@article{dalmau_moody_xie_collazo_sitar_2011, title={Characterization of dislocation arrays in AlN single crystals grown by PVT}, volume={208}, ISSN={1862-6300}, url={http://dx.doi.org/10.1002/pssa.201000957}, DOI={10.1002/pssa.201000957}, abstractNote={Abstract}, number={7}, journal={physica status solidi (a)}, publisher={Wiley}, author={Dalmau, Rafael and Moody, Baxter and Xie, Jinqiao and Collazo, Ramón and Sitar, Zlatko}, year={2011}, month={May}, pages={1545–1547} }
 @article{collazo_mita_xie_rice_tweedie_dalmau_sitar_wetzel_khan_2011, title={Progress on n-type doping of AlGaN alloys on AlN single crystal substrates for UV optoelectronic applications}, volume={8}, ISSN={["1862-6351"]}, DOI={10.1002/pssc.201000964}, abstractNote={Abstract}, number={7-8}, journal={PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 8, NO 7-8}, author={Collazo, Ramon and Mita, Seiji and Xie, Jinqiao and Rice, Anthony and Tweedie, James and Dalmau, Rafael and Sitar, Zlatko and Wetzel, C and Khan, A}, year={2011} }
 @article{feneberg_neuschl_thonke_collazo_rice_sitar_dalmau_xie_mita_goldhahn_2011, title={Sharp bound and free exciton lines from homoepitaxial AlN}, volume={208}, ISSN={["1862-6319"]}, DOI={10.1002/pssa.201000947}, abstractNote={Abstract}, number={7}, journal={PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE}, author={Feneberg, Martin and Neuschl, Benjamin and Thonke, Klaus and Collazo, Ramon and Rice, Anthony and Sitar, Zlatko and Dalmau, Rafael and Xie, Jinqiao and Mita, Seiji and Goldhahn, Ruediger}, year={2011}, month={Jul}, pages={1520–1522} }
 @misc{schlesser_dalmau_noveski_sitar_2009, title={Dense, shaped articles constructed of a refractory material and methods of preparing such articles}, volume={7,632,454}, number={1999 Dec. 15}, author={Schlesser, R. and Dalmau, R. F. and Noveski, V. and Sitar, Z.}, year={2009} }
 @article{mita_collazo_rice_dalmau_sitar_2008, title={Influence of gallium supersaturation on the properties of GaN grown by metalorganic chemical vapor deposition}, volume={104}, ISSN={["0021-8979"]}, DOI={10.1063/1.2952027}, abstractNote={A thermodynamic supersaturation model for gallium (Ga) was developed to describe GaN growth characteristics in low-pressure metalorganic chemical vapor deposition. The model takes into account the simplified GaN chemical reaction that occurs at the growth interface, Ga+NH3=GaN+3/2H2. The supersaturation was varied in two ways: (1) by the V/III ratio and (2) by the choice of the diluent gas. Two diluent gases were considered: H2, a commonly used diluent gas, and N2, a reaction inert gas. The choice of the diluent played a role in the degree of Ga supersaturation; since H2 is the product in the GaN formation, the addition of hydrogen significantly lowered the supersaturation. Atomic force microscopy revealed that surface morphology was associated with the different Ga supersaturation and the Burton–Cabrera–Frank model was used to relate it to the observed spiral size and terrace width. In addition to growth morphology, the degree of Ga supersaturation also influenced the carrier compensation level in n-type GaN. Secondary ion mass spectrometry studies identified that the trend of carbon incorporation followed that of the compensation level and inversely that of the Ga supersaturation. For samples intentionally doped with Si, it was found that GaN became semi-insulating when the carbon concentration exceeded that of silicon. In general, it has been shown that two growth processes at seemingly different conditions will be equivalent if the corresponding Ga supersaturation is the same. This finding showed that the supersaturation is a much more universal parameter than any other growth parameter alone.}, number={1}, journal={JOURNAL OF APPLIED PHYSICS}, author={Mita, S. and Collazo, R. and Rice, A. and Dalmau, R. F. and Sitar, Z.}, year={2008}, month={Jul} }
 @article{kim_choi_yoo_ryou_dupuis_dalmau_lu_sitar_2008, title={Modulated precursor flow epitaxial growth of AlN layers on native AlN substrates by metal-organic chemical vapor deposition}, volume={93}, ISSN={["1077-3118"]}, DOI={10.1063/1.2959064}, abstractNote={We have investigated the growth of AlN layers on bulk AlN substrates by modulated precursor flow epitaxial growth using metal-organic and hydride precursors as well as by conventional metal-organic chemical vapor deposition growth. A nanopit-containing morphology was observed for an AlN layer grown by conventional growth, while atomically smooth and pit-free surface was achieved for an AlN layer created by modulated precursor flow epitaxial growth. For similar growth set-point temperatures, nanopit-free surfaces were observed for AlN layers created by conventional growth on sapphire substrates. This is believed to be due to the difference in the temperature of the growing surface, as evidenced by finite element method thermal profiling and the morphology change of the AlN layer with decreasing temperature observed for growth of AlN on sapphire substrates. The AlN layers grown by modulated precursor flow epitaxial growth on the AlN bulk substrates also have excellent crystalline qualities with narrow x-ray rocking curve peak linewidths of 36 and 61arcsec for the (002) and (102) diffraction conditions, respectively.}, number={2}, journal={APPLIED PHYSICS LETTERS}, author={Kim, Hee Jin and Choi, Suk and Yoo, Dongwon and Ryou, Jae-Hyun and Dupuis, Russell D. and Dalmau, R. F. and Lu, P. and Sitar, Z.}, year={2008}, month={Jul} }
 @article{lu_edgar_cao_hohn_dalmau_schlesser_sitar_2008, title={Seeded growth of AlN on SiC substrates and defect characterization}, volume={310}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2008.01.010}, abstractNote={In this study, seeded sublimation growth of aluminum nitride (AlN) on SiC substrates was investigated. Large diameter (15–20 mm) and thick (1–2 mm) AlN layers were demonstrated on Si-face, 3.5° off-axis 6H-SiC (0 0 0 1). A c-axis growth rate of 15–20 μm/h was achieved at 1830 °C, and the surface morphology was highly textured: step features were formed with a single facet on the top of the layer. High-resolution X-ray diffraction (HRXRD), X-ray photoelectron spectroscopy (XPS), and molten KOH/NaOH etching were employed to characterize the AlN layers. The AlN crystals grew highly orientated along the c-axis, however, the impurities of Si (3–6 at%) and C (5.9–8 at%) from the SiC changed the lattice constants of AlN and shifted the AlN (0 0 .2) 2θ value from pure AlN toward SiC. All the growth surfaces had Al-polarity and the dislocation density decreased from 108 to 106 cm−2 as the film thickness increased from 30 μm to 2 mm.}, number={10}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Lu, P. and Edgar, J. H. and Cao, C. and Hohn, K. and Dalmau, R. and Schlesser, R. and Sitar, Z.}, year={2008}, month={May}, pages={2464–2470} }
 @article{collazo_mita_rice_dalmau_sitar_2007, title={Simultaneous growth of a GaN p/n lateral polarity junction by polar selective doping}, volume={91}, ISSN={["1077-3118"]}, DOI={10.1063/1.2816893}, abstractNote={Differences in surface energy between the Ga-polar orientation and the N-polar orientation of GaN translate into a completely different behavior for the incorporation of intentional and unintentional impurities. Oxygen is found to be an impurity with higher concentrations in N-polar films than in Ga-polar films and is the cause for the high n-type carrier concentration observed in N-polar films. We fabricated a lateral p∕n junction in GaN by the simultaneous growth of p- and n-type regions, utilizing the doping selectivity of the two different polar domains. The N-polar domains were n type with a carrier concentration of 1.7×1019cm−3, predominantly due to high oxygen incorporation, while Ga-polar domains were p type with a carrier concentration of 1.7×1017cm−3. No significant difference in Mg incorporation was observed between the two polar domains. This junction showed the characteristics that define a p∕n junction: current rectification, electroluminescence, and the photovoltaic effect.}, number={21}, journal={APPLIED PHYSICS LETTERS}, author={Collazo, R. and Mita, S. and Rice, A. and Dalmau, R. F. and Sitar, Z.}, year={2007}, month={Nov} }
 @article{adekore_callahan_bouthillette_dalmau_sitar_2007, title={Synthesis of erbium-doped gallium nitride crystals by the ammonothermal route}, volume={308}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2007.07.058}, abstractNote={Gallium nitride (GaN) crystals doped with erbium were grown via the ammonothermal processes on hydride vapor phase epitaxy (HVPE) GaN seeds. The crystallization conducted in alkaline solutions of supercritical ammonia and potassium azide (KN3) at temperatures between 525 and 550 °C yielded growth rates of 15 and 50 μm day on the gallium and nitrogen polar faces, respectively. X-ray diffraction studies indicated single-crystalline growth on the N-polar surface while the Ga-polar surfaces resulted in polycrystalline growth. Photoluminescence spectra acquired at 15 K showed optical transitions corresponding to the inner shell transitions of erbium centers as well as a strong band edge and blue luminescence peaks centered at 3.495 and 2.90 eV, respectively. The incorporation of unintentional impurities, such as oxygen, was found by secondary ion mass spectroscopy (SIMS) to be mitigated to ∼1×1019 and ∼7×1019 cm−3 on the gallium and nitrogen polar faces, respectively.}, number={1}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Adekore, B. T. and Callahan, M. J. and Bouthillette, L. and Dalmau, R. and Sitar, Z.}, year={2007}, month={Oct}, pages={71–79} }
 @article{dalmau_collazo_mita_sitar_2007, title={X-ray photoelectron spectroscopy characterization of aluminum nitride surface oxides: Thermal and hydrothermal evolution}, volume={36}, DOI={10.1007/s11664-006-0044-x}, number={4}, journal={Journal of Electronic Materials}, author={Dalmau, R. and Collazo, Ramon and Mita, S. and Sitar, Z.}, year={2007}, pages={414–419} }
 @article{raghothamachar_bai_dudley_dalmau_zhuang_herro_schlesser_sitar_wang_callahan_et al._2006, title={Characterization of bulk grown GaN and AlN single crystal materials}, volume={287}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2005.11.042}, abstractNote={Sublimation method, spontaneously nucleated as well as seeded on SiC substrates, has been employed for growing AlN bulk crystals. For GaN growth, in addition to the sublimation method using sapphire substrates, ammonothermal growth (analogous to the hydrothermal method) on HVPE GaN seeds is also being used. Thick plates/films of AlN and GaN grown by these methods have been characterized by synchrotron white beam X-ray topography (SWBXT) and high resolution X-ray diffraction (HRXRD). Results from a recent set of growth experiments are discussed.}, number={2}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Raghothamachar, B and Bai, J and Dudley, M and Dalmau, R and Zhuang, DJ and Herro, Z and Schlesser, R and Sitar, Z and Wang, BG and Callahan, M and et al.}, year={2006}, month={Jan}, pages={349–353} }
 @article{dalmau_schlesser_rodriguez_nemanich_sitar_2005, title={AlN bulk crystals grown on SiC seeds}, volume={281}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2005.03.012}, abstractNote={AlN layers with thickness between 0.1 and 3 mm were grown on on-axis and off-axis (0 0 0 1), Si-face SiC seeds by physical vapor transport (PVT) from an AlN powder source. A two-step deposition process was developed for the growth of thick layers. Cracks formed in the AlN layers due to the thermal expansion mismatch between AlN and SiC were observed to decrease with increase in AlN thickness. AlN grown on on-axis SiC was primarily Al-polar, but contained N-polar inversion domains (IDs) revealed by wet etching in hot, aqueous phosphoric acid or potassium hydroxide solutions. Regions of opposite polarity on basal plane surfaces were imaged by piezoresponse force microscopy (PFM). IDs were not observed in crystals grown on off-axis seeds. The influence of SiC seed orientation and stability on the polarity of the AlN layers is discussed.}, number={1}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Dalmau, R and Schlesser, R and Rodriguez, BJ and Nemanich, RJ and Sitar, Z}, year={2005}, month={Jul}, pages={68–74} }
 @article{chen_skromme_dalmau_schlesser_sitar_chen_sun_yang_khan_nakarmi_et al._2004, title={Band-edge exciton states in AlN single crystals and epitaxial layers}, volume={85}, ISSN={["1077-3118"]}, DOI={10.1063/1.1818733}, abstractNote={The band-edge excitonic properties of AlN are investigated using low-temperature (1.7K) optical reflectance and transmission measurements of samples with various crystal orientations. The A, B, and C excitons are found to have energies of 6.025, 6.243, and 6.257eV in unstrained material, which shift with strain. The results are compared to a calculation of exciton energies and oscillator strengths to yield a crystal-field splitting of −230meV in unstrained AlN, in good agreement with previous ab initio calculations.}, number={19}, journal={APPLIED PHYSICS LETTERS}, author={Chen, L and Skromme, BJ and Dalmau, RF and Schlesser, R and Sitar, Z and Chen, C and Sun, W and Yang, J and Khan, MA and Nakarmi, ML and et al.}, year={2004}, month={Nov}, pages={4334–4336} }
 @article{brenner_schlesser_sitar_dalmau_collazo_li_2004, title={Model for the influence of boron impurities on the morphology of AIN grown by physical vapor transport}, volume={560}, ISSN={["1879-2758"]}, DOI={10.1016/j.susc.2004.05.003}, abstractNote={We propose that the reduction of the Schwoebel barrier by trace B impurities is responsible for the relatively flat c-plane morphology observed in AlN crystals that are deposited via physical vapor transport in BN crucibles relative to crystals grown in boron-free conditions. The model is supported by molecular statics calculations that predict that B substitutional impurities are energetically preferred at steps on the (0 0 0 1) surface, and that these impurities enhance binding and incorporation of growth species onto steps.}, number={1-3}, journal={SURFACE SCIENCE}, author={Brenner, DW and Schlesser, R and Sitar, Z and Dalmau, R and Collazo, R and Li, Y}, year={2004}, month={Jul}, pages={L202–L206} }
 @article{strassburg_senawiratne_dietz_haboeck_hoffmann_noveski_dalmau_schlesser_sitar_2004, title={The growth and optical properties of large, high-quality AlN single crystals}, volume={96}, ISSN={["1089-7550"]}, DOI={10.1063/1.1801159}, abstractNote={The effect of impurities and defects on the optical properties of AlN was investigated. High-quality AlN single crystals of more than 20mm2 size were examined. Different crucible materials and growth procedures were applied to the growth of bulk AlN by physical vapor transport method to vary the defect and the impurity concentrations. The crystalline orientation was investigated by Raman spectroscopy. Glow discharge mass spectrometry was used to determine the trace concentration of the incorporated impurities such as oxygen and carbon. The photoluminescence emission and absorption properties of the crystals revealed bands around 3.5 and 4.3eV at room temperature. Absorption edges ranging between 4.1 and 5.95eV were observed. Since no straight correlation of the oxygen concentration was obtained, a major contribution of oxygen or oxygen-related impurities was ruled out to generate the observed emission and absorption bands in the Ultraviolet spectral range. The carbon-related impurities and intrinsic defects might contribute to the observed optical properties. The absorption coefficient for AlN single crystals has been derived for the spectral range below the band edge.}, number={10}, journal={JOURNAL OF APPLIED PHYSICS}, author={Strassburg, M and Senawiratne, J and Dietz, N and Haboeck, U and Hoffmann, A and Noveski, V and Dalmau, R and Schlesser, R and Sitar, Z}, year={2004}, month={Nov}, pages={5870–5876} }
 @article{schlesser_dalmau_sitar_2002, title={Seeded growth of AlN bulk single crystals by sublimation}, volume={241}, ISSN={["0022-0248"]}, DOI={10.1016/S0022-0248(02)01319-2}, abstractNote={AlN bulk single crystals were grown by sublimation of AlN powder at temperatures of 2100–2300°C in an open crucible geometry in a 400 Torr nitrogen atmosphere. Small, single crystalline AlN c-platelets, prepared by vaporization of Al in a nitrogen atmosphere, were used as seeds. Seeded growth occurred preferentially in the crystallographic c-direction, with growth rates exceeding 500 μm/h, while the seed crystals grew only marginally in the c-plane. Transparent, centimeter-sized AlN single crystals were grown within 24 h. Characterization by X-ray diffraction showed that rocking curves around the (0 0 0 2) reflection were very narrow (25 arcsec full-width at half-maximum), thus indicating very high crystalline quality of the material grown on the seeds.}, number={4}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Schlesser, R and Dalmau, R and Sitar, Z}, year={2002}, month={Jun}, pages={416–420} }
 @article{raghothamachar_vetter_dudley_dalmau_schlesser_sitar_michaels_kolis_2002, title={Synchrotron white beam topography characterization of physical vapor transport grown AlN and ammonothermal GaN}, volume={246}, ISSN={["0022-0248"]}, DOI={10.1016/S0022-0248(02)01751-7}, abstractNote={Structural defects in AlN single crystals grown by the sublimation method and GaN single crystals grown by the ammonothermal method are characterized by synchrotron white-beam X-ray topography in conjunction with optical microscopy. AlN platelets are either of (112̄0) or (0 0 0 1) type depending on the growth conditions. Dislocation densities of the order of 103 cm−2 or lower are observed in some crystals. X-ray topographs reveal the presence of growth sector boundaries, inclusions, and growth dislocations that indicate slight impurity contamination. The 2H crystal structure of GaN single crystals obtained by the ammonothermal method was verified by Laue X-ray pattern analysis. GaN crystals grown are of the order of 1 mm in size and are either (0 0 0 1) platelets or [0 0 0 1] prismatic needles. Generally, prismatic needles are characterized by lower degree of mosaicity than (0 0 0 1) platelets.}, number={3-4}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Raghothamachar, B and Vetter, WM and Dudley, M and Dalmau, R and Schlesser, R and Sitar, Z and Michaels, E and Kolis, JW}, year={2002}, month={Dec}, pages={271–280} }