J. Chaudhuri

Growth and Characterization of Low Defect GaN by Hydride Vapor Phase Epitaxy

Low defect density GaN boules were produced using hydride vapor phase epitaxy. A high growth rate of over 100 μm/h enabled the growth of GaN boules longer than 3 mm in length. GaN wafers generated from the boules were characterized by X-ray diffraction, synchrotron white beam X-ray transmission topography, and microscopy of etched surfaces. It was found that the dislocation density decreased with the thickness of the grown material. GaN samples with dislocation density less than 10 4 cm -2 were produced. The high crystal quality of the GaN samples was further demonstrated with a full-width at half-maximum of 38 arcsec for the GaN(0004) double crystal X-ray rocking curve.

Functionalization of graphene sheets through fullerene attachment

Fullerene-functionalized graphene shows a hierarchical structure, and is promising for many potential applications. In this paper, a new method is presented to synthesize such a novel nanostructure. Graphite flakes were intercalated to graphite oxide, and then functionalized with zero-dimensional fullerene nanocrystals. The resultant compounds were characterized by FT-IR spectroscopy, UV–Vis spectroscopy, atomic force microscopy, and transmission electron microscopy. The characterization results confirmed that fullerene crystals were successfully attached onto the single-layer graphene sheets and significantly facilitated the exfoliation of graphite to monolayer graphene. The fullerene grafted on the graphene surface serves as a space impediment to prevent the re-stacking of exfoliated graphene sheets. This attempt provides an effective route for large-scale exfoliation and functionalization of monolayer graphene, and is expected to significantly facilitate the application of graphene in the electronic devices, energy storage, and functional materials.

Tunable photoluminescence and energy transfer of YBO3:Tb3+, Eu3+ for white light emitting diodes

Rare-earth (Eu 3+, Tb 3+) ion co-doped YBO 3 phosphors with a morphology of uniform flower-like assembly are fabricated by a facile hydrothermal method without the use of surfactant, organic ligands, or further sintering treatment. Photoluminescence (PL) studies have demonstrated that under the excitation of 365 nm ultraviolet (UV) light, YBO 3:Tb 3+, Eu 3+ emits a white light, including three emissions: a blue band attributed to self-trapped excitons (STEs), a green band due to the Tb 3+ transition of 5D 4– 7F j ( j = 6, 5, 4, and 3), and a red band due to the Eu 3+ transition of 5D 0– 7F j ( j = 0, 1, 2, 3, and 4). Energy transfers from host YBO 3 to Tb 3+, and Eu 3+ and Tb 3+ to Eu 3+, as well as tunable emission by varying the relative doping ratios are demonstrated. The combination of blue emission from STEs with green and red emissions from activators provides a novel and efficient technique to make white light emitting diodes (WLEDs).

Shear-induced phase transition of nanocrystalline hexagonal boron nitride to wurtzitic structure at room temperature and lower pressure

Disordered structures of boron nitride (BN), graphite, boron carbide (BC), and boron carbon nitride (BCN) systems are considered important precursor materials for synthesis of superhard phases in these systems. However, phase transformation of such materials can be achieved only at extreme pressure–temperature conditions, which is irrelevant to industrial applications. Here, the phase transition from disordered nanocrystalline hexagonal (h)BN to superhard wurtzitic (w)BN was found at room temperature under a pressure of 6.7 GPa after applying large plastic shear in a rotational diamond anvil cell (RDAC) monitored by in situ synchrotron X-ray diffraction (XRD) measurements. However, under hydrostatic compression to 52.8 GPa, the same hBN sample did not transform to wBN but probably underwent a reversible transformation to a high-pressure disordered phase with closed-packed buckled layers. The current phase-transition pressure is the lowest among all reported direct-phase transitions from hBN to wBN at room temperature. Usually, large plastic straining leads to disordering and amorphization; here, in contrast, highly disordered hBN transformed to crystalline wBN. The mechanisms of strain-induced phase transformation and the reasons for such a low transformation pressure are discussed. Our results demonstrate a potential of low pressure–room temperature synthesis of superhard materials under plastic shear from disordered or amorphous precursors. They also open a pathway of phase transformation of nanocrystalline materials and materials with disordered and amorphous structures under extensive shear.

Synthesis and photoluminescence properties of hierarchical architectures of YBO3:Eu3+

We present a general hydrothermal method without any organic solvent or surfactant for the synthesis of YBO3:Eu3+ nano- and micro-structures. The samples were of excellent quality with uniform, well dispersed, self-assembled and self-purified YBO3:Eu3+. Single crystal nanoflakes gathered together to evolve into a hierarchical architecture with eight different types of three dimensional morphologies. Strong photoluminescence spectra were obtained at 592, 611 and 627 nm at excitations wavelengths 254 and 363 nm. The emission spectra are associated with the transitions from the excited 5D0 level to the 7FJ (J = 1, 2, 3, 4) levels of Eu3+ activators. YBO3:Eu3+ prepared using ethanol solvent exhibited the highest chromaticity values yet reported.

Magnetic and optical properties of NaGdF4:Nd3+, Yb3+, Tm3+ nanocrystals with upconversion/downconversion luminescence from visible to the near-infrared second window

We have designed and synthesized NaGdF4:Nd3+, Yb3+, Tm3+ magnetic nanophosphors with combined dual-mode downconversion (DC) and upconversion (UC) photoluminescence upon 800 nm excitation. Hexagonal-phase NaGdF4:Nd3+, Yb3+, Tm3+ nanocrystals (NCs) with an average size of 21 nm were synthesized using a solvothermal approach. Nd3+, Yb3+, Tm3+ triple-doped NaGdF4 NCs exhibit a broad range of photoluminescence peaks covering a near infrared first/second window (860–900, 1,000, and 1,060 nm), and visible emission including blue (475 nm), green (520 and 542 nm) and yellow (587 nm) after excitation at 800 nm. A mechanism involving circulation of energy over Gd3+ sublattices as bridge ions and final trapping by the initial activator ions (Nd3+) has been proposed. Penetration depth studies indicate that NIR emission is easily detected even at a large tissue thickness of 10 mm. These paramagnetic nanophosphors demonstrate a large magnetization value of 1.88 emu/g at 20 kOe and longitudinal relaxivity value of 1.2537 mM−1·S−1 as a T1-weighted magnetic resonance imaging contrast agent. These NaGdF4:Nd3+, Yb3+, Tm3+ NCs are promising for applications in biological and magnetic resonance imaging.

Calculated elastic constants of wide band gap semiconductor thin films with a hexagonal crystal structure for stress problems

The Youngs modulus, E, and Poissons ratio, v, can be used for theoretical calculation or experimental determination of residual stresses arising during the growth of wide band gap semiconductor thin films. In this paper, a generalized expression for v and E(1−v) for the hexagonal closed packed (hcp) structure common to wide band gap semiconductors has been given for the first time. In addition, the specific expressions for E, v, and E(1−v) are given for directions within the c and r planes in the hcp structure. These elastic constants are found to be invariant within the c plane but not within the r plane. Numerical values for these elastic constants are computed and listed for 6H-SiC, Al2O3, and AlN.

X‐ray double crystal characterization of single crystal epitaxial aluminum nitride thin films on sapphire, silicon carbide and silicon substrates

A detailed double crystal x‐ray diffractometry study of epitaxial AlN thin films grown on sapphire, silicon and silicon carbide substrates was carried out to compare the structure, residual stress and defect concentration in these thin films. The structure of AlN is wurtzite with a small distortion in lattice parameters. This results in a small residual stress of the order of 109 dynes/cm2 in the film and can be accounted for from the difference in thermal expansion coefficients between the film and substrate. Both the x‐ray and transmission electron microscopy measurements indicate a low defect density in the AlN thin film grown on 6H‐SiC substrate which could be attributed to the small difference in lattice parameters between AlN and 6H‐SiC.

Comparative study of phosphosilicate glass on (100) silicon by furnace and rapid isothermal annealing

Annealing experiments were carried out on phosphosilicate glass (PSG) films deposited on (100) silicon substrates by using a low‐pressure chemical vapor deposition technique. Rapid isothermal processing and conventional furnace heating were used to study the electrical, structural, and mechanical characteristics of these films and the results of the two processes compared. A refractive index of 1.457 was obtained in the rapid isothermal annealing cycle of 800 °C/15S, but was 1.419 for the furnace annealing cycle (i.e., 800 °C/65S). Spreading resistance analysis has shown that the junction depth remains unchanged for an 800 °C/15S rapid isothermal annealing cycle. Stress measurements show that rapid isothermal annealing leads to less strain compared to furnance annealing. The x‐ray photoelectron spectroscopy analysis shows that as compared to furnance annealing, rapid isothermal annealing provides a chemically homogenous interface. High‐frequency capacitance voltage (C‐V) measurements show that furnance‐an...

Selective epitaxial growth of silicon carbide on SiO2 masked Si(100): The effects of temperature

The effect of substrate temperature on the growth rate, crystal grain size, and SiO2 mask stability in the selective epitaxial growth of silicon carbide deposited from SiH4, C2H4, and HCl1 on silicon dioxide masked silicon (100) was examined. Depositing at atmospheric pressure and a Cl/Si input ratio of 50 to achieve good selectivity, increasing the substrate temperature from 950 to 1000 °C increased the growth rate and the crystal size, and improved the film’s surface morphology, but also enhanced the SiO2 mask degradation rate, causing a loss of selectivity for long deposition times. For prolonged deposition times at 1000 °C, SiC nucleation occurred at both voids formed in the mask from its reaction with the silicon substrate and on the SiO2 mask itself—a consequence of increasing oxide surface roughness.