Optimization of solution composition in hexagonal boron nitride crystal growth via the flux method

Doctor of PhilosophyDepartment of Chemical EngineeringJames H EdgarHexagonal boron nitride (hBN) is an ultrawide bandgap (>6 eV) semiconductor and 2D material that has attracted much attention due to its unique properties and applications in electronics, optoelectronics, and nanophotonics. In all of these applications, large, high quality single crystals of hBN are required and atmospheric pressure solution growth is a consistent method to achieve this. This study was undertaken to improve this process, accelerate its optimization, and enable creation of devices in a wide range of fields. A new methodology was developed to optimize the boron concentration in hBN solution growth using the CALPHAD (CALculation of PHAse Diagrams) method to rapidly predict the optimal boron concentration for a wide range of solvents. Comparison with experimental results validates its accuracy. Deviations from CALPHAD predictions, confirmed with crystal growth by reusing source material, suggest that the hBN crystal growth process from molten metal solutions is kinetically limited. Reusing source material also substantially improves the yield of boron to hBN, which is especially important when using expensive isotopically pure boron for growth of h10BN or h11BN. Increasing the nitrogen solubility of the solvent is often attributed to increasing crystal size, but this work digs deeper into the effects of this property. Five different solvents (Ni-Cr, Co-Cr, Fe, Fe-V, and Cu) were tested and the domain area and thickness of crystals they produced were compared versus their nitrogen solubility. The nitrogen solubility did not affect the hBN domain area but the crystal thickness increased with nitrogen solubility. This suggests that, so long as the boron concentration is properly optimized, similar domain sizes can be obtained from any solvent. Furthermore, the thickness of as-grown crystals may be engineered for specific applications by choosing a solvent that naturally grows hBN of the required thickness. Finally, the optimal boron concentration increased with the nitrogen solubility, provides a shortcut for optimizing future solvents, accelerating research. Crystal defects such as stacking faults, dislocations, and impurities are detrimental to device performance, thus it is important to understand their properties and how they can be avoided or eliminated. Oxygen impurities were greatly reduced in the solvent with the introduction of hydrogen gas while carbon impurities may need to react with oxygen to be removed. Regardless, the impurity content in hBN crystals grown from these solvents was below the detection limit of secondary ion mass spectrometry (SIMS) in all cases, suggesting the purity of the process is already sufficient. Three classes of defects were detected using cathodoluminescence (CL) spectroscopy: spots, invisible lines, and wrinkles, which were determined to be color centers, half-inserted planes, and plastic deformation of hBN single crystals, respectively. A combination of Raman, photoluminescence (PL), and CL spectroscopy indicates that hBN crystals grown from Ni-Cr and Co-Cr tend to have fewer defects than those grown from Fe, Fe-V, or Cu. Monoisotopic hBN is especially useful in applications where coherence of phonons is especially important such as sub-wavelength optical microscopy and quantum sensing. Previously, processes were developed to synthesize hBN enriched with either the 10B or 11B isotopes using naturally abundant 14N2. In this work, a new process was developed to extend the capabilities of solution growth to also produce hBN with the 15N isotope. Raman and PL spectroscopy on these crystals indicate that they are very high quality, on par with crystals grown with 14N. Furthermore, the effect of the reduced mass on the Raman shift of the E2ghigh peak and the energy of the phonon replicas is identified, which is in excellent agreement with theory

Ion Beam Modifications of Boron Nitride By Ion Implantation

The search for alternative methods of synthesizing cubic boron nitride (cBN), one of the hardest known materials, at low thermo-baric conditions has stimulated considerable research interest due to its great potential for numerous practical industrial applications. The practical applications are motivated by the material’s amazing combination of extraordinarily superior properties. The cBN phase is presently being synthesized from graphite-like boron nitride modifications at high thermo-baric conditions in the presence of catalytic solvents or by ion–beam assisted (chemical and physical) deposition methods. However, the potential and performance of cBN have not been fully realized largely due to central problems arising from the aforementioned synthesis methods. The work reported in this dissertation is inspired by the extensive theoretical investigation of the influence of defects in a ecting the transformation of the hexagonal boron nitride (hBN) phase to the cBN phase that was carried out by Mosuang and Lowther (Phys Rev B 66, 014112 (2002)). From their investigation, using an ab-initio local density approach, for the B, C, N, and O simple defects in hBN, they concluded that the defects introduced into hBN could facilitate a low activation–energy hexagonal-to-cubic boron nitride phase transformation, under less extreme conditions. We use ion implantation as a technique of choice for introducing ‘controlled’ defects into the hot–pressed polycrystalline 99.9% hBN powder samples. The reasons are that the technique is non–equilibrium (not influenced by dilusion laws) and controllable, that is the species of ions, their energy and number introduced per unit area can be changed and monitored easily. We investigate the structural modifications of hBN by ion implantation. Emphasis is given to the possibilities of influencing a low activation–energy hBN-to-cBN phase transformation. The characterization of the structural modifications induced to the hBN samples by implanting with He+ ions of energies ranging between 200 keV and 1.2 MeV, at fluences of up to 1.0 1017 ionscm2, was accomplished by correlating results from X-Ray Di raction (XRD), micro-Raman (-Raman) spectroscopy measurements, and two-dimensional X-Y Raman (2D-Raman) mapping measurements. The surface to pography of the samples was investigated using Scanning Electron Microscopy (SEM). E orts to use Surface Brillouin Scattering (SBS) were hampered by the transparency of the samples to the laser light as well as the large degree of surface roughness. All the implantations were carried out at room temperature under high vacuum. 2D-Raman mapping and -Raman spectroscopy measurements done before and after He+ ion irradiation show that an induced hBN-to-cBN phase transformation is possible: nanocrystals of cBN have been observed to have nucleated as a consequence of ion implantation,the extent of which is dictated by the fluences of implantation. The deviationof the measured spectra from the Raman spectra of single crystal cBN is expected, has been observed before and been attributed to phonon confinement e ects. Also observed are phase transformations from the pre-existing hBN modification to: (a) the amorphous boron nitride (aBN), (b) the rhombohedral boron nitride (rBN) modifications, (c) crystalline and amorphous boron clusters, which are a result of the agglomeration of elementary boron during and immediately after ion implantation. These transformations were observed at high energies. Unfortunately, the XRD measurements carried out could not complement the Raman spectroscopy outcomes probably because the respective amounts of the transformed materials were well below the detection limit of the instrument used in the former case

Growth and characterization of BN thin films deposited by PACVD

Boron nitride has become the focus of a considerable amount of interest because of its properties which relate closely to those of carbon. In particular, the cubic boron nitride phase has extreme hardness, chemical inertness, high resistivity, thermal conductivity and transparency and resistance to oxidation up to 1600°C. The conventional methods of synthesis use highly toxic and inflammable source materials at high deposition temperature. A hot filament activated PACVD technique was developed to deposit BN films under a wide range of conditions. The source material was borane-ammonia (BH3-NH3) which is a non-toxic crystalline solid, free of carbon and oxygen. Using this technique, mixed-phase boron nitride (BN) films, containing crystallites of the cubic phase embedded in a hexagonal matrix, were deposited at a substrate temperature of 350°C. These films showed good thermal and chemical stability with smooth surface topography. A good correlation was obtained between the properties of the films and plasma diagnostics. With increasing rf power, the ratio of cubic to hexagonal phase, determined by infra-red spectra, was increased with a shift towards lower wave number. In the growing and phase stabilization of the cubic phase, ion bombardment plays an important role in forming the sp3-microstructure of BN films. The ion-density and thus ion flux to the more negatively biased substrate was increased with rf power corresponding to an increase o f N2+ species in the plasma, determined by OES. This increased ionbombardment contributed in increasing the volume fraction of cubic phase in the film. For a specific substrate and filament temperature, if the deposition conditions are measured in terms of ion-bombardment, a sharp threshold value exists where the phase of the films changes from being hexagonal to being cubic. At a substrate temperature o f 350°C, this threshold value was found to be 160 W of rf power. A substrate temperature o f 300°C was necessary for enhancing the growth of cubic phase. The shift towards lower wave number at higher power was considered to be a transition to the cubic phase from the wurtzite phase of BN which grows under less highly activated conditions. Hydrogen free film was deposited at a temperature greater than 300°C and a rf power greater than 200 W

Thermodynamic Analysis about Nucleation and Growth of Cubic Boron Nitride Crystals in the hBN-Li3N System under High Pressure and High Temperature

The nucleation of cubic boron nitride (cBN) single crystals synthesized with lithium nitride (Li3N) as a catalyst under high pressure and high temperature (HPHT) was analyzed. Many nanometer-sized cubic boron nitride nuclei formed in the near surface layer, as detected by high resolution transmission electron microscopy. Based on the experiment results, the transformation kinetics is described by a nucleation and growth process in the thermodynamic stability region of cBN. A theoretical description is developed based on the heterogeneous nucleation and layer growth mechanism, and the relevant parameters are estimated and discussed. The critical crystal radius, r*, increases with the temperature under constant pressure; the change with temperature more pronounced at lower pressure (such as 4.5 GPa). The crystal growth velocity increased with the temperature, and it is parabolic with temperature under certain pressure. These results are consistent with experimental data