183 research outputs found
Synthesis of atomically thin hexagonal boron nitride films on nickel foils by molecular beam epitaxy
Hexagonal boron nitride (h-BN) is a layered two-dimensional material with
properties that make it promising as a dielectric in various applications. We
report the growth of h-BN films on Ni foils from elemental B and N using
molecular beam epitaxy. The presence of crystalline h-BN over the entire
substrate is confirmed by Raman spectroscopy. Atomic force microscopy is used
to examine the morphology and continuity of the synthesized films. A scanning
electron microscopy study of films obtained using shorter depositions offers
insight into the nucleation and growth behavior of h-BN on the Ni substrate.
The morphology of h-BN was found to evolve from dendritic, star-shaped islands
to larger, smooth triangular ones with increasing growth temperature
Field-dependent nonlinear luminescence response of (In,Ga)N/GaN quantum wells
We have investigated the electric-field- and excitation-density-induced variation of the optical transition energy and cathodoluminescence (CL) as well as photoluminescence intensity of a single (In,Ga)N/GaN quantum well deposited in the depletion region of a p-n junction. The electric-field dependence of the transition energy is significantly influenced by field screening in the depletion region due to the excited carriers and by filling of band tail states of localized excitons. The electric-field dependence of the CL intensity is characterized by an abrupt and strong quenching mainly due to drift of excited carriers in the depletion region. A gradual screening of the p-n junction field with increasing excitation density causes a strongly nonlinear CL response. We describe this nonlinear behavior theoretically by a rate equation model
Field-dependent nonlinear luminescence response of (In,Ga)N/GaN quantum wells
We have investigated the electric-field- and excitation-density-induced variation of the optical transition energy and cathodoluminescence (CL) as well as photoluminescence intensity of a single (In,Ga)N/GaN quantum well deposited in the depletion region of a p-n junction. The electric-field dependence of the transition energy is significantly influenced by field screening in the depletion region due to the excited carriers and by filling of band tail states of localized excitons. The electric-field dependence of the CL intensity is characterized by an abrupt and strong quenching mainly due to drift of excited carriers in the depletion region. A gradual screening of the p-n junction field with increasing excitation density causes a strongly nonlinear CL response. We describe this nonlinear behavior theoretically by a rate equation model
Quenching of the E2 phonon line in the Raman spectra of wurtzite GaAs nanowires caused by the dielectric polarization contrast
We investigate the Raman intensity of EH2 phonons in wurtzite GaAs nanowire
ensembles as well as single nanowires as a function of excitation wavelength.
For nanowires with radii in the range of 25 nm, an almost complete quenching
of the EH2 phonon line is observed for excitation wavelengths larger than 600
nm. The observed behavior is quantitatively explained by the dielectric
polarization contrast for the coupling of light into the GaAs nanowires. Our
results define the limits of Raman spectroscopy for the detection of the
wurtzite phase in semiconductor nanowires
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Phonon anharmonicities and ultrafast dynamics in epitaxial Sb2Te3
In this study we report on the investigation of epitaxially grown Sb2Te3 by employing Fourier-Transform transmission Spectroscopy (FTS) with laser-induced Coherent Synchrotron Radiation (CSR) in the Terahertz (THz) spectral range. Static spectra in the range between 20 and 120 cm−1 highlight a peculiar softening of an in-plane IR-active phonon mode upon temperature decrease, as opposed to all Raman active modes which instead show a hardening upon temperature decrease in the same energy range. The phonon mode softening is found to be accompanied by an increase of free carrier concentration. A strong coupling of the two systems (free carriers and phonons) is observed and further evidenced by exciting the same phonon mode at 62 cm−1 within an ultrafast pump-probe scheme employing a femtosecond laser as pump and a CSR single cycle THz pulse as probe. Separation of the free carrier contribution and the phonon resonance in the investigated THz range reveals that, both damping of the phonon mode and relaxation of hot carriers in the time domain happen on the same time scale of 5 ps. This relaxation is about a factor of 10 slower than expected from the Lorentz time-bandwidth limit. The results are discussed in the framework of phonon scattering at thermal and laser induced transient free carriers
Modulation of the electronic properties of GaN films by surface acoustic waves
We report on the interaction between photogenerated electron-hole pairs and surface acoustic waves (SAW) in GaN films grown on sapphire substrates. The spatial separation of photogenerated carriers by the piezoelectric field of the SAW is evidenced by the quenching of the photoluminescence (PL) intensity. The quenching levels in GaN are significantly smaller than those measured in GaAs under similar conditions. The latter is attributed to the lower exciton ionization efficiency and carrier separation probabilities mediated by the piezoelectric effect. The PL spectra also evidence energy shifts and broadenings of the electronic transitions, which are attributed to the band gap modulation by the SAW strain [email protected]
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