocation in the case of 4H-SiC homoepitaxy) exceeds the sum of the line tension and the Peierls-Nabarro stress as the epilayer grows thicker. Nevertheless, the direct observation of the stress relaxation process in silicon carbide crystals has not yet been reported. In this paper, we present an in-situ study in which a sample cut from a 150mm commercial n-/n+ 4H-SiC homoepitaxial wafer was subject to high-temperature heat treatment while sequential synchrotron white beam X-ray topographs were recorded simultaneously. The heat treatment was carried out in a halogen lamp furnace and the local temperature was elevated to as high as 1600°C. Figure 1 shows a series of topographs recorded in this manner and the time interval between consecutive topographs is 10 seconds. As time advances, a basal plane dislocation (BPD) in the epitaxial layer is observed to glide from left to right, depositing a straight segment of misfit dislocation right at the substrate/epilayer interface. Figure 2 schematically shows the entire process. Such dynamic observations of relaxation process suggest that the lattice misfit once again exceeds the critical value during high temperature heat treatment. Since the epilayer thickness does not change, it can be inferred that the misfit strain simply increases as temperature is raised. The reason may be that the thermal expansion coefficient of 4H-SiC crystal is a function of doping concentration. Sasaki et. al [4] also confirmed this by measuring the lattice constants of both n+ substrate and n- epilayer at elevated temperatures. [1] M. Dudley, H. Wang, J. Guo, Y. Yang, B. Raghothamachar, J. Zhang, B. Thomas, G.Y. Chung, E. Sanchez, D.M. Hansen, S.G. Mueller, MRS Advances, 1(2), 91-102 (2016). [2] J.H. van der Merwe, J. Appl. Phys. 34, 117 (1963). [3] J.W. Matthews, and A.E. Blakeslee, Journal of Crystal Growth, 27, 118–125 (1974). [4] S. Sasaki, J. Suda, and T. Kimoto, Materials Science Forum, 717-720, 481-484 (2012) Figure 1

[formula omitted] leaves mediated combustion synthesis of Tb[formula omitted] doped Zn2V2O7 nanoparticles: Color tunable photoluminescence and electrochemical studies for display and supercapacitor applications

Crystallographic and electro-optic analysis of pure and Cu/Mn-doped Cd0.6Zn0.4O ternary alloy: Role of the defect states and imperfection density

Nitrogen-doped Zn2SnO4 nanostructures were developed via hydrothermal treatment. Urea (CH4N2O) was used as the nitrogen source to achieve post-growth nitrogen doping in Zn2SnO4 nanostructures. Nitrogen doping resulted in morphological distortion. The elemental study proved that nitrogen concentration increased with an increase in the concentration of urea in the precursor solutions. The vibration modes corresponding to the Zn-N and Sn-N bonds confirmed the incorporation of nitrogen into the crystal lattice of Zn2SnO4. XPS analysis revealed that higher nitrogen doping concentrations led to the substitutional incorporation of nitrogen. Nitrogen doping in Zn2SnO4 introduced impurity levels in the electronic band structure and reduced its optical band gap (from 2.7 eV to 2.4 eV). Consequently, the Zn2SnO4 nanostructure with approximately 1.9 at% nitrogen showed the highest photocurrent density of 124 μA cm-2 at 1.23 V vs. RHE, representing approximately 2.6-fold improvement in photocurrent compared to that of undoped Zn2SnO4 nanostructures. Optimized nitrogen doping resulted in approximately 89% charge injection efficiency along with the lowest charge transfer resistance.

Our present knowledge of the atomic scale structural, chemical and electronic properties of semiconductor interfaces is inversely proportional to their importance for a whole generation of novel electronic and photonic quantum well devices. It is the purpose of this paper to demonstrate how wavelength- and time-resolved cathodoluminescence imaging (CLI) provides a one-to-one image of the crystallographic island structure of the heterointerfaces which are the boundaries of the quantum well. A detailed description of the fully computer controlled cathodoluminescence (CL) system is given. The accelerating voltage, which is choosen to be 30 kV in our standard CL work, is lowered to 3 kV in order to reduce the diameter of the carrier generation volume resulting in an increase of the lateral CL resolution. The exciting electron beam is digitally scanned over a sample area divided in up to 512 x 400 pixels. The CL signal is detected in the visible and near infrared regime (GaAs quantum wells) by a cooled photomultiplier using the technique of time resolved single photon counting. The total time resolution is better than 250 ps. Our data acquisition technique has the unique advantage that simultaneously up to 14 time Windows can be activated, enabling us to record up to 14 spectra or up to 14 images corresponding to different times with respect to the start of the exciting pulse in a single run. Results on AlGaAs/GaAs/AlGaAs quantum wells are presented as a typical example. Direct images of growth islands differing by one monolayer height (2.8 A) at GaAs/AlGaAs heterointerfaces and of the columnar structure of GaAs QWs are observed. The dependence of the lateral extension of these islands, which for certain growth conditions exceeds 6-7 µm, on the parameters of crystal growth is investigated. A transition from 2 dimensional to 3 dimensional crystal growth due to an increase of the MBE (molecular beam epitaxy) growth temperature from 600° C to 660° C is clearly observed. Spectrally- and time-resolved CLI experiments directly visualize the lateral diffusion of the quasi two dimensional carriers along the quantum well interfaces and provide a measure of the in-plane diffusion velocity in quantum well structures. The CLI detection setup is further extended to the infrared regime, by adapting a Ge-PIN-diode and avalanche photodiodes to the photon counting System. This enables CLI and time resolved CLI investigation of InGaAs quantum wells and related material Systems. Strain induced dislocations in pseudomorphic strained layer quantum wells (e.g. GaAs/InGaAs/GaAs QWs) are directly visualized by infrared CLI. Time resolved experiments yield lifetime images around these dislocations.

Within the framework of a semi-classical three-valley Monte Carlo electron transport simulation approach, we analyze the steady-state and transient aspects of the electron transport within bulk zinc-blende indium nitride, with a focus on the response to variations in the crystal temperature and the doping concentration. We find that while the electron transport associated with zinc-blende InN is highly sensitive to the crystal temperature, it is not very sensitive to the doping concentration selection. The device consequences of these results are then explored.

The effects of melt-pool geometrical parameters on crystal growth and microstructure development during laser surface melting of single-crystal alloys were studied by means of mathematical modeling and experiments. A mathematical model was developed for the three-dimensional (3-D) melt-pool geometry and single-crystalline melt-pool solidification in laser surface melting. The 3-D melt-pool geometry corresponding to the solidification interface is described by four geometrical parameters (w, l, h, α). The model was used to study the effects of variations in the geometrical parameters on crystal growth and microstructure development in the melt pool. Laser surface melting experiments with single-crystal nickel-base superalloys were conducted to verify the computational results of microstructure development in the melt pool. Results indicate that the melt-pool geometrical parameters have profound influences on the dendrite growth velocity and growth pattern in the melt pool. For the (0 0 1)/[1 0 0] substrate orientation, variations in l/w and α can influence both the number and the relative sizes of growth regions while the variation in h/w can only influence the relative sizes of the growth regions. Unidirectional dendrite growth along the [0 0 1] crystallographic direction can be achieved for an α value of 45° or below. The maximum ratio of dendrite-growth velocity to the beam velocity in the melt pool is related to α and l/w. Experimental microstructure observations agreed well with the computational results. These findings show that the desired dendrite growth velocity and microstructure can be obtained through proper control of the 3-D melt-pool geometry.

We consider the diffusion of a dopant through a moving interface in the suicide film-Si system during silicide layer growth. The dopant concentration distribution is derived in analytical form by the integral Fourier transform method with subsequent reduction of the dopant redistribution problem to numerical solution of two integral equations. The results are presented in the form of curves plotting the time dependence of dopant concentration on both sides of the interface for various values of diffusion coefficients and interface velocity. The effect of physical parameters on the variation of dopant concentration near the interface is demonstrated.

ABSTRACTPhotoinduced conductivity changes and effects of thermal annealing in carbon nanotube transistors have been examined. Low-intensity ultraviolet light significantly reduces the p-channel conductance while simultaneously increasing the n-channel conductance. A combination of optical absorption and electron transport measurements reveals that these changes occur without variations in dopant concentrations. Measurements with different metals reveal that UV induces oxygen desorption from the electrodes rather than from nanotubes. In Ti-nanotube contact where the Schottky barrier plays an important role, photodesorption of oxygen mainly occurs from the native oxide of Ti electrodes. Decrease in the p-channel conductance arises from the metal work function change which causes larger hole Schottky barrier. Non-Schottky Pd-contacted nanotube transistors do not show photodesorption effects with low intensity UV. Thermal annealing of nanotube transistors with Ti/Au electrodes also leads to the disappearance of the photodesorption effects. However, a noticeable p-doping is observed to upon air exposure after thermal annealing.