Expitaxial growth and characterization of hexagonal boron nitride carbon semiconductor alloys
Uddin, Md Rakib
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Hexagonal boron nitride carbon alloys, h-(BN)1-x(C2)x, are layer-structured semiconductor materials with a tunable bandgap energy from 0 eV (graphite) to 6.5 eV (h-BN), which cover the spectral range from far IR to deep UV. Moreover, it allows to tune the electrical conductivity from insulator (h-BN) to semi-metallic (graphite). Despite the identical crystalline structure (hexagonal) and excellent in-plane lattice constant match between h-BN and graphite, it is very difficult to synthesize the alloy due to the difference in bonding energies between the constituent atoms. In this work, thin films of hexagonal boron nitride carbon alloys, h-(BN)1-x(C2)x, were grown on sapphire substrates via metal organic chemical vapor deposition (MOCVD) growth technique. BN-rich h-(BN)1-x(C2)x alloys have been grown with different carbon concentrations. X-ray diffraction (XRD) measurements confirmed the hexagonal phase of the alloys. Optical absorption and x-ray photoelectron spectroscopy (XPS) measurements were used to determine the bandgap energy (Eg) and carbon concentrations, respectively. Bandgap energy (Eg) variation with carbon concentration in the deep UV spectral range has been demonstrated. Experimental results suggest that the critical carbon concentration (xc) to form the homogenous BN-rich h-(BN)1-x(C2)x alloys is about 3.2% at a growth temperature of 1300 0C. C-rich h-(BN)1-x(C2)x alloys with both p- and n-type conductivity have been obtained via controlling the V/III ratio during MOCVD growth. Electrical transport and Raman spectroscopy measurements were used to study the bandgap opening in the C-rich h-(BN)C alloys. Our experimental results revealed evidences that the critical BN concentration, xBN, to open a small bandgap in graphite or to form h-(BN)C homogenous alloy in the C-rich side is ~ 5.0% at a growth temperature of 1300 0C. Thus, h-(BN)1-x(C2)x alloys with x ≤ 0.032 (BN-rich alloys) and x ≥ 0.950 (C-rich alloys) have been realized at our current growth conditions. Furthermore, an enhancement of approximately 15 orders of magnitude in the electrical conductivity has been attained by increasing the carbon concentration (x) from 0 (h-BN) to 1 (graphite). Based on the predicted phase diagram of cubic (BN)1-x(C2)x and the excellent matches in the structural and thermal properties of h-BN and graphite, it is expected that homogenous h-(BN)1-x(C2)x alloys with higher C concentrations (in the BN-rich side) and higher BN concentrations (in the C-rich side) can be achieved and the alloy miscibility gap can be reduced or completely removed by increasing the growth temperature. This is a huge advantage over the InGaN alloy system in which InN decomposes at high temperatures and high growth temperature cannot be utilized to close the mscibility gap. Together with our ability for producing high quality h-BN epilayers, h-(BN)1-x(C2)x alloys, and quantum wells open up new possibilities for realizing novel 2D optoelectronic devices with tunable physical properties.