Kathleen Kash

Low pressure synthesis of bulk, polycrystalline gallium nitride

Thick films of polycrystalline GaN were grown at low pressures by direct reaction of atomic nitrogen with liquid Ga without the presence of a substrate. The crystals were confirmed to be wurtzitic GaN by x-ray diffraction, transmission electron microscopy, Raman spectroscopy, and elemental analysis. Photoluminescence spectra showed near band edge peaks and broad yellow band emission at both 298 and 10 K. The results show that atomic nitrogen is an attractive alternative to high pressure N2 for the saturation of liquid gallium with nitrogen for the synthesis of bulk GaN.

Electrochemical pinning of the Fermi level: mediation of photoluminescence from gallium nitride and zinc oxide.

Charge transfer between diamond and an electrochemical redox couple in an adsorbed water film has recently been shown to pin the Fermi level in hydrogen-terminated diamond. Here we show that this effect is a more general phenomenon and influences the properties of other semiconductors when the band lineup between the ambient and electronic states in the semiconductor is appropriate. We find that the luminescent intensities from GaN and ZnO change in different, but predictable, ways when exposed to HCl and NH3 vapors in humid air. The effect is reversible and has been observed on single crystals, nanowires, flakes, and powders. These observations are explained by electron exchange between the oxygen electrochemical redox couple in an adsorbed water film and electronic states in the semiconductor. This effect can take place in parallel with other processes such as defect formation, chemisorption, and surface reconstruction and may play an important, but previously unrecognized, role when electronic and optical measurements are made in air.

Electrodeposition of polycrystalline InSb from aqueous electrolytes

Layers of the binary compound InSb, with thicknesses in the micron range, were electrodeposited from a sodium citrate–citric acid solution onto polycrystalline Cu substrates and onto single crystal Si. The stoichiometry of the layers was found to depend primarily on the ratio of InCl3 to SbCl3 in the electrolyte, and less sensitively on applied potential and pH, within wide ranges of these parameters. Post-deposition crystallite grain growth and phase separation into zincblende InSb and excess metals were observed. Intraband absorption coefficients were low for near-stoichiometric deposits. Near-stoichiometric deposition onto single crystal Si yielded larger crystallites than did deposition onto polycrystalline Cu or non-stoichiometric deposition onto single crystal Si. These results should encourage efforts to achieve device quality InSb by electrodeposition for photonic crystal, nanostructure and nanodevice applications.

Synthesis of bulk polycrystalline indium nitride at subatmospheric pressures.

Polycrystalline, wurtzitic indium nitride was synthesized by saturating indium with nitrogen from microwave plasma sources. The structure was confirmed by x-ray diffraction, electron diffraction, and elemental analysis. Two types of growth were observed: (i) dendritic crystals on the original melt surface, and (ii) hexagonal platelets adjacent to the In metal source on the upper edge of the crucible. The method does not involve a foreign substrate to initiate growth and is a potential alternative to the high-pressure techniques normally associated with bulk growth of indium nitride. The lattice parameters were a = 3.5366 ± 0.0005 A and c = 5.7009 ± 0.0005 A, with c / a = 1.612 ± 0.0005.

Designs of blue and green light-emitting diodes based on type-II InGaN-ZnGeN2 quantum wells

Type-II InGaN-ZnGeN2 quantum wells (QWs) are studied as improved active regions for light-emitting diodes emitting in the blue (λ ∼ 485 nm) and green (λ ∼ 530 nm) spectral ranges. Both the energy band gap and the lattice parameters of ZnGeN2 are very close to those of GaN. The recently predicted large band offset between GaN and ZnGeN2 allows the formation of a type-II InGaN-ZnGeN2 heterostructure. The strong confinement of holes in the ZnGeN2 layer allows the use of a lower In-content InGaN QW to extend the emission wavelength into the blue and green wavelength regions, as compared to the traditional InGaN QW with uniform In content. In the type-II InGaN-ZnGeN2 QW designs, a thin AlGaN layer was used as a barrier for better carrier confinement. The type-II InGaN-ZnGeN2 QWs lead to a significant enhancement of the electron-hole wave function overlap as compared to those of the conventional QWs. Simulation studies of the proposed type-II QWs promise a significant enhancement of the spontaneous emission rat...

Characterization Of Bulk, Polycrystalline Indium Nitride Grown At Sub-Atmospheric Pressures

Polycrystalline, wurtzitic indium nitride was synthesized by saturating indium metal with atomic nitrogen from a microwave plasma source. Plasma synthesis avoids the high equilibrium pressures required when molecular nitrogen is used as the nitrogen source. Two types of growth were observed: 1) small amounts of indium nitride crystallized from the melt during cooling and 2) hexagonal platelets formed adjacent to the In metal source on the crucible sides. The mechanism of this latter growth is not established, but may involve transport of indium as a liquid film. The crystals were characterized by electron diffraction, X-ray diffraction, elemental analysis, scanning electron microscopy, and Raman spectroscopy. Lattice parameter and Raman active phonon modes are reported and compared with calculations based on the full-potential linear muffin-tin orbital method (FP-LMTO).

High efficiency green light-emitting diodes based on InGaN-ZnGeN2 type-II quantum wells

Strain-compensated type-II InGaN-ZnGeN2-AlGaN quantum wells (QWs) are studied as improved active regions for light-emitting diodes (LEDs). Both the band gap and the lattice parameters of ZnGeN2 are very close to those of GaN. The recently predicted large band offset between GaN and ZnGeN2 allows the formation of a type-II heterostructure. The deep confinement of holes in the ZnGeN2 layer allows the use of a low In-content InGaN QW to extend the emission wavelength into the green wavelength region. A thin layer of AlGaN surrounding the QW is used as a strain compensation layer. Simulation studies of the proposed type-II QW indicate an enhancement of 5.6-6.8 times the spontaneous emission rate compared to InGaN-GaN QWs emitting in the green wavelength region.

Raman study of the vibrational modes in ZnGeN2 (0001)

A Raman spectroscopy study was carried out on ZnGeN2 hexagonal single crystal (0001)-oriented platelets obtained by reaction of gaseous ammonia with a Zn-Ge-Sn liquid alloy at 758 °C. The sample geometry allowed measurement of the A2 and A1 Raman modes. First-principles calculations of the spectra were carried out using an improved pseudopotential. Measurements with crossed polarizers yielded spectra that agreed well with first-principles calculations of the A2 modes. Measurements with parallel polarizers should in principle provide the A1L modes. However, for most of the Raman modes, the LO-TO splitting was calculated to be smaller than could be resolved experimentally, and for the few modes which were predicted to have larger LO-TO splittings, the LO mode was not observed. This absence is tentatively explained in terms of overdamped LO-plasmon coupling.

Growth and Raman Spectroscopy of Single Crystal ZnGeN 2 Rods Grown from a Molten Zn/Ge Alloy

We present evidence for the growth of ZnGeN 2 from a molten Zn/Ge alloy via the vapor-liquid-solid mechanism. Hexagonally faceted, 3-4 microns wide by 20-40 microns long, single crystal rods of ZnGeN 2 capped by a polycrystalline dome of ZnGeN 2 were formed. A micro-Raman spectrum shows several individually resolved peaks and no spectral features above 825 cm −1 , in contrast to a previously published spectrum for polycrystalline ZnGeN 2 , but in excellent agreement with recent theoretical predictions.

Synthesis of bulk, polycrystalline gallium nitride at low pressures

Bulk, polycrystalline gallium nitride was crystallized from gallium saturated with nitrogen obtained from a microwave electron cyclotron resonance source. The polycrystalline samples are wurtzitic and n-type. Well-faceted crystals give near-band-edge and yellow band photoluminescence at both 10 K and 300 K. The results show that atomic nitrogen is an attractive alternative to high pressure molecular nitrogen for saturation of gallium with nitrogen for synthesis of bulk gallium nitride.