Single GaN/Al(Ga)N quantum dots (QDs) have been investigated by means of microphotoluminescence. Emission spectra related to excitons and biexcitons have been identified by excitation power dependence and polarization resolved spectroscopy. All investigated dots exhibit a strong degree of linear polarization (∼90%). The biexciton binding energy scales with the dot size. However, both positive and negative binding energies are found for the studied QDs. These results imply that careful size control of III-Nitride QDs would enable the emission of correlated photons with identical frequencies from the cascade recombination of the biexciton, with potential applications in the area of quantum information processing.
Temperature dependence of intersubband transitions in AlN/GaN multiple quantum wells grown with molecular beam epitaxy is investigated both by absorption studies at different temperatures and modeling of conduction-band electrons. For the absorption study, the sample is heated in increments up to 400 • C. The self-consistent Schrödinger-Poisson modeling includes temperature effects of the band-gap and the influence of thermal expansion on the piezoelectric field. We find that the intersubband absorption energy decreases only by ∼ 6 meV at 400 • C relative to its room temperature value.Intersubband transitions in nitride-based semiconductor heterostructures hold great promise for optoelectronic devices. The high band-offset between gallium nitride (GaN) and aluminum nitride (AlN) enables intersubband transitions in the near-infrared regime (λ = 1-4 µm). Thus, intersubband devices such as modulators, detectors, and quantum cascade lasers (QCLs) have the potential to operate at wavelengths useful for fiberoptic communication. QCLs also have the potential to be used when measuring characteristic absorption of small molecules. There are several studies of intersubband absorption in AlN/GaN heterostructures.[1-6] These typically show a peak at 500 to 900 meV with full width half maximum (FWHM) in the range of 60 to 200 meV. An intersubband device should operate at and above 300 K, often with the condition of a negligible change in transition energy. Temperature dependence of the transition energy, up to room temperature, has been reported for intersubband absorption in InAs/AlSb multiple quantum well (MQW) structures [7] and for interband photoluminescence in nitride-based AlN/GaN MQW structures.[8] The huge piezoelectric fields introduce an additional temperature-dependent effect in the nitride MQW structures, because the different thermal expansions of AlN and GaN induce a temperature dependent strain in the structures.In this letter, we demonstrate that the peak position of intersubband transitions red shifts only ∼ 6 meV as the temperature of the sample is increased from 25 • to 400 • C. This aspect of thermal robustness could be vital for the operation of nitride-based QCLs since their ultrafast LO-scattering rates and high intersubband energy would cause them to heat significantly. The temperature effects are studied for MQW structures both experimentally by measuring the absorption at different temperatures, and theoretically by a self-consistent solution to the Schrödinger-Poisson equations for the conductionband electrons. The nitride-semiconductor heterostructures are characterized by huge internal electric fields, which arise from the exceptionally large spontaneous and piezoelectric fields. A proper account of these fields are crucial for an accurate description and characterization of the quantum well states, and they require that the complete structure is considered in the design of heterostructures.The set of MQW structures (A -C) are grown in a Varian Gen II modular molecular beam epitaxy (MBE) system....
The hydrogen gas sensing ability of n-type PtSi∕porous Si Schottky junctions is investigated at room temperature. These junctions exhibit a breakdown-type current-voltage curve, whose breakdown voltage depends on the gas content inside the pores. Hydrogen replaces gases with an inherent dipole moment and increases the breakdown voltage. The response time is very fast, around 6 s, and the recovery time is about 60 s. Concentrations as low as 10 ppm have been detected in the presence of methanol and acetone. Detection has been tested at different temperatures ranging from 273 to 300 K.
Elemental boron was thermally deposited, using the MBE‐technique, on surfaces of AlN and GaN. To suppress boron clustering, nitrogen was supplied from an RF nitrogen plasma source. Reflection high energy electron diffraction was used to monitor the surface before and during the growth. On the GaN surface, low concentrations of boron (∼0.1 ML) resulted in additional 6‐fold highly streaky reflection rods indicating a reconstructed GaN(0001) surface. By increasing the boron concentration to ∼0.5 ML, however, the growth resulted in the formation of 3D islands as observed by spots in the RHEED pattern. Islands were also observed by atomic force microscopy and scanning electron microscopy. The AlN surface produced surface morphological features already for the lowest boron concentration, i.e. ∼0.1 ML coverage. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
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