Defect engineering and growth optimization of single crystal large area hexagonal boron nitride and graphite using atmospheric pressure and high temperatures

Abstract

In this study, the growth of large area single crystal graphite and hexagonal boron nitride (hBN) through atmospheric pressure and high temperature metal flux precipitation (APHT) is optimized using process parameters ranging from source material purity to cover gas composition. The effects of each of these parameters were assessed with a variety of characterization techniques including Raman spectroscopy, X-ray diffraction (XRD), and defect sensitive etching (DSE). Additionally, defects are intentionally introduced into these materials to engineer their electronic and optical properties. Graphite was doped with boron while hBN was prepared with boron vacancies and doped with carbon. To maximize graphite crystal grain size and minimize its defect density, the source carbon concentration, temperature, and cover gas (N2 or Ar) were optimized. The optimum starting source composition to grow graphite single crystals was 5.8 wt.% carbon in iron. At temperatures <1400 °C, graphite crystals were ~20% smaller than those grown at temperatures above 1400 °C. Defect sensitive etching (DSE) revealed that nitrogen leads to larger etch pits but significantly fewer in quantity compared to argon. Boron-doped and undoped graphite crystals were of comparable quality, showing no additional strain due to boron. The addition of a small amount of silicon (0.25 at. %) consistently produced larger graphite crystals. Like graphite, high quality large area single crystal hBN was pursued through APHT process parameters like temperature, cover gas, time, source material, and crucible materials. Compared to previous studies, larger (6 mm2) and thicker (>80 µm) hBN single crystals were grown by hydrogen-rich crystal growth and standard crystal growth and tripling the dwell time in hot pressed boron nitride (HPBN) crucibles, respectively. To determine the temperature at which the hBN crystals begin to form, a series of experiments was conducted in which the nitrogen source was switched from 15N2 enriched to natural nitrogen during the crystal growth process. In this manner, it was determined that when cooling from an initial temperature of 1550 °C, the majority of the hBN crystals formed at 1508 °C. Source materials such as traditional Ni-Cr or Fe and new flux compositions including Ga and Cu, were explored for hBN. Ga reduced the surface tension of the metal flux sufficiently for the metal to occupy more area of the crucible, directly allowing more area for hBN growth; however, Ga2O3 posed a challenge at Ga concentrations >15 wt. %. Gallium also significantly reduced vapor pressure of the metal flux, eliminating metal condensation onto the crystal surface. Copper proved useful for producing thin films with lower quality than hBN precipitated from other transition metals. Boron vacancies in hBN was created by neutron irradiation, causing the 10B isotope to undergo transmutation to 7Li. This study used boron-10 and nitrogen-15 isotopically enriched h10B15N. The 7Li is ejected from the lattice site, resulting in either an absence of electrons or leaving its valence electrons behind. The former is known as the VB0 or neutral boron vacancy. The latter is known as the negatively charged boron vacancy (VB-), which has useful quantum sensing properties. Different fluences of neutrons and annealing conditions proved effective for optimizing the photoluminescence (PL) brightness from the VB- and thus the signal integrity which are vital for magnetic sensing. Lower fluences and post-irradiation annealing temperatures of 600 °C to improve these properties proved best. hBN was doped with carbon in three ways: flux addition, diffusive annealing, and ion implantation. Carbon induces numerous electronic changes to hBN permitting a wide range of applications and property changes. After hBN:C was grown using fifteen different carbon concentrations from 0.25 to 2.32 wt. % carbon, the crystals themselves did not change appearance, but their photoluminescence spectral properties changed. Diffusively doped hBN:C was obtained at 1700°C, lower than previously observed, leading to yellow crystals, and saturation was reached after 9 hours of annealing. In both methods, the most abundant defect was the carbon dimer, CBCN; however, other signatures in addition to this dimer appeared in diffusive annealing. Ion implantation generated a wide array of spectral light emission signatures between 500 and 800 nm in wavelength that could not be assigned without further characterization due to the convolution of photoluminescent peaks.