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1. Electronic and optical characterization of bulk single crystals of cubic boron nitride (cBN)

   https://doi.org/10.1063/5.0092557  

   Milas, Peker; Mathab, Sheikh; Sam Abraham, John Bishoy; Alam, Jahangir; Chandrashekar, M. V.; Robinson, Adam J.; Vora, Patrick M.; Ozturk, Birol; Spencer, Michael G. (September 2022, AIP Advances)

   Cubic boron nitride (cBN) is a relatively less studied wide bandgap semiconductor despite its many promising mechanical, thermal, and electronic properties. We report on the electronic, structural, and optical characterization of commercial cBN crystal platelets. Temperature dependent transport measurements revealed the charge limited diode behavior of the cBN crystals. The equilibrium Fermi level was determined to be 0.47 eV below the conduction band, and the electron conduction was identified as n-type. Unirradiated dark and amber colored cBN crystals displayed broad photoluminescence emission peaks centered around different wavelengths. RC series zero phonon line defect emission peaks were observed at room temperature from the electron beam irradiated and oxygen ion implanted cBN crystals, making this material a promising candidate for high power microwave devices, next generation power electronics, and future quantum sensing applications. 

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2. Size-Tunable Band Structure and Optical Properties of Colloidal Silicon Nanocrystals Synthesized via Thermal Disproportionation of Hydrogen Silsesquioxane Polymers

   https://doi.org/10.1021/acs.jpcc.4c01462  

   Pate, David S; Spence, Griffin C; Graves, Lisa S; Arachchige, Indika U; Özgür, Ümit (June 2024, The Journal of Physical Chemistry C)

   Dodecane-capped silicon nanocrystals (NCs) were synthesized using a low-temperature (800 – 1100 °C) polymer variant of traditional hydrogen silsesquioxane (HSQ) thermal disproportionation. Highly crystalline Si NCs having tunable diameters (3.0 – 6.7 nm) and thus photoluminescence (PL) peaks (1.68 – 1.29 eV) were attained via changes in maximum annealing temperature. Modifications in NC band structure with diameter were explored by comparison of emission with absorption spectra obtained from diffuse reflectance spectroscopy. Large apparent energy shifts between onsets and PL were noted, being significant for smaller NCs (≤ ~4.0 nm). This, along with comparatively “softer” onsets, is commensurate with density of states (DOS) elongation around PL peaks associated with increasing confinement predicted for indirect semiconductor nanostructures. Tauc analyses of absorption additionally revealed three distinguishable optical transitions in all NCs; attributed to indirect Γ25′-Δ1 in lower energy ranges (likely the emission progenitor), indirect Γ25′-L1 overtaken by quasi-direct Γ-X wavefunction mixing for NC diameters ≤ ~4.0 nm within the mid-energy regime, and direct Γ25′-Γ15 transitions at energies nearing and above ~3 eV. 

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3. Identification of Defect Levels in Copper Indium Diselenide (CuInSe2) Thin Films via Photoluminescence Studies

   https://doi.org/10.1557/adv.2018.556  

   Shrestha, Niraj; Sapkota, Dhurba R.; Subedi, Kamala K.; Pradhan, Puja; Koirala, Prakash; Phillips, Adam B.; Collins, Robert W.; Heben, Michael J.; Ellingson, Randy J. (January 2018, MRS Advances)

   Photoluminescence (PL) spectroscopy has been used to study the defect levels in thin film copper indium diselenide (CuInSe 2 , CIS) which we are developing as the absorber layer for the bottom cell of a monolithically grown perovskite/CuInSe 2 tandem solar cell. Temperature and laser power dependent PL measurements of thin film CIS for two different Cu/In ratios (0.66 and 0.80) have been performed. The CIS film with Cu/In = 0.80 shows a prominent donor-to-acceptor peak (DAP) involving a shallow acceptor of binding energy ∼22 meV, with phonon replica at ∼32 meV spacing. In contrast, PL measurement of CIS film for Cu/In = 0.66 taken at 20 K exhibited an asymmetric and broad PL spectrum with peaks at 0.845 eV and 0.787 eV. Laser intensity dependent PL revealed that the observed peaks 0.845 eV and 0.787 eV shift towards higher energy (aka j-shift) at ∼11.7 meV/decade and ∼ 8 meV/decade with increase in laser intensity respectively. The asymmetric and broad spectrum together with large j-shift suggests that the observed peaks at 0.845 eV and 0.787 eV were related to band-to-tail (BT) and band-to-impurity (BI) transition, respectively. Such a band-tail-related transition originates from the potential fluctuation of defect states at low temperature. The appearance of band related transition in CIS film with Cu/In = 0.66 is the indicator of the presence of large number of charged defect states. 

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4. Identification of Defect Levels in Copper Indium Diselenide (CuInSe2) Thin Films via Photoluminescence Studies

   https://doi.org/10.1557/adv.2018.556  

   Niraj Shrestha, Dhurba R. (August 2018, MRS advances)

   Photoluminescence (PL) spectroscopy has been used to study the defect levels in thin film copper indium diselenide (CuInSe2, CIS) which we are developing as the absorber layer for the bottom cell of a monolithically grown perovskite/CuInSe2 tandem solar cell. Temperature and laser power dependent PL measurements of thin film CIS for two different Cu/In ratios (0.66 and 0.80) have been performed. The CIS film with Cu/In = 0.80 shows a prominent donor-to-acceptor peak (DAP) involving a shallow acceptor of binding energy ~22 meV, with phonon replica at ~32 meV spacing. In contrast, PL measurement of CIS film for Cu/In = 0.66 taken at 20 K exhibited an asymmetric and broad PL spectrum with peaks at 0.845 eV and 0.787 eV. Laser intensity dependent PL revealed that the observed peaks 0.845 eV and 0.787 eV shift towards higher energy (aka j-shift) at ~11.7 meV/decade and ~ 8 meV/decade with increase in laser intensity respectively. The asymmetric and broad spectrum together with large j-shift suggests that the observed peaks at 0.845 eV and 0.787 eV were related to band to-tail (BT) and band-to-impurity (BI) transition, respectively. Such a band-tail-related transition originates from the potential fluctuation of defect states at low temperature. The appearance of band related transition in CIS film with Cu/In = 0.66 is the indicator of the presence of large number of charged defect states. 

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5. Homoepitaxial growth of isotopically enriched h10BN layers on h11BN crystals by high-temperature molecular beam epitaxy

   https://doi.org/10.1038/s41699-025-00619-4  

   Bradford, Jonathan; Collins, Amy_F M; Cheng, Tin S; Shen, Jialiang; Kerfoot, James; Rance, Graham A; Li, Jiahan; Mellor, Christopher J; Beton, Peter H; Cassabois, Guillaume; et al (December 2025, npj 2D Materials and Applications)

   Abstract Isotope-enriched bulk hexagonal boron nitride (hBN) crystals have enhanced properties that improve the performance of nanophotonic and quantum technologies. Developing methods to deposit epitaxial layers on such crystals enables the exciting prospects of producing isotope-engineered hBN layers and heterostructures. Here, we demonstrate the homoepitaxial growth of hBN with phase-separated¹⁰B and¹¹B isotopes by high-temperature molecular beam epitaxy (HT-MBE). Controlled nucleation, improved surface uniformity, and step-flow growth of an h¹⁰BN epilayer were achieved by etching the h¹¹BN bulk crystals with molecular hydrogen. The alignment of the h¹⁰BN epilayer and host h¹¹BN lattices was confirmed by lattice-resolved atomic force microscopy. Micro-Raman spectroscopy and scattering-type scanning near-field optical microscopy show that the bulk h¹¹BN and h¹⁰BN epilayer have distinct phonon energies, with no intermixing of the van der Waals layers, thus enabling the different boron isotopes to be spatially separated in the heterostructure. This work demonstrates the potential of HT-MBE to produce isotopic heterostructures of hBN to advance future nanophotonic and quantum technologies. 

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