S. Majety

Epitaxially grown semiconducting hexagonal boron nitride as a deep ultraviolet photonic material

Hexagonal boron nitride (hBN) has emerged as an important material for various device applications and as a template for graphene electronics. Low-dimensional hBN is expected to possess rich physical properties, similar to graphene. The synthesis of wafer-scale semiconducting hBN epitaxial layers with high crystalline quality and electrical conductivity control has not been achieved but is highly desirable. Large area hBN epitaxial layers (up to 2 in. in diameter) were synthesized by metal organic chemical vapor deposition. P-type conductivity control was attained by in situ Mg doping. Compared to Mg-doped wurtzite AlN, which possesses a comparable energy band gap (∼6 eV), dramatic reductions in Mg acceptor energy level and P-type resistivity (by about six to seven orders of magnitude) have been realized in hBN epilayers. The ability of conductivity control and wafer-scale production of hBN opens up tremendous opportunities for emerging applications, ranging from revolutionizing p-layer approach in III-ni...

Dielectric strength, optical absorption, and deep ultraviolet detectors of hexagonal boron nitride epilayers

Hexagonal boron nitride (hBN) epilayers have been synthesized by metal organic chemical vapor deposition and their dielectric strength, optical absorption, and potential as a deep ultraviolet (DUV) detector material have been studied. Based on the graphene optical absorption concept, the estimated band-edge absorption coefficient of hBN is about 7 × 105/cm, which is more than 3 times higher than the value for wurtzite AlN (∼2 × 105 /cm). The dielectric strength of hBN epilayers exceeds that of AlN and is greater than 4.4 MV/cm based on the measured result for an hBN epilayer released from the host sapphire substrate. The hBN epilayer based DUV detectors exhibit a sharp cut-off wavelength around 230 nm, which coincides with the band-edge photoluminescence emission peak and virtually no responses in the long wavelengths. Based on the present study, we have identified several advantageous features of hBN DUV photodetectors: (1) low long wavelength response or high DUV to visible rejection ratio; (2) requirin...

Epitaxial growth and demonstration of hexagonal BN/AlGaN p-n junctions for deep ultraviolet photonics

Recent advances in epitaxial growth and demonstration of p-type conductivity in hexagonal boron nitride (hBN) epilayers represent an exceptional opportunity to revolutionize p-layer approach and overcome the intrinsic problem of low p-type conductivity in Al-rich AlGaN for deep ultraviolet (DUV) device applications. Nevertheless, the ability of epitaxial growth of hBN on AlGaN is a prerequisite for the incorporation of p-type hBN in AlGaN DUV device structures. We report on the epi-growth of hBN on Al-rich AlGaN/AlN/Al2O3 templates using metal organic chemical vapor deposition. X-ray diffraction measurement revealed a 2θ peak at 26.5° which indicates that the BN epilayers are hexagonal and consist of a single phase. Mg doped hBN epilayers were also grown on highly insulating AlN and n-type AlGaN templates with an attempt to demonstrate hBN/AlGaN p-n junctions. Mg doped hBN epilayers grown on insulating templates were p-type with an in-plane resistivity of ∼2.3 Ω cm. Diode behavior in the p-n structures of...

Band-edge transitions in hexagonal boron nitride epilayers

Hexagonal boron nitride (hBN) epilayers have been synthesized on sapphire substrates by metal-organic chemical vapor deposition (MOCVD). These MOCVD grown epilayers exhibit highly efficient band-edge photoluminescence (PL) emission lines centered at around 5.5 eV. The results represent a remarkable improvement over the optical qualities of hBN films synthesized by different methods in the past. It was observed that the emission of hBN at 10 K is about 500 times stronger than that of high quality AlN epilayers. Polarization-resolved PL spectroscopy revealed that hBN epilayers are predominantly a surface emission material, in which the band-edge emission with electric field perpendicular to the c-axis (E⇀emi⊥c⇀) is about 1.7 times stronger than the component along the c-axis (E⇀emi‖c⇀). This is in contrast to AlN, in which the band-edge emission is known to be polarized along the c-axis, (E⇀emi‖c⇀). Time-resolved PL measurements revealed a decay lifetime of around 4.3 ns at 10 K for the dominant band-edge t...

Hexagonal boron nitride and 6H-SiC heterostructures

Hexagonal boron nitride (hBN) epilayers were grown on n-type 6H-SiC substrates via metal organic chemical vapor deposition. X-ray diffraction measurements confirmed that the epilayers are of single hexagonal phase. Photoluminescence (PL) studies revealed a dominant band edge emission at around 5.5 eV, similar to the PL spectra of hBN epilayers grown on sapphire. The current-voltage (I-V) characteristics of the hBN/6H-SiC heterostructure were measured and the results were utilized to determine the band offsets of the hBN/6H-SiC heterojunctions. The analysis yielded the conduction and valence band offsets (ΔEC and ΔEV) of the hBN/6H-SiC heterointerface of about 2.3 and 0.7 (±0.2) eV, respectively, giving a ΔEC/ΔEg value of around 76%. The measured band offsets are in reasonable agreement with values deduced from the band alignments between hBN, AlN, and 6H-SiC obtained from independent experimental data and theoretical calculations.

Electrical transport properties of Si-doped hexagonal boron nitride epilayers

The suitability of Si as an n-type dopant in hexagonal boron nitride (hBN) wide bandgap semiconductor has been investigated. Si doped hBN epilayers were grown via in-situ Si doping by metal organic chemical vapor deposition technique. Hall effect measurements revealed that Si doped hBN epilayers exhibit n-type conduction at high temperatures (T > 800 K) with an in-plane resistivity of ∼12 Ω·cm, electron mobility of μ ∼ 48 cm2/V·s and concentration of n ∼ 1 × 1016 cm−3. Temperature dependent resistivity results yielded a Si energy level in hBN of about 1.2 eV, which is consistent with a previously calculated value for Si substitutionally incorporated into the B sites in hBN. The results therefore indicate that Si is not a suitable dopant for hBN for room temperature device applications.

Layer-structured hexagonal (BN)C semiconductor alloys with tunable optical and electrical properties

Hexagonal boron nitride carbon, h(BN)1-x(C2)x, semiconductor alloys have been grown on sapphire substrates by metal-organic chemical vapor deposition. Bandgap tuning through compositional variation has been demonstrated via optical absorption measurements. Furthermore, an enhancement of approximately 10 orders of magnitude in the electrical conductivity has been attained by increasing the carbon concentration (x) from 0 to 0.21. Experimental results revealed evidences that the critical carbon concentration xc to form the homogenous h(BN)1-x(C2)x alloys, or the carbon solubility in hBN is about 3.2% at a growth temperature of 1300 °C before carbon clusters form. Based on the predicted phase diagram of cubic (BN)1-x(C2)x and the excellent matches in the structural and thermal properties of hBN and graphite, it is expected that homogenous h(BN)1-x(C2)x alloys with higher x can be achieved and the alloy miscibility gap can be reduced or completely removed by increasing the growth temperature. This is a huge a...

Metal-semiconductor-metal neutron detectors based on hexagonalboron nitride epitaxial layers

Hexagonal boron nitride (hBN) possesses extraordinary potential for solid-state neutron detector applications. This stems from the fact that the boron-10 (10B) isotope has a capture cross-section of 3840 barns for thermal neutrons that is orders of magnitude larger than other isotopes. Epitaxial layers of hBN have been synthesized by metal organic chemical vapor deposition (MOCVD). Experimental measurements indicated that the thermal neutron absorption coefficient and length of natural hBN epilayers are about 0.0036 μm-1 and 277 μm, respectively. To partially address the key requirement of long carrier lifetime and diffusion length for a solid-state neutron detector, micro-strip metal–semiconductor–metal detectors were fabricated and tested. A good current response was generated in these detectors using continuous irradiation with a thermal neutron beam, corresponding to an effective conversion efficiency approaching ~80% for absorbed neutrons.

Semiconducting hexagonal boron nitride for deep ultraviolet photonics

Hexagonal boron nitride (hBN) has been recognized as an important material for various device applications and as a template for graphene electronics. Low-dimensional hBN is expected to possess rich physical properties, similar to graphene. The synthesis of wafer-scale semiconducting hBN epitaxial layers with high crystalline quality and electrical conductivity control is highly desirable. We report the successful synthesis of large area hBN epitaxial layers (up to 2-inch in diameter) by metal organic chemical vapor deposition. Ptype conductivity control was also attained by in-situ Mg doping. Compared to Mg doped wurtzite AlN, which possesses a comparable energy band gap (~6 eV), dramatic reductions in Mg acceptor energy level and p-type resistivity have been realized in hBN epilayers. Our results indicate that (a) hBN epitaxial layers exhibit outstanding semiconducting properties and (b) hBN is the material of choice for DUV optoelectronic devices. The ability of conductivity control and wafer-scale production of hBN opens up tremendous opportunities for emerging applications, ranging from revolutionizing p-layer approach in III-nitride deep ultraviolet optoelectronics to graphene electronics.