POLARIZATION OF HOT PHOTOLUMINESCENCE IN QUANTUM-WELLS AND HEAVY-LIGHT HOLE MIXING

Hot electrons excited from the valence band by linearly polarized laser light are characterized by certain angular distributions in momenta. Owing to such angular distributions in momenta, the photoluminescence from the hot electrons shows a certain degree of polarization. A theoretical treatment of this effect observed in the photoluminescence in quantum wells is given, showing that the effect depends strongly on heavy and light hole mixing. The very large disparity between the experimentally observed and theoretically expected values of the degree of polarization in the hot-electron photoluminescence suggests the presence of random quasielastic scattering. The effects of such additional scattering and the presence of a perpendicular magnetic field are incorporated into the theory. it is shown that the measurements of the degree of polarization observed in the hot electron photoluminescence, with and without an applied perpendicular magnetic field can serve to determine the time constants for both LO-phonon inelastic and random quasielastic scattering. As an example, these time constants are determined for the experiments reported in the literature.

Photoluminescence from the nitrogen-perturbed above-bandgap states in dilute GaAs1-xNx alloys: A microphotoluminescence study

Using microphotoluminescence (mu-PL), in dilute N GaAs1-xNx alloys, we observe a PL band far above the bandgap E-0 with its peak energy following the so-called E+ transition, but with contribution from perturbed GaAs host states in a broad spectral range (> 100 meV). This finding is in sharp contrast to the general understanding that E+ is associated with a well-defined conduction band level (either L-1c or N-x). Beyond this insight regarding the strong perturbation of the GaAs band structure caused by N incorporation, we demonstrate that a small amount of isoelectronic doping in conjunction with mu-PL allows direct observation of above-bandgap transitions that are not usually accessible by PL.

Multiband photoluminescence from carbon nanoflakes synthesized by hot filament CVD: towards solid-state white light sources

Carbon nanoflakes (CNFLs) are synthesized on silicon substrates deposited with carbon islands in a methane environment using hot filament chemical vapor deposition. The structure and composition of the CNFLs are studied using field emission scanning electron microscopy, high-resolution transmission electron microscopy, micro-Raman spectroscopy, and X-ray photoelectron spectroscopy. The results indicate that the CNFLs are composed of multilayer graphitic sheets and the area and thickness of CNFs increase with the growth time. The photoluminescence (PL) of CNFLs excited by a 325 nm He-Cd laser exhibits three strong bands centered at 408, 526, and 699 nm, which are related to the chemical radicals terminated on the CNFLs and the associated interband transitions. The PL results indicate that the CNFLs are promising as an advanced nano-carbon material capable of generating white light emission. These outcomes are significant to control the electronic structure of CNFLs and contribute to the development of next-generation solid-state white light emission devices. © 2014 the Partner Organisations.

Room-temperature photoluminescence from nitrogenated carbon nanotips grown by plasma-enhanced hot filament chemical vapor deposition

Nitrogenated carbon nanotips with a low atomic concentration of nitrogen have been synthesized by using a custom-designed plasma-enhanced hot-filament plasma chemical vapor deposition system. The properties (including morphology, structure, composition, photoluminescence, etc.) of the synthesized nitrogenated carbon nanotips are investigated using advanced characterization tools. The room-temperature photoluminescence measurements show that the nitrogenated carbon nanotips can generate two distinct broad emissions located at ∼405 and ∼507 nm, respectively. Through the detailed analysis, it is shown that these two emission bands are attributed to the transition between the lone pair valence and bands, which are related to the sp3 and sp2 C-N bonds, respectively. These results are highly relevant to advanced applications of nitrogenated carbon nanotips in light emitting optoelectronic devices.

The effect of temperature on the mechanism of photoluminescence from plasma-nucleated, nitrogenated carbon nanotips

Nitrogenated carbon nanotips (NCNTPs) have been synthesized using customized plasma-enhanced hot filament chemical vapor deposition. The morphological, structural, and photoluminescent properties of the NCNTPs are investigated using scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and photoluminescence spectroscopy. The photoluminescence measurements show that the NCNTPs predominantly emit a green band at room temperature while strong blue emission is generated at 77 K. It is shown that these very different emission behaviors are related to the change of the optical band-gap and the concentration of the paramagnetic defects of the carbon nanotips. The studies shed light on the controversies on the photoluminescence mechanisms of carbon-based amorphous films measured at different temperatures. The relevance of the results to the use of nitrogenated carbon nanotips in light-emitting optoelectronic devices is discussed.

Analysis of photoluminescence background of Raman spectra of carbon nanotips grown by plasma-enhanced chemical vapor deposition

Carbon nanotips with different structures were synthesized by plasma-enhanced hot filament chemical vapor deposition and plasma-enhanced chemical vapor deposition using different deposition conditions, and they were investigated by scanning electron microscopy and Raman spectroscopy. The results indicate that the photoluminescence background of the Raman spectra is different for different carbon nanotips. Additionally, the Raman spectra of the carbon nanotips synthesized using nitrogen-containing gas precursors show a peak located at about 2120 cm-1 besides the common D and G peaks. The observed difference in the photoluminescence background is related to the growth mechanisms, structural properties, and surface morphology of a-C:H and a-C:H:N nanotips, in particular, the sizes of the emissive tips.

Synthesis of GaN nanowires and nano-pyramids in a two-hot-boat chemical vapor deposition system via an in-doping technique

A two-hot-boat chemical vapor deposition system was modified from a thermal evaporation equipment. This system has the advantage of high vacuum, rapid heating rate and temperature separately controlled boats for the source and samples. These are in favor of synthesizing compound semiconducting nano-materials. By the system, we have synthesized high-quality wurtzite single crystal GaN nanowires and nanotip triangle pyramids via an in-situ doping indium surfactant technique on Si and 3C-SiC epilayer/Si substrates. The products were analyzed by x-ray diffraction, field emission scanning electron microscopy, highresolution transmission electron microscopy, energy- dispersive x-ray spectroscopy, and photoluminescence measurements. The GaN nanotip triangle pyramids, synthesized with this novel method, have potential application in electronic/ photonic devices for field-emission and laser.

Blue-violet photoluminescence from large-scale highly aligned boron carbonitride nanofibers

We report on the strong blue-violet photoluminescence (PL) at room temperature from the large-scale highly aligned boron carbonitride (BCN) nanofibers synthesized by bias-assisted hot filament chemical vapor deposition. The photoluminescence peak wavelength shifts in the range of 470-390 nm by changing the chemical composition of the BCN nanofibers, which shows an interesting blue and violet-light-emitting material with adjustable optical properties. The mechanism for the shift of the PL peaks at room temperature is also discussed. (C) 2000 American Institute of Physics. [S0003-6951(00)04427-2].

Homoepitaxial growth and characterization of 4H-SiC epilayers by low-pressure hot-wall chemical vapor deposition

Horizontal air-cooled low-pressure hot-wall CVD (LP-HWCVD) system is developed to get high quality 4H-SiC epilayers. Homoepitaxial growth of 4H-SiC on off-oriented Si-face (0001) 4H-SiC substrates purchased from Cree is performed at a typical temperature of 1500 degrees C with a pressure of 40 Torr by using SiH4+C2H4+H-2 gas system. The surface morphologies and structural and optical properties of 4H-SiC epilayers are characterized with Nomarski optical microscope, atomic force microscopy (AFM), x-ray diffraction, Raman scattering, and low temperature photoluminescence (LTPL). The background doping of 32 pm-thick sample has been reduced to 2-5 x 10(15) cm(-3). The FWHM of the rocking curve is 9-16 arcsec. Intentional N-doped and B-doped 4H-SiC epilayers are obtained by in-situ doping of NH3 and B2H6, respectively. Schottky barrier diodes with reverse blocking voltage of over 1000 V are achieved preliminarily.

Growth and characterization of 4H-SiC by horizontal hot-wall CVD

4H-SiC layers have been homoepitaxially grown at 1500 degrees C with the use of a horizontal hot-wall chemical vapor deposition (CVD) system, which was built in the author's group. The typical growth rate was 2 mu m/h at a pressure of 40 Torr. The background donor concentration has been reduced to 2.3 x 10(15) cm(-3) during a prolonged growth run. It confirmed the idea that the high background concentration of thin films was caused by the impurities inside the susceptor and thermal insulator The FWHM of x-ray co-rocking curves show 9 similar to 15 aresecs in five different areas of a 32-mu m-thick 4H-SiC epilayer The free exciton peaks dominated in the near-band-edge low-temperature photoluminescence spectrum (LTPL), indicating high crystal quality.

Luminescence properties of ZnSe films grown by hot wall epitaxy

The photoluminescence (PL) properties of ZnSe films grown by hot wall epitaxy are reported. The PL spectra show clear neutral donor-bound exciton peak; donor acceptor pair (DAP) peak, conduction band to acceptor (CA) peak, and their phonon replicas until fourth order. The conduction band to acceptor peak and it's phonon replicas exist until room temperature. From the ratio of PL intensities of DAP and CA peaks and their replicas, we obtain the Huang-Rhys factor S = 0.58, in agreement with other experiments for acceptor-bound exciton transitions. From the temperature dependence of PL intensities we derive the activation energy of thermal quenching process for the DAP transitions as about 7 meV.

Homoepitaxial Growth and Characterization of 4H-SiC Epilayers by Low-Pressure Hot-Wall Chemical Vapor Deposition

Horizontal air-cooled low-pressure hot-wall CVD (LP-HWCVD) system is developed to get highly qualitical 4H-SiC epilayers.Homoepitaxial growth of 4H-SiC on off-oriented Si-face (0001) 4H-SiC substrates is performed at 1500℃ with a pressure of 1.3×103Pa by using the step-controlled epitaxy.The growth rate is controlled to be about 1.0μm/h.The surface morphologies and structural and optical properties of 4H-SiC epilayers are characterized with Nomarski optical microscope,atomic force microscopy (AFM),X-ray diffraction,Raman scattering,and low temperature photoluminescence (LTPL).N-type 4H-SiC epilayers are obtained by in-situ doping of NH3 with the flow rate ranging from 0.1 to 3sccm.SiC p-n junctions are obtained on these epitaxial layers and their electrical and optical characteristics are presented.The obtained p-n junction diodes can be operated at the temperature up to 400℃,which provides a potential for high-temperature applications.